_Inclusions._—Minute fluid, gas and mineral inclusions, often in irregular trains, are very characteristic of quartz in granite rocks and crystalline schists. The inclusions are not so abundant in porphyritic rocks, but a few glass inclusions may occur, filling up “negative” crystals in the quartz. Rutile, amphibole, etc., may occur as needle-like inclusions in quartz. _Polarized Light:_ _Pleochroism._—None. _▄Crossed Nicols:▄_ _Double Refraction_.—Weak (γ − α = 0.009). _Interference Colors._—The middle 1st order, white, yellow, etc. _Extinction._—As quartz is uniaxial, basal sections remain dark during a complete rotation of stage. In the other sections extinction is not characteristic, due to the absence of cleavage and crystallographic outlines. Thin sections do not show circular polarization. _▄Convergent Light:▄_ Basal sections show a dark cross, without any rings. Optical character (+). ▄Alteration:▄ Does not take place, so quartz always appears fresh and unweathered in sections. ▄Distinguished from:▄ (_a_) SANIDINE (in fresh grains).—By use of convergent light. Feldspar is biaxial, or sections which appear uniaxial are (−). (_b_) NEPHELITE.—By almost entire absence of hexagonal outline, stronger double refraction, fresh, unweathered appearance and (+) optical character. (_c_) IOLITE (Cordierite), SCAPOLITE and TOPAZ.—See under the latter minerals. Quartz may be distinguished from all silicates by being dissolved without residue in hydrofluoric acid. REMARKS: Quartz occurs widely distributed, as in the great sandstone formations. It is also a characteristic mineral of all acidic rocks, being common in granite, aplite, rhyolite, quartz-porphyry, quartz-diorite, dacite, etc. Quartz is very brittle and hence is a good indicator of the dynamic forces which have affected the rocks. It may show traces of mechanical deformation by peripheral shattering of the larger grains or by “wavy extinction”;[85] and also evidences of chemical corrosion by curved and looped contours. In some diabases the quartz may be surrounded by a rim of hornblende or augite needles (“quartz augen”). “Cataclastic” quartz may be biaxial. The “secondary enlargement” of quartz in clastic rocks may be noticed by the deposition of silica in crystallographic orientation around the clastic grains,[86] the new portion extinguishing at the same time as the core. Quartz is not attacked by ordinary acids. H., 7. Sp. gr., 2.6 to 2.7. _Chalcedony._—This variety of SiO_{2} has a radially fibrous structure and shelly parting. It may form sphærulites, central sections through which show a dark cross between crossed nicols, or line cavities in rocks. The index of refraction is a little lower than for ordinary quartz. The optical character is (−), which must be determined by a mica or gypsum plate, _ć_ = a. Elongation ∥ a′.