Chalcedony occurs in the ground mass of very silicious porphyritic rocks, which have microfelsitic development; and is found as a secondary mineral in all kinds of silicate rocks. TRIDYMITE. ▄Usual Appearance in Sections▄: This form of SiO_{2}, which is soluble in boiling caustic soda, appears in “tile-like” aggregates of minute colorless plates (pseudo-hexagonal) and is always secondary. The refractive index is extremely low (_n′_ = 1.477), hence the surface appears rough. Between _crossed nicols_ the interference colors are very low in order (γ-α = 0.002), and the tablets may show a division into different areas (optical anomalies). In _convergent light_ an indistinct biaxial figure is generally seen. REMARKS: Chiefly a volcanic mineral, found in rhyolite, trachyte and andesite. Commonly associated with opal and chalcedony. CALCITE. ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: CaCO_{3}. Ca may be replaced by small quantities of Mg, Fe, Mn, etc. _ć_ = a. ▄Usual Appearance in Sections:▄ Grains and aggregates. May be fibrous or oölitic. Only in crystals in certain rocks.[87] [Illustration: FIG. 40.—Calcite, crossed twin lamellæ, in granular limestone. (From Cohen.) ] _Twinning._—Polysynthetic, parallel to one or more faces of −½ R. (10̄12). Very common in crystalline limestones, and may have been produced by pressure or by the grinding of the section. Shows itself between crossed nicols as a series of light and dark bands, parallel or intersecting, Fig. 40, about parallel to longer diagonal of cleavage rhombs. When the composition face of the twins is oblique to the face of the section, interference colors can be seen without the analyzer. _Color._—Colorless when pure, but may appear colored by transmitted light, due to organic pigments. _Index of Refraction._—_n′_ = 1.601 (α = 1.487, γ = 1.659) hence with ordinary light _relief_ not marked. Due to the great variation in refractive indices of the two rays, with polarized light, the surface will appear either quite smooth or rather rough, depending upon which vibration direction lies over the plane of the polarizer. This marked variation in appearance (sometimes called “twinkling”) serves as a good test for calcite. [Illustration: FIG. 41.—Calcite, section parallel to face of rhombohedron, showing rhombohedral cleavage. (From Cohen.) ] _Cleavage._—Parallel to unit rhombohedron (10̄11), appearing in thin sections as many sharp cracks, whose angles of intersection depend on the position of the section, Fig. 41. Newton’s colors may be seen along cleavage cracks. ▄Polarized Light:▄ _Pleochroism._—None. _▄Crossed Nicols:▄_ _Double Refraction._—Very strong (γ − α = 0.172). _Interference Colors._—Pale, iridescent colors of very high order.