(_a_) STAUROLITE.—In convergent light the axial plane is shown to be in the shorter diagonal of the rhombic cross-section, while in staurolite it is in the longer diagonal. (_b_) RUTILE.—By biaxial character. (_c_) CALCITE.—The light colored titanite (sphene), in absence of twinning, by higher index of refraction. Titanite may easily be confused with some of the rarer minerals. REMARKS: Titanite is always an accessory mineral and is found distributed in all rocks, except SiO_{2} rich eruptive and magnesia silicate rocks. As a secondary mineral it forms rims around other titanium minerals or pseudomorphs after them and also the principal part of _leucoxene_. It is partly soluble in hot hydrochloric acid and completely decomposed by sulphuric acid. H., 5 to 5.5. Sp. gr., 3.3 to 3.7. In a specific gravity separation it falls with the ferruginous minerals (on account of its density) and from these can generally be separated by electromagnetic methods. FELDSPAR GROUP. Orthoclase, Microcline and the Plagioclases. ORTHOCLASE. ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION (∥ cleavage) ∥ a′. COMPOSITION: KAlSi_{3}O_{8}, with some replacement by Na. ▄Usual Appearance in Sections▄: In crystals and grains. In porphyritic rocks habit of crystals more or less tabular parallel to clino pinacoid (010), or rectangular much extended parallel to clino axis _à_, with cross-sections six-sided, or rectangular to long lath shape, Figs. 65 and 66. Crystals may be changed into rounded or looped grains by chemical corrosion, Fig. 6. Adularia crystals more prismatically developed, giving rhombic sections. Dimensions of crystals vary extremely; microlites occur, at times forming sphærulitic structure. The very fine grained ground-mass “microfelsite” (not resolved by the microscope) consists largely of feldspar. [Illustration: Orthoclase cleavage plates. FIG. 65.—Basal. FIG. 66.—Clino pinacoid. ] Intergrowths with microcline and plagioclase common, forming “microperthite” when lamellæ are microscopic. May be in zonal formation with plagioclase (the orthoclase on the periphery). Also intergrown with quartz[97] forming “pegmatite” and “micro-pegmatite,” Fig. 67. [Illustration: FIG. 67.—Micro-pegmatitic structure, in granophyric quartz-porphyry. (From Cohen.) ] Zonal structure often seen, Fig. 20, especially when decomposition has commenced; and in fresh crystals may be indicated by zonal arrangement of inclusions.