▄Usual Appearance in Sections▄: Minute, scaly aggregates, which may incline to radial grouping; or in minute grains as a pigment (veridite) in other minerals. _Twinning._—May be seen as in mica. _Color._—Generally green, varying from greenish white to dark green, rarely colorless or red. _Index of Refraction._—_n′_ = 1.567 to 1.589 (α = 1.560 to 1.585, γ = 1.571 to 1.596), hence no marked relief and only slightly rough surface. _Cleavage._—Like mica, very perfect; parallel to flat face, which is considered to be the basal plane. This cleavage may not be noticed, especially in secondary chlorite in rocks. ▄Polarized Light▄: _Pleochroism._—In green and yellow tints (green ∥ cleavage), being more marked in dark colored varieties. Basal sections are non-pleochroic, the mineral being practically uniaxial. Pleochroic halos may be seen. _▄Crossed Nicols▄_: _Double Refraction._—Generally very weak (γ − α = 0.001 to 0.011). _Interference Colors._—Very low first order, gray or white, at times scarcely noticed. Anomalous colors, however, often seen (deep blue or brown.) _Extinction._—Plates parallel to cleavage generally appear isotropic or only show faint color. In other sections extinction is apparently parallel to the cleavage in the uniaxial type, but extinction angles may be noticed when type is biaxial. Complete extinction may not be noticed, due to aggregate structure. _▄Convergent Light▄_: Plates parallel to cleavage show, at times, an indistinct interference cross, which may open into two hyperbolas, indicating biaxial nature of crystallization. Ax. pl. ∥ (010); Bx_{_a_}. Λ _ć_ = 0° to 15°; 2_E_ variable. Optical character (±). ▄Distinguished from▄: SERPENTINE.—By general green color (serpentine, with exception of Fe rich variety, is colorless), pleochroism and frequent anomalous interference colors; but the distinction between these two minerals may be very difficult. Chlorite may resemble decomposed or green mica (mica has, however, strong double refraction). The different species in the chlorite group cannot usually be distinguished in rocks. REMARKS: The chlorites are essentially secondary minerals, derived from the aluminous silicates, biotite, augite, garnet, feldspar, etc. They are found abundantly in chlorite-schists, contact rocks, etc., and as pigment (veridite) in altered eruptive rocks. May occur as a primary constituent of eruptive rocks, often in parallel growth with biotite. Chlorides are acted on by hot hydrochloric acid, and decomposed easily by sulphuric acid. H., 2 to 3. Sp. gr., 2.6 to 2.96. A thin section heated to redness on platinum foil loses water and becomes opaque. Ferruginous varieties are turned reddish-brown to black by heating (serpentine, as it contains less iron, may give negative results with this test). _Delessite._—Found in sphærulites, filling cavities in amygdaloidal basic rocks, and in pseudomorphs. It appears to be much altered to other minerals.