_Color._—Colorless to light greenish, except the Fe rich variety which is green. _Index of Refraction._—_n′_ = 1.55 to 1.56 (α = 1.56, γ = 1.571 for antigorite), hence no _relief_ and surface smooth. ▄Polarized Light▄: _Pleochroism._—Not seen or very feeble, except in the Fe rich variety. _▄Crossed Nicols▄_: _Double Refraction._—Rather weak (γ − α = 0.009 to 0.011). _Interference Colors._—Middle first order, gray, white, yellow, etc. Anomalous colors do not appear. The aggregate structure is distinctly seen between crossed nicols. Due to compensation aggregates may appear isotropic. ▄Distinguished from▄: CHLORITE.—By more usual absence of color, pleochroism and anomalous interference colors; but this distinction may be very difficult. REMARKS: Serpentine (both antigorite and chrysotile) is essentially a secondary mineral, resulting in most cases from the alteration of chrysolite (olivine), Fig. 22, more rarely of pyroxene or amphibole.[122] The alteration of olivine to antigorite leads to the characteristic “lattice structure,” the alteration to chrysotile to “mesh structure.” In the case of the “mesh” formation the alteration starts from the surface and cracks, producing fibres of chrysotile, which stand at right angles to these edges and cracks. As serpentinization proceeds new cracks form, due to increase in volume, and the process may continue until complete pseudomorphism takes place. When this subsequent serpentinization of the meshes takes place the resulting serpentine may appear almost isotropic[123] and is certainly different from the chrysotile of the first formed veins (Weinschenk). Pieces of the parent mineral are often present. Serpentine is found in ophiolites, the altered basic igneous rocks, pyroxenites, peridotites, etc., and as a primary mineral in the Central Alps peridotite, intergrown with fresh olivine (Weinschenk). It may also form a rock by itself. Serpentine is attacked quite strongly by hydrochloric acid, still more so by sulphuric acid. Common serpentine is not altered by heating (distinction from chlorite), but the Fe rich variety becomes brown and opaque. H., 2.5 to 4. Sp. gr., 2.5 to 2.7. CLAY, Kaolin. COMPOSITION: AGGREGATE. H_{4}Al_{2}Si_{2}O_{9} (kaolinite). ▄Usual Appearance in Sections▄: Fine, scaly, colorless aggregates, which appear opaque (due to porous structure). The scales show basal cleavage. Index of refraction is about the same as balsam (_n′_ = 1.55), hence no _relief_. The double refraction is weak (γ − α = 0.008). ▄Distinguished from▄: Colorless MICA and HYDRARGILLITE [(Al(OH)_{3}), which as an alteration product of the feldspars is often confused with clay] by weak double refraction.