crystals, which may have sphærulitic structure, showing a dark cross
between crossed nicols. Index of refraction lower than balsam (_n′_ =
1.483, α = 1.478, γ = 1.490), hence (in large crystals) the surface
would appear rather rough.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.012). Interference
colors the middle first order (yellow, etc.), a little higher than
those of quartz. _Extinction_ parallel to fibres. Optical character
(+).
REMARKS: Never a primary mineral in rocks, but found in igneous rocks
filling amygdaloidal cavities, and also as a very common alteration
product of sodalite, noselite, nephelite and acid plagioclases. It
gelatinizes easily with hydrochloric acid. H., 5 to 5.5. Sp. gr., 2.2.
OTHER ZEOLITES.
COMPOSITION: Hydrous silicates; Al, Ca and Na being the chief bases.
▄Usual Appearance in Sections▄: The form depends on the individual
mineral species, but the majority appear in elongated crystals or
fibers. They are all colorless and most of them have a small index of
refraction, hence no _relief_ (prehnite has distinct relief).
▄Crossed Nicols▄: The double refraction is generally very weak
(between that of nephelite and quartz), giving very low order
interference colors (prehnite and thomsonite have strong double
refraction).
REMARKS: The zeolites are always secondary minerals in rocks. They
gelatinize with hydrochloric acid.
GYPSUM.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
COMPOSITION: CaSO_{4} + 2H_{2}O.
▄Usual Appearance in Sections▄: Colorless grains or fibers. May be
colored, however, by inclusions of carbonaceous matter, iron oxides,
etc. Index of refraction about the same as orthoclase (_n′_ = 1.525, α
= 1.521, γ = 1.531), hence no _relief_ and surface smooth. Twinning
lamellæ abundant. Cleavage parallel to (010) gives abundant cracks,
other cleavages may also be noticed.
▄Crossed Nicols▄: Double refraction weak (γ − α = 0.010), the same as
quartz. Interference colors middle first order, white to yellow.
_Extinction_ parallel to most perfect cleavage cracks in sections
parallel to _b_ axis; large extinction angles noticed with reference
to less perfect cleavages. In _convergent light_, Ax. pl. ∥ (010),
_i.e._, ∥ to most distinct cleavages; Bx_{_a_}. (c) Λ = 54° front;
2_E_ = 104°; optical character (+). As the characters of gypsum are
not always very marked it may be necessary to employ micro-chemical
tests.
REMARKS: Forms a rock by itself, often associated with rock salt. It
also occurs as an alteration product of anhydrite. Gypsum is soluble
in hydrochloric acid. H., 1.5 to 2. Sp. gr., 2.2 to 2.4.
MONOCLINIC PYROXENES, Augite, etc.
Including the monoclinic minerals of the Pyroxene Group, which show
distinctly the characteristic cleavage parallel to an almost
right-angled prism.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
ELONGATION ∥
c′.[91]
COMPOSITION: RSiO_{3}, R = Ca, Mg, Mn, Fe, Al chiefly, with the Ca
predominating over the Mg.
▄Usual Appearance in Sections▄: Both in crystals and more or less
irregular grains, Figs. 4 and 12, the habit varying with the chemical
composition as follows:
_Diopside_ (Ca, Mg varieties), long columnar crystals and grains.
_Augite_ (ditto, but containing also Al and Fe), short prismatic
crystals and grains.
_Diallage_, granular or lamellar (|| (100)), may show fibrous structure
∥ _ć_.
Prism angle = 87° 06′ (important in cross-sections). Sections of
crystals nearly at right angles to the vertical axis _ć_ are octagonal
or square with truncated corners, Figs. 4 and 53, while those parallel
to the _ć_ axis are lath-shaped. Pyroxene also occurs in skeleton
crystals and acicular microlites in eruptive rocks.
[Illustration:
FIG. 52.—Augite, section parallel to _ć_ axis showing prismatic
cleavage, in leucite-basal. (From Cohen.)
]
Zonal structure (especially in augites) may be marked by differences in
color or extinction, and in some basalts the crystals have the
“hour-glass” structure.
_Twinning._—Common, usually the twinning plane being the ortho pinacoid
(100). Twin lamellæ may be noticed. Intergrowths occur with orthorhombic
pyroxene and amphibole.
_Color._—From almost colorless through green (diopsides, Na pyroxenes,
etc.) to brown (augites); the red to brownish-red color of certain
augites has been considered due to manganese. Yellow color very rare.
_Index of Refraction._—_n′_ = 1.68 to 1.72 (α = 1.671 to 1.706, γ =
1.700 to 1.728), hence _relief_ high and surface rough.
_Cleavage._—More or less perfect parallel to prism of 87° 06′. Cleavage
cracks distinct and numerous, but not generally running uninterruptedly
through crystal, Figs. 12 and 52. Cleavage not so perfect as that of
amphibole.
_Parting._—Diallage and diopside have distinct parting parallel to ortho
pinacoid (100), Fig. 53. Some crystals may show parting parallel to base
(001).
_Inclusions._—Tabular microscopic interpositions, similar to those in
bronzite, may occur in diallage. The iron ores, apatite, etc., may occur
in augite.
▄Polarized Light▄:
[Illustration:
FIG. 53.—Diallage, cross-section.
]
_Pleochroism._—Usually not noticed, and in general only appearing as
different shades of the same color. In some cases (diallage, fassaite
and Na rich augite) well marked, a and c green to yellowish green and b
brownish to reddish-brown; hence pleochroism not intense in sections
showing extinction angles. When Ti is present, violet parallel to _b_.
▄_Crossed Nicols_▄:
_Double Refraction._—Strong (γ − α = 0.022 to 0.029), being stronger in
the pale or colorless pyroxenes.
_Interference Colors._—Second order, hence always bright tints.
_Extinction._—Symmetrical in sections (through _b_ axis) showing
intersecting cleavage lines, in such cases bisecting the angles of the
cleavage. In sections showing parallel cleavage lines, only parallel in
ortho pinacoid (100) sections, in all other sections an extinction angle
being observed. The maximum extinction angle is large, lies in the
obtuse angle, varies with the chemical composition from 36° 30′ to 54°,
and is only obtained when the section of the crystal is parallel to the
clino pinacoid (010), Fig. 54, varying from this angle to 0°, when the
section is parallel to the ortho pinacoid (100). In Ti and Na pyroxenes
the inclined dispersion is so great that extinctions are not sharp, but
instead a change takes place in the interference color from bluish to
brownish.
▄_Convergent Light_▄: Axial plane parallel to clino pinacoid (010). Fig.
54. A cleavage flake parallel to ortho pinacoid (100) shows the
emergence of an optic axis (orthorhombic pyroxene parallel to best
pinacoidal cleavage would not show figure). Bx_{_a_}.(c) Λ _ć_ = 36° to
54° front. Axial angles large (2_E_ = 70° to 112°). Optical character
(+). The interference figures are distinct on account of the strong
double refraction.
▄Alteration▄: May take place to chlorite, serpentine or amphibole
(uralitization[92]), depending on the chemical composition and the
conditions producing the change.
[Illustration:
FIG. 54.—Diopside, clino pinacoid section.
]
▄Distinguished from▄:
(_a_) ORTHORHOMBIC PYROXENES.—By extinction angle, the orthorhombic
pyroxenes having always parallel or symmetrical extinction in sections
parallel to _a_, _b_, or _c_, and by higher order interference colors.
Also from hypersthene by absence of, or much fainter, pleochroism.
Diallage and bronzite might be confused on account of pronounced
pinacoidal parting, fibrous structure and inclusions; but may be
distinguished by the presence or absence of extinction angles and also
by the position of the optic axes relative to the best cleavage plates.
(_b_) AMPHIBOLE.—See under amphibole.
(_c_) EPIDOTE and CHRYSOLITE (Olivine). When light colored and granular,
by examination in convergent light. The plane of the optic axes is
parallel to the clino pinacoid (010), hence to the longitudinal axis and
prismatic cleavage cracks, while in epidote it is at right angles to
these directions and in chrysolite parallel to the base. Also yellow
color is common in epidote but rare in pyroxene.
REMARKS: Next to the feldspars pryoxene is the most common constituent
of the igneous rocks. Diopside and fassaite (green) are found in
contact rocks; also, what appear to be the same pyroxenes, in many
eruptive rocks, as andesites, monzonites, etc. Malacolite (light
green) is found in amphibolites and eclogites, where it may be
associated with a greenish amphibole (smaragdite). Diallage (bladed
and twinned) occurs in gabbros and pyroxenites. Common augite (brown)
is found in the remaining basic eruptive rocks. In the schists the
pyroxene is colorless.
Finally augite occurs as a secondary product resulting from the
“magmatic resorption” of hornblende and biotite.
Chemical corrosion and mechanical deformation may occur. The green and
brown augites when heated to redness on platinum foil may become red
in color. In general the pyroxenes are not attacked by acids. H., 5 to
6. Sp. gr., 3.3 to 3.5. The sp. gr. of the pyroxenes is considerably
higher than that of the amphiboles of similar composition, hence
mechanical separations are possible.
_Acmite_ (_Ægirine_) (Na pyroxenes).—Occur in green or brown, elongated
prismatic crystals, often not very transparent and with marked
pleochroism (like amphibole). Zonal coloring is common. When zonally
intergrown with pyroxene the outer zone is ægirine. The elongation is ∥
a′ (distinction from amphibole whose elongation is ∥ c′). The index of
refraction is higher than in the other pyroxenes (_n′_ = 1.792, α =
1.763, γ = 1.813) and the double refraction stronger (γ − α = 0.050).
The _extinction_ angle is small (5°) and the optical character (—).
The term _Ægirine-augite_ may be used to describe a soda, pleochroic
augite with a large extinction angle.
These pyroxenes are only found in the eruptive rocks rich in alkalies,
as elæolite-syenite, phonolite, certain trachytes, etc.; hence are
associated with elæolite, sodalite, leucite, etc. The small, second
generation, crystals, in the ground mass of a rock, are always the
richest in Na of the pyroxenes in that rock.
AMPHIBOLE, Hornblende, etc.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
ELONGATION ∥ c′.
COMPOSITION: RSiO_{3}. R = Mg, Ca, Fe chiefly also may contain Al, Na,
Mn. The Mg predominates over the Ca.
[Illustration:
FIG. 55.—Hornblende, showing twinning between crossed nicols, in
amphibole-biotite-granite. (From Cohen.)
]
▄Usual Appearance in Sections▄: Both in crystals and more or less
irregular grains, often fibrous, Figs. 55 and 56, the habit varying with
the chemical composition as follows:
_Tremolite_ (Mg_{3}Ca) and _Actinolite_ ((MgFe)_{3}Ca varieties), in
long columnar to needle-like individuals, with no terminal planes or
with frayed out ends. May be in dense aggregates.
_Pargasite_ in well developed crystals.
_Common green Hornblende_ (aluminous varieties) in crystals, compact
grains or shreds.
_Basaltic Hornblende_ (iron rich, aluminous varieties) in prismatic
crystals of varying length, which may often show “magmatic resorption”
(to augite and magnetite) around outer zone or throughout whole crystal.
[Illustration:
FIG. 56.—Hornblende, section parallel to _ć_ axis, showing prismatic
cleavage, in hornblende-diorite. (From Cohen.)
]
Crystals are simple in form of prismatic habit, with prism angle 124°
30′. Cross-sections are acutely rhombic, generally with acute angles
truncated, hence six-sided (pyroxene being eight-sided). Longitudinal
sections are lath-shaped and fibrous structure may be noticed. Skeleton
crystals may also occur, being very fine in certain pitchstones.
Zonal structure and parallel growth may be noticed in the amphiboles.
_Twinning._—Frequent, parallel to ortho pinacoid (100). Twins dual, less
often multiple, Fig. 55. Intergrowths with pyroxene and biotite occur.
_Color._—From colorless (tremolite), through green (actinolite,
pargasite and hornblende) to brown (basaltic hornblende). Yellow in some
varieties and bluish in the soda varieties.
_Index of Refraction._—_n′_ = 1.621 to 1.641 (α = 1.607 to 1.629, γ =
1.634 to 1.653) (1.719, in the basaltic hornblende), hence _relief_
distinct and surface rough.
_Cleavage._—Perfect, parallel to prism of 124° 30′. Generally appears in
thin sections as sharp cracks crowded close together, Figs. 56 and 57.
More perfect than in pyroxene.
Some of the long prisms (actinolite and tremolite) may show transverse
parting.
_Inclusions._—The iron ores, apatite, etc., may be found in hornblende.
[Illustration:
FIG. 57.—Hornblende, cross-section.
]
▄Polarized Light▄:
_Pleochroism._—All colored amphiboles show pleochroism, which in general
is stronger the darker the color of the variety (actinolite and
pargasite show but little). The absorption is very marked in the
hornblendes, being greatest in the general direction of the cleavage
lines in longitudinal sections. Marked differences in absorption are
also characteristic of the mineral species biotite, tourmaline and
allanite. Pleochroic halos (brownish) surrounding inclusions may be
noticed.
_▄Crossed Nicols▄_:
_Double Refraction._—Quite strong, but a little weaker than in pyroxene
(γ-α = 0.019 to 0.027). Ferruginous basaltic hornblende has strong
double refraction (γ-α = 0.072).
[Illustration:
FIG. 58.—Actinolite, clino pinacoid section.
]
_Interference Colors._—Second order, hence bright tints, but in darker
colored varieties not so noticeable as in pyroxenes, due to the stronger
absorption of parts of the light. The colors of basaltic hornblende are
so high that they show no bright tints.
_Extinction._—Always symmetrical in sections (through _b_ axis) showing
intersecting cleavage lines, in such cases bisecting the angles of the
cleavage. In sections showing parallel cleavage lines, only parallel in
ortho pinacoid (100) sections, in all other sections an extinction angle
being observed. The maximum extinction angle lies in the acute angle and
is much smaller than in pyroxene, varying with the chemical composition
from 0°–20°. In hornblende, actinolite and tremolite 12°–20°, Fig. 58;
in the basaltic hornblende 0°–10°. The maximum extinction angle is only
obtained when the section of the crystal is parallel to the clino
pinacoid (010), varying from this angle to 0°, when the section is
parallel to the ortho pinacoid (100).
_▄Convergent Light▄_: Axial plane parallel to clino pinacoid (110), Fig.
58. Bx_{_o·_}(c) Λ _ć_ = 0°–20° behind. Axial angles large (2_E_ = 77°
to >180°). Optical character (−). Pargasite is (+).
▄Alteration▄: May take place to chlorite, talc, serpentine, asbestus,
etc., depending on the chemical composition. Amphibole frays out and
becomes fibrous during alteration, and may also lose color.
▄Distinguished from▄:
(_a_) PYROXENE.—By usually much stronger pleochroism in the colored
varieties, and by cleavage and extinction angle. In pyroxene the
cleavage (parallel to prism of 87° 06′) is less perfect; and the
extinction angle is much larger, varying from 36° to 54°.
(_b_) BIOTITE.—By the extinction in the mica being always about parallel
to the cleavage. Both have strong absorption, but biotite shows very
slight pleochroism in sections parallel to the cleavage, and has only
the one cleavage parallel to the base. Also the biotite has lower index
of refraction and generally shows uniaxial interference figure.
Colorless tremolite may be distinguished from muscovite and talc by
extinction angles, relief and lower order interference colors.
(_c_) TOURMALINE.—By presence of cleavage, and by the fact that
absorption is most marked about parallel to the elongation (also
parallel to cleavage lines), while in tourmaline the absorption is
strongest at right angles to the elongation.
(_d_) The ORTHORHOMBIC PYROXENES.—By extinction angles, the latter
having parallel extinction in all sections parallel to _a_, _b_ and _c_,
and by prismatic cleavage of 124° 30′. Pleochroism is strong in the
colored varieties of both species, but in amphibole it appears more
generally as a variation of the same color; while in hypersthene a
change in color is often noticed, from brownish-red to greenish parallel
to _ć_ axis.
(_e_) SILLIMANITE and CYANITE.—See under the latter.
REMARKS: Amphibole comes next to pyroxene in importance and
distribution of the dark colored ferruginous rock-forming minerals. As
a rule it occurs in rocks with a large percentage of SiO_{2},
associated with quartz and orthoclase; while augite generally occurs
in rocks of a basic nature, associated with plagioclase and little or
no free SiO_{2}. Furthermore amphibole contains hydroxyl and is
therefore naturally found in the deep eruptive rocks; its place being
taken by augite in the effusives. By application of heat hornblende
changes to augite, while hydrochemical processes bring about the
opposite result “uralitization.”
Tremolite and actinolite are found in contact rocks and crystalline
schists, also as a result of the alteration of olivine into
serpentine. Pargasite occurs in contact rocks. Common green hornblende
is found in the plutonic rocks (Na poor and SiO_{2} rich), also in
contact rocks and crystalline schists (amphibolites). Brown hornblende
replaces the green variety in the basic plutonic rocks. Basaltic
hornblende is found in many effusive rocks.
The hornblende crystals in eruptive rocks, being among the first
formed constituents, have often suffered subsequent corrosion by the
magma, giving rise to the dark border already mentioned. The brown
primary hornblende in some rocks may be changed by a process analogous
to “uralitization” into a green, reed-like hornblende. Mechanical
deformations are found in massive and schistose rocks. Light green
amphiboles, with weak pleochroism, may often be colored intensely
reddish-brown and made strongly pleochroic by heating to redness on
platinum foil. In general the amphiboles are not affected by acids.
H., 5 to 6. Sp. gr., 2.9 to 3.3.
_Glaucophane_, _Arfvedsonite_, etc. (Na rich amphiboles).—Occur blue to
bluish-green in color, with pleochroism and weaker double refraction
than the other amphiboles. Extinction angles vary from 4°–6°
(glaucophane) to 14° (arfvedsonite). They are found in contact rocks,
crystalline schists, eclogite, etc.
For the rarer and less known members of the amphibole group, resource
should be had to more elaborate works.
_Uralite._—Pyroxene altered to amphibole, having the outward crystal
form of pyroxene and the physical characters and cleavage of amphibole.
The change usually commences on the surface and the uralite does not
form a single compact crystal, but consists of numerous slender columns
parallel to one another. These little columns or fibers have their _c_
and [_=b_] axes parallel to the positions of these axes in the parent
mineral. The color is green and the pleochroism weak to strong.
This change is called “_uralitization_” and results from hydrochemical
processes. When the alteration is not complete, portions of the original
pyroxene may be left, having all the characteristic optical properties
of this latter mineral.
_Anthophyllite_, the orthorhombic amphibole, with always parallel
extinction, is sometimes found in colorless to brownish, blade- to
rod-like aggregates in crystalline schists and serpentine.
MICA GROUP.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
May appear hexagonal or orthorhombic.
COMPOSITION: ELONGATION (∥ cleavage) ∥ c′.
_Biotite_ (black or ferro-magnesium mica) =
(H.K)_{2}(Mg.Fe)_{2}Al_{2}(SiO_{4})_{3}, approx.
_Phlogopite_ = a magnesium mica, near biotite, but containing little Fe.
[Illustration:
FIG. 59.—Mica. _A_, biotite, showing hexagonal cross-section and zonal
markings. _Minette_, Freiburg. _B_, Biotite, showing strong
absorption parallel to cleavage and also zonal marking (_P_ = plane
of vibration of polarizer). _Minette_, Cumberland. _C_, Muscovite in
bent shreds in gneiss.
]
_Muscovite_ (white or potash mica) = H_{2}(K.Na)Al_{3}(SiO_{4})_{3},
with some replacement by Mg or Fe.
▄Usual Appearance in Sections▄: Scales, which may be notched or jagged,
with lateral sections lath-shaped; or shreds, Fig. 59 C. When distinctly
crystallized (magnesium micas) the thin hexagonal plates have plane
angles of 120°, Fig. 59 A. Phlogopite crystals may be extended in
direction of _ć_ axis.
Zonal structure not uncommon in the magnesium micas, Fig. 59 A, which
may also have dark iron ore border like hornblende.
_Twinning._—Common, generally parallel to base; seen in sections showing
cleavage by variations in extinction, in basal sections by distorted
interference figures.
Micas of different kinds often associated together in parallel position,
also intergrown with hornblende, pyroxene, chlorite and quartz.
_Color._—Depends on chemical composition. Biotites, brown, green or red
to almost opaque. Phlogopites, colorless or yellowish. Muscovites
colorless.
_Index of Refraction._—_n′_ = 1.564 to 1.619 (α = 1.541 to 1.580, γ =
1.575 to 1.638), hence somewhat marked _relief_ and surface varies in
appearance from slightly rough to fairly rough. In polarized light the
surface appears roughest when the cleavage cracks are parallel to the
plane of the polarizer.
Biotite has more marked relief.
_Cleavage._—Very perfect, parallel to base (001), Fig. 60. Basal
sections show no cleavage, but all other sections show many sharp,
parallel cleavage cracks.
[Illustration:
FIG. 60.—Biotite, showing basal cleavage, in biotite-granite. (From
Cohen).
]
For percussion and pressure figures, see reference given below.[93]
_Inclusions._—May be arranged parallel to lines of pressure figure.
Rutile needles, tourmaline, apatite, etc., common in magnesium mica.
Zircon inclusions often surrounded by pleochroic halos.
▄Polarized Light▄:
_Pleochroism._—Varies with the color, being very marked in the colored
varieties (from pale yellow to chestnut-brown or black). The strong
absorption, about parallel to the cleavage lines, is very characteristic
of the colored mica, Fig. 59 B. Strong absorption is also noticed in
hornblende, tourmaline and allanite. Absorption may even be noticed
around inclusions (pleochroic halos) in colorless, non-pleochroic micas.
Cleavage plates of biotite are not pleochroic unless the axial angle is
large.
_▄Crossed Nicols▄_:
_Double Refraction._—Very strong (γ − α = 0.034 to 0.058).
_Interference Colors._—High order (third). May be very bright in thin
sections of the colorless micas, and at times be so high in order as not
to show any marked color tints. May not be noticeable in sections of the
colored varieties, due to absorption of parts of the light.
_Extinction._—About parallel to cleavage lines. Very small extinction
angles may be noticed in biotites. Basal sections of biotite (the
approximately hexagonal mica) usually appear isotropic.
Mottled appearance (like “Birds-eye” maple) characteristic, caused by
distortion of the flexible laminæ during grinding. Most noticeable in
sections inclined to cleavage and near position of extinction.
_▄Convergent Light▄_: Axial plane[94] and Bx_{_a_}, practically at right
angles to basal cleavage; therefore cleavage plates always show well
defined interference figures, generally biaxial in character. The axial
angles vary greatly, being usually small for biotite and phlogopite (may
appear uniaxial) and large for muscovite (2_E_ = 55° to 90°). Optical
character for all micas (−).
▄Alteration▄: Biotites decompose quite easily, lose color and may become
completely bleached, which appears to be due to a leaching out of the
iron. May also alter to green chlorite, with a fraying out of the mica
and a change to chloritic structure.
Phlogopites may alter to fibrous, scaly masses, apparently chiefly talc.
“Sagenite” webs of rutile may accompany the alteration.
Muscovites are characterized by their freshness, and do not seem to
suffer from weathering.
▄Distinguished from▄:
(_a_) HORNBLENDE.—Magnesium mica has extinction about parallel to the
cleavage, while hornblende may have extinction angles of from 0° to 20°.
Both have strong absorption, but biotite shows very slight pleochroism
in basal sections, which also give approximately uniaxial interference
figures in convergent light.
(_b_) TOURMALINE.—Magnesium mica shreds show absorption parallel to
elongation, while in tourmaline the absorption is at right angles to
elongation. There is also an absence of cleavage in tourmaline.
(_c_) CHLORITE.—By strong double refraction, the very high order colors,
however, being often not noticed. Chlorite also shows aggregate
structure and is almost always greenish in color.
(_d_) TALC.—White mica by large axial angle of scales in convergent
light and by micro-chemical tests. The distinction may be very
difficult.
REMARKS: Muscovite is a rare primary mineral in eruptive rocks, except
in two-mica granite, etc. As a secondary mineral it occurs in dense
scaly aggregates or as pseudomorphs after feldspar, nephelite, etc. It
is frequent in crystalline schists and is probably also the mica in
amphibolite and eclogite. Phlogopite is found chiefly in contact
metamorphic limestone; and may be distinguished from muscovite by
nearly uniaxial character and less sharp cleavage. Biotite is much
more widely distributed, occurring especially in eruptive rocks,
crystalline schists and contact rocks.
Chemical corrosion occurs in original biotite of porphyritic rocks,
producing a “resorption border” of augite and magnetite. Mechanical
deformations, producing bending and slipping along “gliding” planes
(oblique to cleavage), are common to all varieties of mica and may
produce change to chlorite. The muscovites, together with the
feldspars, are the most characteristic minerals of dynamo metamorphic
origin. Biotites and phlogopites are attacked by sulphuric acid at
high temperatures. Muscovite is but slightly attacked by acids. H., 2
to 3. Sp. gr., 2.7 to 3.2. The specific gravity separation between the
micas is difficult on account of the scaly nature of the minerals.
Other micas occur, some being alteration products of those already
described. Among these may be mentioned:
_Lithia Mica._—Both light and dark colored, occurring in granitic rocks
and often only distinguished chemically from muscovite and biotite.
_Damourite_ (_Sericite_) (hydrous K mica).—A secondary product usually
in colorless, fine scaly aggregates in phillites, sericite-schists, etc.
CHLORITE GROUP.
Embracing the members of the Chlorite Group, commonly occurring in
rocks.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
The minerals of this group usually appear uniaxial, and crystallize in
part with hexagonal symmetry.
COMPOSITION: May be considered as isomorphous mixtures of
H_{4}(MgFe)_{3}Si_{2}O_{9} and H_{4}(MgFe)_{2}(AlFe)_{2}SiO_{9}
(Rosenbusch).
▄Usual Appearance in Sections▄: Minute, scaly aggregates, which may
incline to radial grouping; or in minute grains as a pigment (veridite)
in other minerals.
_Twinning._—May be seen as in mica.
_Color._—Generally green, varying from greenish white to dark green,
rarely colorless or red.
_Index of Refraction._—_n′_ = 1.567 to 1.589 (α = 1.560 to 1.585, γ =
1.571 to 1.596), hence no marked relief and only slightly rough surface.
_Cleavage._—Like mica, very perfect; parallel to flat face, which is
considered to be the basal plane. This cleavage may not be noticed,
especially in secondary chlorite in rocks.
▄Polarized Light▄:
_Pleochroism._—In green and yellow tints (green ∥ cleavage), being more
marked in dark colored varieties. Basal sections are non-pleochroic, the
mineral being practically uniaxial. Pleochroic halos may be seen.
_▄Crossed Nicols▄_:
_Double Refraction._—Generally very weak (γ − α = 0.001 to 0.011).
_Interference Colors._—Very low first order, gray or white, at times
scarcely noticed. Anomalous colors, however, often seen (deep blue or
brown.)
_Extinction._—Plates parallel to cleavage generally appear isotropic or
only show faint color. In other sections extinction is apparently
parallel to the cleavage in the uniaxial type, but extinction angles may
be noticed when type is biaxial. Complete extinction may not be noticed,
due to aggregate structure.
_▄Convergent Light▄_: Plates parallel to cleavage show, at times, an
indistinct interference cross, which may open into two hyperbolas,
indicating biaxial nature of crystallization. Ax. pl. ∥ (010); Bx_{_a_}.
Λ _ć_ = 0° to 15°; 2_E_ variable. Optical character (±).
▄Distinguished from▄: SERPENTINE.—By general green color (serpentine,
with exception of Fe rich variety, is colorless), pleochroism and
frequent anomalous interference colors; but the distinction between
these two minerals may be very difficult. Chlorite may resemble
decomposed or green mica (mica has, however, strong double refraction).
The different species in the chlorite group cannot usually be
distinguished in rocks.
REMARKS: The chlorites are essentially secondary minerals, derived
from the aluminous silicates, biotite, augite, garnet, feldspar, etc.
They are found abundantly in chlorite-schists, contact rocks, etc.,
and as pigment (veridite) in altered eruptive rocks. May occur as a
primary constituent of eruptive rocks, often in parallel growth with
biotite. Chlorides are acted on by hot hydrochloric acid, and
decomposed easily by sulphuric acid. H., 2 to 3. Sp. gr., 2.6 to 2.96.
A thin section heated to redness on platinum foil loses water and
becomes opaque. Ferruginous varieties are turned reddish-brown to
black by heating (serpentine, as it contains less iron, may give
negative results with this test).
_Delessite._—Found in sphærulites, filling cavities in amygdaloidal
basic rocks, and in pseudomorphs. It appears to be much altered to other
minerals.
TALC.
ANISOTROPIC. MONOCLINIC
(Pseudo-Hexagonal).
COMPOSITION: ELONGATION ∥ c′.
H_{2}Mg_{3}(SiO_{3})_{4}.
▄Usual Appearance in Sections▄: In fine scaly, colorless aggregates.
Sections, cutting across the scales, would show rod-like forms. Index
of refraction only a little higher than balsam (_n′_ = 1.572, α =
1.539, γ = 1.589), hence no marked _relief_ and only slightly rough
surface. Cleavage perfect parallel to base, like mica.
▄Crossed Nicols▄: Double refraction very strong (γ − α = 0.050).
Interference colors third order, like muscovite. _Extinction_ parallel
to basal cleavage lines. In _convergent light_, Ax. pl. ∥ (100),
Bx_{_a_}. ∥ _ć_; axial angle small (2_E_ = small); optical character
(−).
▄Distinguished from▄: MUSCOVITE (with which it is easily confused) by
micro-chemical tests, proving absence of alkalies and Al; and often by
the more aggregate structure of the talc. Also by smaller axial angle
of scales in convergent light.
REMARKS: Found mainly in metamorphic schists, etc., always as a
secondary product. It is insoluble in hydrochloric acid. H., 1 to 1.5.
Sp. gr., 2.6 to 2.8.
EPIDOTE.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
ELONGATION ∥ a′ or
c′.
COMPOSITION: Ca_{2}Al_{2}(AlOH)(SiO_{4})_{3}, with some Fe replacing Al.
▄Usual Appearance in Sections▄: Columnar to thick tabular crystals, more
or less elongated parallel to ortho axis [_=b_], Fig. 61, or in granular
aggregates.
[Illustration:
Epidote.
FIG. 61. FIG. 62.
Ortho pinacoid section. Clino pinacoid section.
]
_Twinning._—May occur but rarely noticed. Irregular interpenetrations
common with other members of the group; also parallel growths.
_Color._—Colorless to yellowish (Fe poor) or yellow to greenish to
yellow-brown (Fe rich).
_Index of Refraction._—_n′_ = 1.751 but may be lower (α = 1.731, γ =
1.768), hence _relief_ high and surface rough.
_Cleavage._—Parallel to base (001), Figs. 61 and 62, imperfect parallel
to ortho pinacoid (100). Basal cleavage cracks not very numerous and
appear parallel to general direction of elongation.
▄Polarized light▄:
_Pleochroism._—Varies with the color, being faint in the light colored
varieties, but strong when the color is marked (Fe rich).
_▄Crossed Nicols▄_:
_Double Refraction._—Variable, often very strong (γ − α = 0.037).
_Interference Colors._—Variable, often high (third) orders. Intergrowths
with other members of the group are clearly shown by the “flecked”
interference colors.
_Extinction._—Parallel to cleavage in sections parallel to [_=b_] axis.
In other sections extinction angles vary, see Fig. 62.
_▄Convergent Light▄_: Axial plane ∥ (010), _i. e._, at right angles to
the elongation of crystal and cleavage cracks, Fig. 61. Bx_{_a_.}-(a) Λ
_ć_= 3° behind. Basal cleavage flakes show the almost ⟂ emergence of an
optic axis. Axial angles very large (2_E_ > 180°). Optical character
(−).
▄Alteration▄: Does not take place readily.
▄Distinguished from▄: Light colored MONOCLINIC PYROXENE.—By having plane
of optic axes at right angles to cleavage cracks and direction of
elongation; while in pyroxene plane of optic axes is parallel to
parallel prismatic cleavage cracks or bisects the angle between
intersecting cracks. Furthermore the yellow color is rare in pyroxene.
REMARKS: Epidote is essentially a secondary mineral, resulting from
the alteration of the feldspars and the ferro-magnesium silicates. It
is found in crystalline schists (especially those containing
hornblende), gneiss, gabbro, diorite, diabase, lime-silicate
hornstones, contact rocks, etc. Epidote is partially decomposed by
hydrochloric acid. The Fe rich epidote can be changed to an intense
color by “glowing” in the air. H., 6 to 7. Sp. gr., 3.32 to 3.45.
_Piedmontite_ (containing Mn).—Red in color. Very pleochroic, red to
yellow. Found in crystalline schists, the porphyrite of Scotland, the
famous “Porfido rosso antico” of Egypt and in certain Japanese mica
schists.
ZOISITE.
Essentially orthorhombic? members of Epidote group.[95]
COMPOSITION: Like epidote but without any Fe.
▄Usual Appearance in Sections▄: Similar to epidote or in columnar
aggregates. Often intergrown with epidote.
Distinguished from epidote by general absence of color (colorless to
yellowish) and pleochroism; by slightly lower refractive index (_n′_ =
1.699 to 1.720) and by much weaker double refraction (γ − α = 0.005
and less). The interference colors are very low order, gray to white,
but anomalous colors are often seen (yellow or prussian blue).
Extinction is in general parallel to pinacoidal cleavage cracks
(except in clinozoisite).
The plane of the optic axes may be parallel or at right angles to
cleavage cracks, and the optical character is (+).
REMARKS: Generally a secondary mineral. Found in crystalline schists,
amphibolites, contact rocks, eclogite, etc., and in “saussurite.” May
be hard to distinguish from vesuvianite and apatite.
ALLANITE, Orthite.
Monoclinic member of Epidote group.
COMPOSITION: Like epidote but containing cerium.
▄Usual Appearance in Sections▄: Similar to epidote in form; but
distinguished by brown color (may be also almost colorless), strong
pleochroism and absorption, and medium to weak double refraction (γ −
α = 0.002 to 0.030). Lamellar twinning clearly seen on account of
oblique extinction. When included in hornblende and mica it is
surrounded by pleochroic halos. _n′_ = 1.78 about. Optical character
(±). Usually perfectly fresh.
REMARKS: Found as an accessory mineral in SiO_{2} rich eruptive rocks
and connected crystalline schists, and (light colored) in amphibolite
and eclogite.
TITANITE, Sphene.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
COMPOSITION: CaTiSiO_{5}. ELONGATION ∥
a′.[96]
▄Usual Appearance in Sections▄: Wedge-shaped crystals (in Na rich rocks,
more prismatically developed); grains, which may be elongated; and
aggregates of small rounded particles, which appear nearly opaque.
Sections of crystals commonly acute rhombs, Fig. 63.
_Twinning._—Occurs, the twinning boundary bisecting the acute angles of
the rhomb (only noticed between crossed nicols), Fig. 64.
_Color._—Reddish-brown to yellowish to colorless.
[Illustration:
FIG. 63.—Titanite, showing acute rhombic cross-section.
]
_Index of Refraction._—_n′_ = 1.920 to 1.963 (α = 1.888 to 1.913, γ =
1.978 to 2.054), hence _relief_ very marked and surface very rough.
_Cleavage._—Imperfect and not parallel to predominant form, hence only
appears as a few rough cracks, which are not parallel to any
crystallographic boundary, Fig. 63. Cleavage rarely observed in
secondary grains.
▄Polarized Light▄:
_Pleochroism._—Varies with the color, being more distinct in colored
crystals, yellowish (the lighter color) ∥ a′ and reddish-brown ∥ c′.
Scarcely noticed when the color is light.
_▄Crossed Nicols▄_:
_Double Refraction._—Very strong (γ − α = 0.090 to 0.141).
[Illustration:
FIG. 64.—Titanite, showing twinning in nepheline-syenite. (From
Cohen.)
]
_Interference Colors._—Very high order, like those of calcite. Due to
the fact that the refractive indices of two of the rays are very nearly
alike, some sections may show very low order colors.
_Extinction._—Extinction angles not characteristic. There may be no
complete extinction in white light, owing to dispersion.
_▄Convergent Light▄_: On account of the very strong characteristic
dispersion of the optic axes (ρ > ν), the axial angle varies a good deal
with the color of the light used, (St. Gotthard) 2_E_{IA}_ = 57°,
2_E_{Tl}_ = 47°. By using colored glasses this variation in the axial
angle can be seen. The axial plane lies in the clino pinacoid (010),
hence bisects the obtuse angle in the rhombic cross-section, Fig. 63.
Bx_{_a_}.(c) Λ _ć_ = 51° front. The optical character is (+).
▄Alteration▄: May take place.
▄Distinguished from▄:
(_a_) STAUROLITE.—In convergent light the axial plane is shown to be in
the shorter diagonal of the rhombic cross-section, while in staurolite
it is in the longer diagonal.
(_b_) RUTILE.—By biaxial character.
(_c_) CALCITE.—The light colored titanite (sphene), in absence of
twinning, by higher index of refraction.
Titanite may easily be confused with some of the rarer minerals.
REMARKS: Titanite is always an accessory mineral and is found
distributed in all rocks, except SiO_{2} rich eruptive and magnesia
silicate rocks. As a secondary mineral it forms rims around other
titanium minerals or pseudomorphs after them and also the principal
part of _leucoxene_. It is partly soluble in hot hydrochloric acid and
completely decomposed by sulphuric acid. H., 5 to 5.5. Sp. gr., 3.3 to
3.7. In a specific gravity separation it falls with the ferruginous
minerals (on account of its density) and from these can generally be
separated by electromagnetic methods.
FELDSPAR GROUP.
Orthoclase, Microcline and the Plagioclases.
ORTHOCLASE.
ANISOTROPIC. BIAXIAL. MONOCLINIC.
ELONGATION (∥
cleavage) ∥ a′.
COMPOSITION: KAlSi_{3}O_{8}, with some replacement by Na.
▄Usual Appearance in Sections▄: In crystals and grains. In porphyritic
rocks habit of crystals more or less tabular parallel to clino pinacoid
(010), or rectangular much extended parallel to clino axis _à_, with
cross-sections six-sided, or rectangular to long lath shape, Figs. 65
and 66. Crystals may be changed into rounded or looped grains by
chemical corrosion, Fig. 6. Adularia crystals more prismatically
developed, giving rhombic sections. Dimensions of crystals vary
extremely; microlites occur, at times forming sphærulitic structure. The
very fine grained ground-mass “microfelsite” (not resolved by the
microscope) consists largely of feldspar.
[Illustration:
Orthoclase cleavage plates.
FIG. 65.—Basal. FIG. 66.—Clino pinacoid.
]
Intergrowths with microcline and plagioclase common, forming
“microperthite” when lamellæ are microscopic. May be in zonal formation
with plagioclase (the orthoclase on the periphery). Also intergrown with
quartz[97] forming “pegmatite” and “micro-pegmatite,” Fig. 67.
[Illustration:
FIG. 67.—Micro-pegmatitic structure, in granophyric quartz-porphyry.
(From Cohen.)
]
Zonal structure often seen, Fig. 20, especially when decomposition has
commenced; and in fresh crystals may be indicated by zonal arrangement
of inclusions.
_Twinning._—Very common, generally after _Carlsbad_ law, Figs. 18 and
69; the twinning boundary, dividing the section longitudinally, either
being parallel to edges of crystal or bent or jagged. Twinning after
_Baveno_ (twinning boundary diagonal, with the two parts extinguishing
at the same time, but having a and c directions crossed in the two
portions) and _Manebach_ laws less common.[98]
[Illustration:
FIG. 68.—Orthoclase, ortho pinacoid section showing cleavages
intersecting at 90°, in augite-syenite.
]
_Color._—Colorless or tinged by oxide of iron. Cloudy if decomposed.
_Index of Refraction._—_n′_ = 1.523 (α = 1.519, γ = 1.526), hence no
_relief_ and surface smooth.
_Cleavage._—Varies and sometimes only seen in very thin sections, but is
an important character and should always be searched for. It occurs
perfect, parallel to base (001), and almost as perfect parallel to clino
pinacoid (010). The two cleavages intersect at 90° in section parallel
to the [_=b_] axis, Fig. 68.
_Inclusions._—May be present and arranged in regular or zonal order, but
not important. Do not occur in individuals of a second generation.
▄Polarized Light▄:
_Pleochroism._—None.
_▄Crossed Nicols▄_:
_Double Refraction._—Very weak (γ − α = 0.007).
_Interference Colors._—Lower first order, gray, white, etc., not quite
so bright as colors of quartz and plagioclase.
_Extinction._—Being monoclinic the extinction angle on base (001), with
reference to clino pinacoid (010) cleavage cracks, is 0°. On clino
pinacoid with reference to basal cleavage cracks, it is 5°. Some
sections (notably in glassy sanidine grains) may appear dark during
complete rotation. This is due to the fact that the axial angle is very
small and the sections act approximately like those of a uniaxial
mineral at right angles to the optic axis.
_▄Convergent Light▄_:[99] Plane of optic axes in general at right angles
to clino pinacoid (010) (plane of symmetry), Fig. 65, hence parallel to
trace of basal cleavage; but in some sanidines parallel to plane of
symmetry. Bx_{_a_} (a) Λ _a_ = 5° above. Axial angles vary, 2_E_ = 125°
(orthoclase), 0°–50° (sanidine).[100] May appear uniaxial when axial
angle is very small. Optical character (−).
▄Alteration▄: Very common to clay,[101] muscovite, hydrargillite, etc.
Generally commences along the cleavage cracks, and when it has
progressed very far the whole feldspar appears opaque or cloudy, and no
perceptible change may take place between crossed nicols. As
decomposition is very prevalent in many rocks, the orthoclase is rarely
clear or pellucid. Epidote is often formed when accessory solutions are
present.
▄Distinguished from▄:
(_a_) The other FELDSPARS and MELILITE.—See under the latter minerals.
(_b_) QUARTZ.—Feldspar is biaxial but when occurring in clear glassy
grains (notably sanidine), which appear uniaxial in convergent light,
may resemble quartz. When tested the optical character is (−), while
that of quartz is (+).
REMARKS: The members of the feldspar group are the widest distributed
of the rock-forming minerals and their recognition is of the utmost
importance on account of their bearing on the systematic
classification of rocks. Orthoclase is found as an essential
constituent in the more acid plutonic and older volcanic rocks, as
granite, syenite, trachyte, porphyry, and also in gneiss, crystalline
schists, more seldom in contact rocks and subordinate in clastic
rocks.
Chemical corrosion (producing rounded or looped grains), Fig. 6, and
mechanical deformation (producing angular, sharp-edged, broken grains,
bending and undulatory extinction[102]), Fig. 7, occur in orthoclase.
When a rock containing feldspar crystals is shattered, the orthoclase
breaks parallel to basal cleavage and plagioclase parallel to twinning
plane. Orthoclase is practically insoluble in acids. H., 6 to 6.5. Sp.
gr., 2.56.
[Illustration:
FIG. 69. —Sanidine, showing Carlsbad twin and cross-parting, in
nepheline-phonolite. (From Cohen.)
]
_Sanidine._—This clear, glassy variety of orthoclase occurs in the later
eruptive rocks, rhyolite, trachyte, obsidian, etc. Sanidine often has a
parting parallel to ortho pinacoid (100), which may be noticed in
sections so thick that the cleavage is not seen, Fig. 69. In general it
shows no sign of decomposition, and has a smaller axial angle than
orthoclase. Inclusions of glass are more abundant than in orthoclase.
MICROCLINE.
ANISOTROPIC. BIAXIAL. TRICLINIC.
COMPOSITION: KAlSi_{3}O_{8}.
▄Usual Appearance in Sections▄: As a rock constituent in irregular
grains.
In general characters like orthoclase and _distinguished_ from it and
the plagioclases by characteristic “_gridiron_” structure between
crossed nicols, resulting from the polysynthetic twinning after both
_Albite_ and _Pericline_ laws, Fig. 19. This crossed twinning will show
in all sections except those parallel to the brachy pinacoid (010). The
lamellæ are generally thinner than in the plagioclases and more
“spindle-shaped.”[103]
Furthermore the rather obscure triclinic crystallization is shown by an
extinction angle of + 15° on basal cleavage plates with reference to
brachy pinacoid (010) cleavage lines (distinction from orthoclase, which
has O° extinction angle).
REMARKS: Found with orthoclase, often almost replacing it, in granite,
syenite, gneiss, etc., and is one of the last minerals to form. It is
notably resistant to decomposition. A structure like the microcline
twin structure may be produced in orthoclase by dynamic action.[104]
THE PLAGIOCLASES.
Albite, Oligoclase, Labradorite, Anorthite.
ANISOTROPIC. BIAXIAL. TRICLINIC.
ELONGATION (∥ Albite twin lamellæ) ∥ a′ (except in anorthite when it may
be ∥ a′ or c′).
COMPOSITION:[105]
Albite, NaAlSi_{3}O_{8}.
Oligoclase, _n_(NaAlSi_{3}O_{8}) + CaAl_{2}Si_{2}O_{8}, or _n_ Ab + An,
_n_ = 2 to 6.
Labradorite, NaAlSi_{3}O_{8} + _n_(CaAl_{2}Si_{2}O_{8}), or Ab + _n_ An,
_n_ = 1, 2 or 3.
Anorthite, CaAl_{2}Si_{2}O_{8}.
▄Usual Appearance in Sections▄: Much the same as orthoclase.
Lath-shaped[106] forms and microlites very common, especially in the
acid series.
[Illustration:
FIG. 70.—Plagioclase, showing narrow lamellæ, in diabase. (From
Cohen.)
]
[Illustration:
FIG. 71.—Plagioclase, showing broad lamellæ, in gabbro. (From Cohen.)
]
_Twinning._—Polysynthetic, after _Albite_ law, almost universal; the
twinning appearing between crossed nicols as a series of dark and light
bands, bounded by parallel edges, Figs. 70 and 71. The twin lamellæ are
parallel to brachy pinacoid (010), hence not observed in sections
parallel to this pinacoid. The lamellæ may appear irregular and
interrupted, and seem to be broader in the basic than in the acid
series. When this twinning fails, however, as in the basic plagioclases
in certain metamorphic rocks, the determination becomes very difficult.
In some cases polysynthetic twinning, after both _Albite_ and
_Pericline_ laws, may take place at the same time, giving rise to a
structure somewhat similar to that of microcline, Fig. 72. In addition
the polysynthetic crystals may be twinned like orthoclase after
_Carlsbad_ and _Baveno_ laws.
The general characters are the same as in orthoclase with the following
differences:
_Indices of Refraction_: _n′_ = 1.535 (α = 1.532, γ = 1.540) Albite.
_n′_ = 1.541 (α = 1.537, γ = 1.545)
Oligoclase, Ab_{4}An_{1}.
_n′_ = 1.559 (α = 1.555. γ = 1.563)
Labradorite, Ab_{1}An_{1}.
_n′_ = 1.582 (α = 1.575, γ = 1.588) Anorthite.
The surface of anorthite appears slightly rougher than that of
orthoclase.
_Cleavages_, parallel to base (001) and brachy pinacoid (010), never
intersect at right angles, as is the case in sections of orthoclase
parallel to [_=b_] axis. This is due to the triclinic system of
crystallization, but the divergence from a right angle is small (93° 36′
to 94° 10′).
[Illustration:
FIG. 72.—Plagioclase, showing crossed lamellæ, in olivine-gabbro.
(From Cohen.)
]
_Inclusions_ at times may be quite important, as the vitreous inclusions
of oligoclase in andesites, etc., and the iron ore inclusions and other
microlites in labradorite. The arrangement of these inclusions may be
zonal or in parallel orientation.
_Double refraction_ is a little stronger than for orthoclase (γ − α =
0.008 to 0.013 (anorthite)), hence producing slightly brighter
interference colors in sections of the same thickness.
_Extinction_ takes place in all sections unsymmetrically with respect to
crystallographic, twinning or cleavage lines (as these minerals are
triclinic); hence extinction angles are always observed.
_▄Convergent Light▄_: All plagioclases show the emergence of a
bisectrix,[107] more or less oblique, on brachy pinacoid (010) cleavage
faces. These cleavage faces show no twin lamellæ, unless twinning after
_Pericline_ law occurs, in which case the determination is much more
complicated. The axial angle is large, 2_E_ = 155° (Albite). Optical
character, depending on variety, (+) or (−).
▄Alteration▄: Partly the same as in orthoclase, forming clay, muscovite,
etc. Calcite and epidote are more common as side-products, and
zeolitization also occurs in some rocks. The plagioclases decompose more
easily than orthoclase.
▄Distinguished from▄:
(_a_) ORTHOCLASE.—By repeated twinning after _Albite_ law, giving
between crossed nicols a series of alternate dark and light bands.
When _Albite_ twinning is absent the distinction is very difficult.
(_b_) MICROCLINE.—By common absence of the microcline “_gridiron_”
structure between crossed nicols.
Methods for Optical Determination of the Plagioclases.[108]
The correct determination of the particular plagioclase is of the
greatest importance in the classification of rocks, and it is no longer
sufficient to simply determine the feldspar as either orthoclase or
plagioclase.
A quantitative analysis of isolated material would lead most surely to
the desired result, but has many objections.
Modern optical methods now permit of a very accurate and convenient
determination under ordinary circumstances. But of course these methods
involve a knowledge of the approximate orientation of the section
tested. When this section is not a definite cleavage fragment, its
orientation can best be determined by convergent light tests.
Only an outline of these methods can be here given, and reference should
be made to more complete works for an elaborate discussion of the
subject.
It is very convenient to have at hand a set of glass models of the
plagioclases, showing location of plane of optic axes, vibration
directions and crystal axes.[109]
(1) _Schuster’s_ method of recognizing the different feldspars by
extinction angles measured on the cleavage plates[110] is very precise,
but not always applicable for crystals in rock sections.
EXTINCTION ANGLES:
EXTINCTION ANGLES:
ON BASE, MEASURED FROM TRACE OF ON BRACHY PINACOID, MEASURED FROM
PINACOIDAL CLEAVAGE. TRACE OF BASAL CLEAVAGE.
Albite + 4° Albite +19½°
Oligoclase, Ab_{4}An_{1} + 2° Oligoclase, Ab_{4}An_{1} + 8°
Labradorite, Ab_{1}An_{1} − 5½° Labradorite, Ab_{1}An_{1} −20°
Anorthite −36½° Anorthite −41½°
Confusion may here arise between albite and labradorite if disregard be
had to signs, but the more acid oligoclase is readily distinguished from
the basic anorthite.
By convention the angles on base and pinacoids are (+) when the
direction of extinction has apparently moved as the hands of a watch,
with reference to the upper right hand edge (between base and pinacoid)
of the crystal. When the reverse is true the angles are (−), see Fig.
73.
(2) The statistical method of _Michel Lévy_ and others is often
applicable, especially in the following case:
Sections at right angles to the brachy pinacoid (010) and hence showing
_Albite_ twinning.—These sections, as nearly perpendicular to the
lamellæ as possible, are known by the sharp dividing lines, by the
extinction angles on each side of the trace of the twinning plane being
approximately equal and by the fact that the two adjacent lamellæ are of
the same color when the trace of the twinning plane is parallel to the
plane of vibration of either nicol. Also in the 45° position the lamellæ
are exactly the same color and the dividing lines disappear. Measure the
extinction angles in as many sections thus selected as possible and take
the maximum value.[111] This should be very close to the maximum
extinction angle, which is a constant for each kind of feldspar.
[Illustration:
FIG. 73.—Showing conventional signs of extinction angles.
]
MAXIMUM EXTINCTION ANGLES IN
SECTIONS PERPENDICULAR TO
ALBITE TWINS.
Albite 16°
Oligoclase, Ab_{4}An_{1} 5°
Labradorite, Ab_{1}An_{1} 27°
Anorthite 53°
In the determination of rod-like microlites,[112] oligoclase
extinguishes almost parallel to its length, while anorthite may show
extinction angles of over 27°. When these microlites show _Albite_
twinning use the method just described.
(3) _Fouqué’s_ method[113] can be used when the optical orientation of
the section is known (as the result of a test with convergent light).
The extinction angles of these known sections are of great diagnostic
importance.
The best sections are those at right angles to the two bisectrices, and
these may be obtained by rapidly testing those sections, in the rock,
which show an interference color about half as high as the maximum color
in the rock section, in this way avoiding the sections parallel to the
optic axes.
Having found such a section, test it with a gypsum or ¼ undulation mica
plate to prove whether the bisectrix is ⟂ a or c. If ⟂ a (these sections
show sharp twinning striations) measure extinction angle between trace
of axial plane and _albite_ twinning; if ⟂ c measure extinction angle
between trace of axial plane and basal cleavage cracks.
EXTINCTION ANGLES IN SECTIONS:[114]
⟂ a, MEASURED FROM ALBITE STRIATIONS. ⟂ c, MEASURED FROM
CLEAVAGE CRACKS.
Albite 74° 19½°
Oligoclase, Ab_{4}An_{1} 88° 5°
Labradorite, Ab_{1}An_{1} 60° 22°
Anorthite 55½° 48°
When both extinction angles can be obtained, the determination of the
plagioclase is very certain, but the result cannot be regarded as
definite when only one is found; and the method becomes more difficult
as the crystals become smaller.
The position of the axial plane should be determined by convergent light
test and not simply by the direction of extinction in parallel polarized
light.
(4) _Michel Lévy’s_ method[115] can be employed when twinning is present
after both _Carlsbad_ and _Albite_ laws.
[Illustration:
FIG. 74.—Extinction Angles in the Zone normal to (010) in Carlsbad
Twins of the Plagioclases.
]
The section to be tested should be in the zone perpendicular to (010).
Such sections show sharp boundary lines between the twin lamellæ and,
between crossed nicols, the _Albite_ lamellæ show the same interference
color when the trace of (010) is parallel to the cross-wires of the
ocular, and also in the 45° position the lamellæ are the same color and
no dividing lines show. The two parts of the _Carlsbad_ twin exhibit
different interference colors in the 45° position, this difference being
more marked as the composition approaches that of Anorthite. The
extinction angles are measured from the trace of the _Albite_ twinning
plane (010), paying regard to the + and − signs, and the _concurrent
series_ of angles are to be obtained from the two parts of the
_Carlsbad_ twin. The range of these angles (for four type compositions,
Ab, Ab_{4}An_{1}, Ab_{1}An_{1} and An) is given in the accompanying
diagram for all positions of the section in the zone normal to (010).
In the curves of this diagram (Fig. 74) the vertical distances are the
extinction angles for every ten degrees measured from the trace of the
_Albite_ twinning plane, and the horizontal distances represent varying
positions of the section in the zone normal to (010) for every ten
degrees of rotation from the position ∥ to the edge (100)(010) (that is
∥ _ć_ axis), through a revolution of 180° to again ∥ to the same
position. The concurrent angles in one part of a _Carlsbad_ twin are
represented by a heavy line and in the other part by a broken line. It
will be observed that the difference between these concurrent angles is
very small in Albite (3°) and increases markedly towards Anorthite
(60°).
TABLE FOR BECKE METHOD.[116]
Orthoclase │ α │< ω Quartz
Microcline │ β │ „
Albite │ γ │ „
───────────────────────────────┼───────┼───────────────────────────────
Oligoclase, Ab_{4}An_{1} │ α │< ω; γ > ω Quartz.
───────────────────────────────┼───────┼───────────────────────────────
Labradorite, Ab_{1}An_{1} │ α │>ε Quartz
Anorthite │ β │ „
„ │ γ │ „
(5) _Becke’s_ method may be employed to identify the feldspar, by
determining the relative values of the indices of refraction of the
feldspar grain when it lies in contact with a quartz grain (best
results) or with the balsam (not such good results.) The grains should
have vibration directions in parallel position.
Other methods that may be employed are here simply referred to:
Determination (in convergent light) of the emergence of an optic axis
with reference to a known plane, the basic plagioclases show an optic
axis about parallel to _ć_ of the crystal; determination of total
reflection by Wallerant’s total reflectometer; determination of the
value of the mean index of refraction of crushed isolated grains by
Schrœder van der Kolk’s method;[117] determinations by specific gravity
separations with use of heavy solutions, and by chemical and
micro-chemical tests (for the relative amounts of K, Na and Ca).[118]
REMARKS: The plagioclases may have the same two general habits as
orthoclase, being glassy and colorless in the younger eruptive rocks,
and dull and cloudy in the granular and porphyritic, older, massive
and schistose rocks. They occur in rocks of intermediate and basic
composition.
Albite is found in granite (commonly intergrown with orthoclase),
gneiss, etc., and frequently as a secondary constituent (secondary
feldspar[119]) in the feldspar-quartz mosaic of mechanically
metamorphosed rocks. It may also be present in acid eruptive rocks.
Oligoclase is very frequent in granite, syenite, gneiss, diorite,
trachyte, andesite, diabase, etc.; and particularly accompanies
orthoclase.
Labradorite is confined more to the gabbros,[120] basic eruptive rocks
and crystalline schists, rich in amphibole and pyroxene.
Anorthite occurs in gabbros, the most basic porphyrites, basalts, etc.
Chemical corrosion and mechanical deformation[121] may take place as
in orthoclase.
Anorthite and labradorite are more or less decomposed by hydrochloric
acid, while albite and oligoclase are not acted on by the acid.
Especially interesting is the alteration of the plagioclase that takes
place in gabbros, accompanied by “uralitization” of the pyroxene,
forming “_saussurite_.” This consists of a white to greenish confused
aggregate, chiefly of zoisite, grossularite, vesuvianite, chlorite,
secondary feldspar (albite), etc.
_Anorthoclase_ (a Na K, triclinic, feldspar).—Shows between crossed
nicols intersecting areas of exceedingly fine composite twin structure
and others of homogeneous structure, producing a watery or “moiré”
appearance. The twin structure may be only seen in very thin sections.
All possible kinds of perthitic intergrowth occur. Further distinguished
from orthoclase by small extinction angle (4°) on base and by smaller
axial angle (2_E_ = 72° to 88°).
Replaces orthoclase in the Na rich eruptives. Found in augite-syenite
and “Rhombenporphyr” of Norway (with rhombic cross-section), acid
augite-andesite of Pantelleria and in the porphyries of the Hartz.
CYANITE, Disthene.
ANISOTROPIC. BIAXIAL. TRICLINIC.
COMPOSITION: Al_{2}SiO_{5}. ELONGATION ∥ c′.
[Illustration:
FIG. 75.—Cyanite, macro pinacoid cleavage section.
]
▄Usual Appearance in Sections▄: Blade-like crystals without terminal
planes, but with cross-section (six-sided) showing two long parallel
edges and four shorter edges; also in columnar aggregates. Twinning
common, with generally twinning plane parallel to macro pinacoid
(100). Colorless or bluish and spotted. The index of refraction is
high (_n′_ = 1.720, α = 1.712, γ = 1.728), hence _relief_ marked and
surface rough. Cleavage perfect, parallel to macro pinacoid (100),
appearing as sharp cracks, parallel to longest edges in
cross-sections; less distinct, parallel to brachy pinacoid (010).
Fibrous parting parallel to base (001), Fig. 75. Pleochroism
(colorless to blue ∥ c′) not noticed except in colored crystals.
▄Crossed Nicols▄: Double refraction quite strong (γ − α = 0.016).
Interference colors upper first order, yellow, red, violet, etc.
_Extinction_ angles observed in all sections (being triclinic),
reaching a maximum of 30° on macro pinacoid (100), Fig. 75. Extinction
on base, about parallel to most perfect cleavage. In _convergent
light_ axial angle large; axial plane and Bx_{_a_}. about
perpendicular to best cleavage (100); optical character (−).
▄Alteration▄: Seldom observed, but may take place to mica.
▄Distinguished from▄:
(_a_) AMPHIBOLE by cleavage (intersecting cleavages at 124° in
amphibole and 90° in cyanite) and by (100) cleavage plates of cyanite
showing emergence of acute bisectrix.
(_b_) CORUNDUM by being biaxial.
Distinction from similar appearing minerals may be difficult.
REMARKS: Found in gneiss, granulite, metamorphic schists, ecolgite,
etc., commonly associated with garnet. It is not attacked by acids.
H., 5 to 7. Sp. gr., 3.6.
SERPENTINE.
AGGREGATE.
ELONGATION (of fibers) ∥ c′.
COMPOSITION: H_{4}Mg_{3}Si_{2}O_{9}, with replacement by Fe.
▄Usual Appearance in Sections▄: Dense, fibrous (chrysotile) or scaly
(antigorite) aggregates.
_Color._—Colorless to light greenish, except the Fe rich variety which
is green.
_Index of Refraction._—_n′_ = 1.55 to 1.56 (α = 1.56, γ = 1.571 for
antigorite), hence no _relief_ and surface smooth.
▄Polarized Light▄:
_Pleochroism._—Not seen or very feeble, except in the Fe rich variety.
_▄Crossed Nicols▄_:
_Double Refraction._—Rather weak (γ − α = 0.009 to 0.011).
_Interference Colors._—Middle first order, gray, white, yellow, etc.
Anomalous colors do not appear. The aggregate structure is distinctly
seen between crossed nicols. Due to compensation aggregates may appear
isotropic.
▄Distinguished from▄: CHLORITE.—By more usual absence of color,
pleochroism and anomalous interference colors; but this distinction may
be very difficult.
REMARKS: Serpentine (both antigorite and chrysotile) is essentially a
secondary mineral, resulting in most cases from the alteration of
chrysolite (olivine), Fig. 22, more rarely of pyroxene or
amphibole.[122] The alteration of olivine to antigorite leads to the
characteristic “lattice structure,” the alteration to chrysotile to
“mesh structure.” In the case of the “mesh” formation the alteration
starts from the surface and cracks, producing fibres of chrysotile,
which stand at right angles to these edges and cracks. As
serpentinization proceeds new cracks form, due to increase in volume,
and the process may continue until complete pseudomorphism takes
place. When this subsequent serpentinization of the meshes takes place
the resulting serpentine may appear almost isotropic[123] and is
certainly different from the chrysotile of the first formed veins
(Weinschenk). Pieces of the parent mineral are often present.
Serpentine is found in ophiolites, the altered basic igneous rocks,
pyroxenites, peridotites, etc., and as a primary mineral in the
Central Alps peridotite, intergrown with fresh olivine (Weinschenk).
It may also form a rock by itself. Serpentine is attacked quite
strongly by hydrochloric acid, still more so by sulphuric acid. Common
serpentine is not altered by heating (distinction from chlorite), but
the Fe rich variety becomes brown and opaque. H., 2.5 to 4. Sp. gr.,
2.5 to 2.7.
CLAY, Kaolin.
COMPOSITION: AGGREGATE.
H_{4}Al_{2}Si_{2}O_{9}
(kaolinite).
▄Usual Appearance in Sections▄: Fine, scaly, colorless aggregates,
which appear opaque (due to porous structure). The scales show basal
cleavage. Index of refraction is about the same as balsam (_n′_ =
1.55), hence no _relief_. The double refraction is weak (γ − α =
0.008).
▄Distinguished from▄: Colorless MICA and HYDRARGILLITE [(Al(OH)_{3}),
which as an alteration product of the feldspars is often confused with
clay] by weak double refraction.
REMARKS: Clay results from the alteration of the feldspars (especially
the plagioclases), elæolite, scapolite and other silicates. Kaolinite
is insoluble in hydrochloric but decomposed by sulphuric acid. H.,
2.5. Sp. gr., 2.6.