▄Distinguished from▄: The OPAQUE ORES by adamantine lustre with reflected light; ZIRCON and CASSITERITE in concentrates by chemical tests. May not be possible to distinguish from cassiterite in sections. REMARKS: Found in the metamorphic schists, amphibolites, slates, contact and fragmentary rocks, etc.; also as inclusions in quartz and mica. Especially common as a secondary product of titaniferous hornblende and biotite. The “sagenite” webs of the decomposed micas are probably secondary. Rutile is insoluble in hydrochloric acid. H., 6 to 6.5. Sp. gr., 4.2. It is easily separated from rock powder by its insolubility in acid and its high sp. gr. [Illustration: Rutile twins. FIG. 38.—Twin plane (101). FIG. 39.—Twin plane (301). ] ZIRCON. ANISOTROPIC. UNIAXIAL. TETRAGONAL. COMPOSITION: ZrSiO_{4}. _c_ = ć. ELONGATION ∥ c′. ▄Usual Appearance in Sections▄: Small, short prismatic crystals, Fig. 33A, and grains. Shell-like (zonal) structure may be noticed. When enclosed in black-mica, hornblende, cordierite, etc., often surrounded by characteristic pleochroic halos.[83] _Color._—Colorless, rarely pale brownish. _Index of Refraction._—_n′_ = 1.95, (α = 1.931, γ = 1.993) hence _relief_ very high and surface rough. ▄Polarized Light▄: _Pleochroism._—Not usually noticeable. ▄_Crossed Nicols_▄: _Double Refraction._—Very strong (γ − α = 0.062). _Interference Colors._—Very high (4th) order, minute crystals show brilliant colors. _Extinction._—As zircon is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_. ▄_Convergent Light_▄: Basal sections, which are large enough to give interference figures, show several rings in addition to dark cross. Optical character (+). ▄Alteration▄: Very rarely takes place. ▄Distinguished from▄: (_a_) APATITE.—By much higher relief and stronger double refraction. (_b_) TITANITE.—By uniaxial character. (_c_) RUTILE.—See under the latter. Easily confused with xenotime, which, however, has higher interference colors and more distinct pleochroism; but chemical tests may be necessary. REMARKS: Found widely distributed but not in quantity in eruptive and metamorphic rocks. Occurs in granite, syenite, diorite, gabbro, gneiss, etc. It is one of the oldest constituents of the rocks in which it occurs, and may often be found as inclusions in the ferro-magnesium minerals. Zircon is insoluble in hydrochloric acid. H., 7.5. Sp. gr., 4.5 to 4.7. It can easily be separated from rock powder on account of its high sp. gr., insolubility in acid and non-magnetic properties. The crystals can then be examined separately, or chemical tests made to prove the presence of Zr. SCAPOLITE GROUP, Wernerite, etc.