_Pleochroism._—All colored amphiboles show pleochroism, which in general is stronger the darker the color of the variety (actinolite and pargasite show but little). The absorption is very marked in the hornblendes, being greatest in the general direction of the cleavage lines in longitudinal sections. Marked differences in absorption are also characteristic of the mineral species biotite, tourmaline and allanite. Pleochroic halos (brownish) surrounding inclusions may be noticed. _▄Crossed Nicols▄_: _Double Refraction._—Quite strong, but a little weaker than in pyroxene (γ-α = 0.019 to 0.027). Ferruginous basaltic hornblende has strong double refraction (γ-α = 0.072). [Illustration: FIG. 58.—Actinolite, clino pinacoid section. ] _Interference Colors._—Second order, hence bright tints, but in darker colored varieties not so noticeable as in pyroxenes, due to the stronger absorption of parts of the light. The colors of basaltic hornblende are so high that they show no bright tints. _Extinction._—Always symmetrical in sections (through _b_ axis) showing intersecting cleavage lines, in such cases bisecting the angles of the cleavage. In sections showing parallel cleavage lines, only parallel in ortho pinacoid (100) sections, in all other sections an extinction angle being observed. The maximum extinction angle lies in the acute angle and is much smaller than in pyroxene, varying with the chemical composition from 0°–20°. In hornblende, actinolite and tremolite 12°–20°, Fig. 58; in the basaltic hornblende 0°–10°. The maximum extinction angle is only obtained when the section of the crystal is parallel to the clino pinacoid (010), varying from this angle to 0°, when the section is parallel to the ortho pinacoid (100). _▄Convergent Light▄_: Axial plane parallel to clino pinacoid (110), Fig. 58. Bx_{_o·_}(c) Λ _ć_ = 0°–20° behind. Axial angles large (2_E_ = 77° to >180°). Optical character (−). Pargasite is (+). ▄Alteration▄: May take place to chlorite, talc, serpentine, asbestus, etc., depending on the chemical composition. Amphibole frays out and becomes fibrous during alteration, and may also lose color. ▄Distinguished from▄: (_a_) PYROXENE.—By usually much stronger pleochroism in the colored varieties, and by cleavage and extinction angle. In pyroxene the cleavage (parallel to prism of 87° 06′) is less perfect; and the extinction angle is much larger, varying from 36° to 54°. (_b_) BIOTITE.—By the extinction in the mica being always about parallel to the cleavage. Both have strong absorption, but biotite shows very slight pleochroism in sections parallel to the cleavage, and has only the one cleavage parallel to the base. Also the biotite has lower index of refraction and generally shows uniaxial interference figure. Colorless tremolite may be distinguished from muscovite and talc by extinction angles, relief and lower order interference colors.