ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: _ć_ = a. ELONGATION ∥ a′. Ca_{5}(Cl.F)(PO_{4})_{3}. ▄Usual Appearance in Sections▄: Minute, slender hexagonal prisms, cross-sections having regular hexagonal boundaries, needles, and grains. Figs. 14 _a_ and 42. [Illustration: FIG. 42.—Apatite, showing cross fracture, in nepheline-basalt. (From Cohen.) ] _Color._—Generally colorless, seldom bluish or brownish (only in eruptive rocks). _Index of Refraction._—_n′_ = 1.635 (α = 1.634, γ = 1.637), hence _relief_ more marked than that of the colorless associated minerals. _Cleavage._—Seldom observed microscopically. _Parting._—Long columnar crystals generally show a transverse jointing, so that the pieces may be more or less separated. _Inclusions._—Gas and fluid may be present. ▄Polarized Light▄: _Pleochroism._—None shown by the colorless crystals, the colored crystals show stronger absorption parallel to _ć_. _▄Crossed Nicols▄_: _Double Refraction._—Weak (γ − α = 0.003). _Interference Colors._—The lower first order, generally grayish-blue or white. _Extinction._—As apatite is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_ axis. ▄_Convergent light_▄: Basal sections show a cross, without rings. Optical character (−). ▄Alteration▄: Does not usually take place, apatite being found perfectly fresh in decomposed rocks, which is quite remarkable considering its easy solubility in acids. ▄Distinguished from▄: (_a_) SILLIMANITE and TREMOLITE.—By weak double refraction and elongation ∥ a′. (_b_) NEPHELITE.—By being relatively much smaller and longer than the nephelite crystals, which are often decomposed. Also by higher relief and negative results with gelatinization test. (_c_) ZIRCON.—By lower relief and much weaker double refraction. (_d_) FELDSPARS (when granular and undecomposed).—By higher relief and uniaxial interference figure. (_e_) VESUVIANITE and ZOISITE.—May be only possible by chemical tests. (_f_) CORUNDUM.—See under the latter. REMARKS: Found in most igneous rocks and crystalline schists. In the eruptive rocks it appears as one of the oldest secretions from the magma, and hence is often found as inclusions in other minerals, especially biotite, hornblende, etc. Apatite is easily soluble in hydrochloric and nitric acids. H., 4.5 to 5. Sp. gr., 3.19. On account of its high sp. gr., apatite, in rock powder, comes down in heavy solutions with the metallic minerals, and can be separated from them by the use of a magnet. This residue can also be tested for phosphorus in the wet way with ammonium molybdate. NEPHELITE,[88] Nepheline, Elæolite. ANISOTROPIC. UNIAXIAL. HEXAGONAL. COMPOSITION: 7NaAlSiO_{4} + _ć_ = α. NaAl(SiO_{3})_{2}, with partial replacement of Na by K.