THE MICRO-STRUCTURE OF BRICKS (_Continued_).
Turning now to the actual appearance of minerals commonly found in
bricks as they are examined under the microscope, we may remind the
reader, that the physical aspect of the majority of them has already
been described in those chapters dealing with the “Mineral Constitution
of Brick Earths” and “Minerals: their behaviour in the Kiln,” and the
particulars that follow may be read in conjunction with what was there
said.
It will be convenient now to describe the appearance of certain
well-known minerals, as they are seen (A) in reflected light and (B)
in thin sections in transmitted light, whilst the latter will be
subdivided into _1_ denoting the phenomena observed in ordinary light,
and _2_ in polarised light. To save repetition, the letters and figures
will be used to denote the methods of examination as indicated.
QUARTZ.--Present in nearly all rubber-bricks, and in the vast majority
of common stocks, as well as in vitrified goods and fire-bricks. In the
last mentioned, the grains are usually partially agglutinated, and are
extremely minute.
A. As more or less rounded, or sub-angular fragments, white and
crystalline, like clear window glass.
B. 1--Clear white, often broken up by thin hair-like lines running
in various directions, and rows and patches of minute specks, which,
as previously remarked, have been shown to contain fluid, &c. 2--On
revolving the stage of the microscope, the crystals are usually
seen to present beautiful, clear transparent colours, which in
characteristic sections are very vivid--red, blue, yellow, &c.
FLINT.--Found in the same class of bricks as quartz.
A. Bluish horn colour; irregular fragments and splinters.
B. 1--Translucent; often melted more thoroughly than quartz in hard
burnt bricks; colourless. 2--Opaque unless in some such form as
chalcedony, when an extremely minute granular aspect results, becoming
slightly transparent. Melted portions always opaque.
FELSPAR.--The alteration which the different kinds of felspar have
undergone in a hard burnt brick, when present, render it almost
impossible to recognise them specifically.
A. Milk white, or more rarely light pink; the mineral, even when red in
the raw earths, becomes white on the application of moderate heat, as
in the burning of common bricks. It is often closely fractured, and but
rarely powdered.
B. The characteristic parallel lines of the triclinic varieties may
often be observed, especially in rubber bricks; but great heat, such
as leads to partial peripheral fusion, frequently obliterates them
to a large extent, and in a well-burnt brick it is quite impossible
in the majority of cases to determine whether the felspars present
are triclinic or monoclinic. More particularly is this the case when
the mineral has been more or less decomposed prior to its having been
burnt. The bulk of the fragments of the mineral can only be alluded to
in the general term “felspars,” and in ordinary light these are opaque
or “fleecy,” whilst in polarised light minute portions may be found to
be slightly birefringent. In a decomposed state it forms a prominent
constituent of brick-earths in the first place, and that is precisely
the material which most readily agglutinates in presence of a suitable
flux. Crystallites are not uncommon in the melted peripheries, as may
be seen in a hard-burnt brick in ordinary light.
MICA.--In minute flakes, shining, or glistening, and commonly black,
silvery or bronze-coloured.
A. Detected at once by its thin shining scales, which frequently have
not suffered much in the kiln except near the outside of the brick.
B. 1--The darker micas are usually citron coloured or light brown,
and unless cut parallel to the cleavage of the mineral, exhibit a
number of closely-set parallel lines, the fragments being much “frayed
out” and “ragged” at the edges. 2--Using one nicol only, the mineral
changes from dark to light on the revolution of the stage, and is said
(in common with other minerals exhibiting a similar property) to be
dichroic. With both nicols in position but little further difference is
noted, except that in changing tint the whole is darker. Vivid colours
are not observed except in yellows and browns. Muscovite mica is often
quite white and transparent.
IRON.--Common except in white bricks made from the purest china-clays.
A. Brown or reddish-brown specks; sometimes as blue black films in
fire-bricks; dull and frequently powdery in common bricks. Surrounding,
film-like, grains of mineral matter of which the brick is composed.
A grain of quartz, for instance, is frequently seen enveloped by a
film of red iron. Other metallic iron is more lustrous and whiter than
magnetite when seen in reflected light, but such unaltered particles of
the mineral could only occur in a brick that had not been subjected to
great heat.
B. Opaque either in 1 or 2.
IRON PYRITE only occurs as such in bricks that have not been thoroughly
burnt, or in common “baked” bricks. Higher temperatures lead to the
separation of the iron from the sulphur and the general incorporation
of both in the agglutination of the brick during partial fusion.
A. Brassy yellow particles.
B. Opaque both in 1 and 2.
CALCITE.--Not found in burnt bricks, nor indeed in any except those
that have been sun-dried, or have been subjected to very little heat.
Small pellets of lime are of common occurrence in poorly-burnt bricks.
In reflected light such pellets are generally of a dirty white tint;
opaque in transmitted light.
DOLOMITE.--Practically the same observations apply as to calcite,
crystals of dolomite not being found except in sun-dried bricks and
the like. Under the action of much heat the mineral, like calcite, is
reduced to lime.
SELENITE.--This is not rare in the commoner class of bricks, though
the application of much heat reduces it to the state of powder. In
reflected light it is found to be present as extremely minute specks or
“tears” of whitish powdery plaster. Opaque, of course, in transmitted
light.
The description of the micro-appearance of many other minerals which
occur but rarely in bricks does not fall within the scope of the
present elementary treatise; for practical purposes they may be
ignored.