Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . le forcomplete development. Such crystals are often called Idiomorp/iic,see Fig. 4. Crystals that are large in comparison with other * With the Seibert microscope. Fig. 2, the No. 0 eye-piece and the No. II. objec-tive will prove most satisfactory for the following tests. With the Fuess microscopeuse No. 4 objective. With an English microscope use an ordinary eye-piece and a i^or j^^ objective. f Twins may be recog
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . le forcomplete development. Such crystals are often called Idiomorp/iic,see Fig. 4. Crystals that are large in comparison with other * With the Seibert microscope. Fig. 2, the No. 0 eye-piece and the No. II. objec-tive will prove most satisfactory for the following tests. With the Fuess microscopeuse No. 4 objective. With an English microscope use an ordinary eye-piece and a i^or j^^ objective. f Twins may be recognized just as in macroscopic specimens, and zonal structurenoticed if the zones differ in color. When a colorless mineral is surrounded by othercolorless minerals, of about the same index of refraction, its outline is often best broughtout by observation between crossed nicols. 13 14 LWESTIGATION OF CHARACTERS OF MINERALS. accompanying crystals may be called PJienockrysts. Of coursecrystals, which are not wholly contained within the rock section,will only show the outline of the bounding planes cut by this par-ticular section. In some cases, by a careful study of the outline. Fig. 4. — Idiomorphic augite crystal in camptonite. The section is about at rightangles to the vertical axis c, and shows the intersecting cleavages parallel to the prismof 87° Ob. Keene Valley, N. Y. of several sections of the same mineral and by a measurement ofthe angles, it is possible to determine the common crystallo-graphic forms of the mineral. Very misleading outlines may,however, be observed ; as for example the triangular outline of a
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