These pyroxenes are only found in the eruptive rocks rich in alkalies, as elæolite-syenite, phonolite, certain trachytes, etc.; hence are associated with elæolite, sodalite, leucite, etc. The small, second generation, crystals, in the ground mass of a rock, are always the richest in Na of the pyroxenes in that rock. AMPHIBOLE, Hornblende, etc. ANISOTROPIC. BIAXIAL. MONOCLINIC. ELONGATION ∥ c′. COMPOSITION: RSiO_{3}. R = Mg, Ca, Fe chiefly also may contain Al, Na, Mn. The Mg predominates over the Ca. [Illustration: FIG. 55.—Hornblende, showing twinning between crossed nicols, in amphibole-biotite-granite. (From Cohen.) ] ▄Usual Appearance in Sections▄: Both in crystals and more or less irregular grains, often fibrous, Figs. 55 and 56, the habit varying with the chemical composition as follows: _Tremolite_ (Mg_{3}Ca) and _Actinolite_ ((MgFe)_{3}Ca varieties), in long columnar to needle-like individuals, with no terminal planes or with frayed out ends. May be in dense aggregates. _Pargasite_ in well developed crystals. _Common green Hornblende_ (aluminous varieties) in crystals, compact grains or shreds. _Basaltic Hornblende_ (iron rich, aluminous varieties) in prismatic crystals of varying length, which may often show “magmatic resorption” (to augite and magnetite) around outer zone or throughout whole crystal. [Illustration: FIG. 56.—Hornblende, section parallel to _ć_ axis, showing prismatic cleavage, in hornblende-diorite. (From Cohen.) ] Crystals are simple in form of prismatic habit, with prism angle 124° 30′. Cross-sections are acutely rhombic, generally with acute angles truncated, hence six-sided (pyroxene being eight-sided). Longitudinal sections are lath-shaped and fibrous structure may be noticed. Skeleton crystals may also occur, being very fine in certain pitchstones. Zonal structure and parallel growth may be noticed in the amphiboles. _Twinning._—Frequent, parallel to ortho pinacoid (100). Twins dual, less often multiple, Fig. 55. Intergrowths with pyroxene and biotite occur. _Color._—From colorless (tremolite), through green (actinolite, pargasite and hornblende) to brown (basaltic hornblende). Yellow in some varieties and bluish in the soda varieties. _Index of Refraction._—_n′_ = 1.621 to 1.641 (α = 1.607 to 1.629, γ = 1.634 to 1.653) (1.719, in the basaltic hornblende), hence _relief_ distinct and surface rough. _Cleavage._—Perfect, parallel to prism of 124° 30′. Generally appears in thin sections as sharp cracks crowded close together, Figs. 56 and 57. More perfect than in pyroxene. Some of the long prisms (actinolite and tremolite) may show transverse parting. _Inclusions._—The iron ores, apatite, etc., may be found in hornblende. [Illustration: FIG. 57.—Hornblende, cross-section. ] ▄Polarized Light▄: