Feldspar is the family name of several minerals closely related and
indeed grading into each other, but distinguished by mineralogists by
separate specific terms. These minerals are all silicates of aluminum,
with some alkali or alkali earth, having a hardness of about 6 in the
scale in which quartz is 7 and a specific gravity varying from 2.5 to
2.7. They are fusible with difficulty before the blowpipe, crystallize
in the monoclinic or triclinic system and cleave in two well-marked
directions nearly or quite at right angles to each other. It is this
latter property, probably, which led to the grouping of these minerals
as spar, since this term is applied in common language to any minerals
which break with bright crystalline surfaces. Thus calc spar is a common
name for calcite, heavy spar for barite, needle spar for aragonite, and
so on. The term field spar, of which Feldspar is probably a corruption,
was perhaps given the minerals of this group because of their widespread
occurrence. The English spelling of the word is Felspar. The Feldspars
form an essential part of nearly all eruptive rocks and by their
decomposition produce clays and other soils which may harden into great
areas of sedimentary rocks. They are thus of great geological importance
and interest. Usually the white crystals to be seen in an eruptive rock
in contrast to the dark green or black of the pyroxene or hornblende, or
the glassy, nearly colorless quartz, are Feldspar. The Feldspar may,
however, contain more or less iron and then take on a flesh color or
become even darker. Feldspar crystals can best be recognized by their
prominent cleavage, which appears as numerous bright flat surfaces
extending in any given crystal in the same direction. The crystals,
while they may be of so minute dimensions as to be visible only with the
microscope, may on the other hand reach in veins in coarse-grained
granites a length of a foot or more.
As ornamental stones only certain varieties of Feldspar are valued and
their value depends on accidents of color or structure. The first of the
Feldspars which may be mentioned as being prized as an ornamental stone
is amazonstone or green Feldspar. This in composition is what is called
a potash Feldspar, potash being the alkali which in combination with
alumina and silica goes to make up the mineral. The percentages of each
in a pure amazonstone are silica 64.7, alumina 18.4 and potash 16.9. The
mineralogical name of the species is micro-cline, meaning small
inclination, and refers to the fact that the angle between the two
cleavages of the mineral is not quite a right angle. The common color of
microcline is white to pale yellow, but occasionally green and red
occur.
It is only to the green variety that the name of amazonstone is applied,
a name meaning stone from the Amazon river. It first referred probably
to jade or some such green stone from that locality and then came to
include green Feldspar. No occurrence of green Feldspar in that region
is now known.
Practically all the amazonstone now used for ornamental purposes comes
from three localities. These are the vicinity of Miask in the Ural
Mountains, Pike’s Peak, Colorado, and Amelia Court House, Virginia. In
all these places the amazonstone occurs in coarse-grained granite and is
closely accompanied by quartz and Feldspar. All gradations are found in
color from the deep green to white, only the bright green being prized
for ornamental purposes. The Feldspar is usually well crystallized and
crystals of several pounds weight may be found. A crystal will rarely be
of a uniform color, streaks of paler green or white being commonly
present. Only the uniformly colored portions are prized for ornamental
purposes. The green often takes on a bluish tone and blue sometimes even
predominates. The color is doubtless due to some organic matter, as it
disappears, leaving the stone white, on heating. The stone is always
opaque. Its use is not extensive, its sale being greater to tourists in
the vicinity of the regions where it is found than to gem cutters.
Several other localities in the United States besides those mentioned
afford the mineral, though not in large quantities. It occurs in two or
three localities in North Carolina; in Paris, Maine; Mount Desert,
Maine; Rockport, Massachusetts; and Delaware county, Pennsylvania. The
finest comes from the Pike’s Peak locality. Mr. G. F. Kunz states in
regard to these crystals that when they were first exhibited at the
Centennial Exposition in Philadelphia in 1876 they were a great surprise
to Russian dealers who had brought over some amazonstone from the Urals,
expecting to sell it at what would now be considered fabulously high
prices.
[Illustration: FELDSPAR.
LOANED BY FOOTE MINERAL CO.]
Top row:
Amazonstone, crystallized (Colorado).
Amazonstone, crystallized (Colorado).
Amazonstone (Colorado).
Center row:
Labradorite, polished (Labrador).
Labradorite, polished (Labrador).
Bottom row:
Sunstone (Norway).
Moonstone, polished (Norway).
The second species of Feldspar which may be mentioned as of use as an
ornamental stone is labradorite. This differs in composition from
amazonstone in containing soda and lime in place of potash, the
percentages in a typical labradorite being, silica 53.7, alumina 29.6,
lime 11.8 and soda 4.8. Labradorite has the typical cleavage of Feldspar
and cleavage surfaces in the direction of easiest cleavage are usually
marked by rows of parallel striae. These show that the mass is made up
of a series of crystal twins in parallel position and afford an
excellent criterion for determining a triclinic Feldspar. Labradorite is
a common rock-forming mineral, especially in the older rocks. It is
only, however, when it occurs in large pieces which exhibit a play of
colors that it is prized as an ornamental stone. The labradorite
exhibiting the latter property in the most remarkable degree and hence
most valued is that found on the coast of Labrador near Nain and the
adjacent island of St. Paul. It was first found here by a Moravian
missionary named Wolfe and brought to Europe in the year 1775. It occurs
together with the form of pyroxene known as hypersthene, in a
coarse-grained granite, or perhaps a gneiss. From these it is weathered
out by wave and atmospheric action and occurs as beach pebbles. It is
also mined from veins. Labradorite of pleasing color and opalescence
occurs in a few other localities in Canada, and in Essex county, New
York, in the United States. Two localities occur in Russia, one near St.
Petersburg and the other in the region of Kiew. The labradorite of the
latter locality is the better, its occurrence being in a coarse-grained
gabbro. The Labrador occurrence exceeds all others, however, in
abundance and beauty and by far the larger quantity used in the arts
comes from there. The play of colors which gives labradorite its
attractiveness is rarely seen to advantage except upon a polished
surface, but whether polished or unpolished it only appears when the
surface is held at a particular angle with reference to the eye. Emerson
thus describes it in his essay on Experience as illustrating the
limitations of the individual: “A man is like a bit of Labrador spar,
which has no lustre as you turn it in your hand until you come to a
particular angle; then it shows deep and beautiful colors.”
The play of colors seen in labradorite is not like that of the opal,
which presents to the eye fragments of different colors varying in
different positions, but appears as broad surfaces of a single color. It
is only rarely that these colors change with a change of position. Bauer
remarks that the appearance is similar to that seen on the wings of some
tropical butterflies. The colors over any given surface are not
necessarily alike, but more than two or three tints are rare. Each tint
is uniform where it occurs. A surface may be interspersed with many
spots exhibiting no sheen. Both colored and uncolored portions have only
vague outlines and merge into each other at the edges. Bauer mentions a
labradorite from Russia the colored portions of which formed a striking
likeness of Louis XVI, the head being a beautiful blue against a gold
green background, while above appeared a beautiful garnet red crown.
Excellent effects are sometimes produced in labradorite by cutting it in
the form of cameos so as to make the base of different color from the
figure in relief. Of the different colors shown by labradorite blue and
green are the most common, yellow and red least so. These colors are
regarded by Vogelsang as of different origin, the blue being, in his
opinion, a polarization phenomenon due to the lamellar structure of the
Feldspar, and the yellows and reds the result of the reflection of light
from minute included crystals of magnetite, hematite and ilmenite. These
lying in parallel position in great numbers in the labradorite give the
colors.
The gems known as moonstone and sunstone owe the play of colors which
gives them their respective names to similar causes. These gems are
generally some form of Feldspar, although any mineral giving a similar
sheen of color might be included under them. The moonstone of commerce
comes chiefly from Ceylon, where it occurs in large pieces the size of a
fist in a clay resulting from the decomposition of a porphyritic rock.
Pieces of these when polished exhibit the beautiful pale blue light
coming from within which makes the stone prized as a gem. The cause of
this light is undoubtedly minute tabular crystals lying in parallel
position through the stone.
The stone varies from translucent to opaque, and from colorless to
white, the essential feature being the blue opalescent light or
chatoyancy exhibited from a polished surface. Good moonstones are worth
from three to five dollars a carat.
The Ceylon moonstone is sometimes known as Ceylon opal, but it is the
variety of Feldspar known as orthoclase, which is a potash Feldspar,
differing from the microcline just described in being monoclinic in
crystallization and in having two cleavages meeting at right angles.
Another species of Feldspar used as moonstone is albite. This is a soda
Feldspar and is triclinic, but exhibits the color characteristic of
moonstone. One variety is known as peristerite, from the Greek word for
pigeon, and is applied on account of the resemblance of the sheen to
that of a pigeon’s neck. It is found at Macomb, St. Lawrence county, New
York. Albite found at Mineral Hill, Pennsylvania, also exhibits the
chatoyancy of moonstone. Amelia Court House, Virginia, is another
locality whence come pieces either of orthoclase or oligoclase
exhibiting this property. Like most of the more or less opaque gems,
moonstone is cut chiefly in the rounded form known as en cabochon. It is
of late, however, cut in the form of balls, which are quite popular, the
bringing of good luck being attributed to them. The brilliancy of
moonstone is considerably increased by mounting it against black.
Sunstone is the term by which those kinds of Feldspar are known which
reflect a spangled yellow light. The appearance comes from minute
crystals of iron oxide, hematite or gothite, which are included in the
stone and both reflect the light and give it a reddish color. Like
labradorite the sheen is visible only when the stone is held at a
certain angle. Some specimens of the mineral carnallite, which is a
chloride of potassium and magnesium, exhibit a similar sheen, and being
soluble in water the crystals of hematite can be separated out. They are
then seen to be perfect little hexagons of a blood-red color. The sheen
of sunstone is best visible when the stone is held in the sunlight or
strong artificial light. The variety of Feldspar to which the sunstone
most in use at the present time belongs is oligoclase, a soda-lime
triclinic Feldspar. Like labradorite it usually exhibits on the surface
of easiest cleavage parallel striations due to twinning structure. The
best sunstone at the present time comes from Tvedestrand, in southern
Norway, where it occurs in compact masses together with white quartz, in
veins, in gneiss. Some also comes from Hittero, Norway. In Werchne
Udinsk, Siberia, another occurrence was discovered in 1831. Previous to
this Bauer states that all the sunstone known came from the Island of
Sattel in the White Sea, and was very costly, although of a quality
which would not now be deemed desirable. At the present time, although
stones of fine quality can be obtained, sunstone is little used in
jewelry, and its market value is very low. Statesville, North Carolina,
and Delaware county, Pennsylvania, are two localities in the United
States where good sunstone has been obtained.
Both sunstone and moonstone can be accurately imitated in glass and the
distinction of the artificial from the real by ocular examination alone
would be almost impossible. Glass, however, lacks the cleavage of
Feldspar and is somewhat heavier and softer. The discovery of the method
of making artificial sunstone is said to have been accidental, and was
made at Murano, near Venice, when a quantity of brass filings by chance
fell into a pot of melted glass. The product was for a long time and is
still used in the arts under the name of goldstone. Sunstone is
sometimes known as aventurine Feldspar, in distinction from aventurine
quartz, which presents a similar appearance, owing to the inclusion of
scales of mica. The term aventurine is from the Italian avventura,
meaning chance, and refers to the chance discovery above referred to.
Gems are occasionally cut from other forms of Feldspar than those here
described, which are transparent and colorless and valued for their
lustre. The varieties chiefly employed in this manner are adularia, a
variety of orthoclase which is often transparent, the best specimens
being obtained in Switzerland, and oligoclase, in the transparent form
in which it is found near Bakersville, North Carolina.
Oliver Cummings Farrington.