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............