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 the129 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 a130 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 mineral131 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.
