_Cleavage._—Very perfect, parallel to base (001), Fig. 60. Basal sections show no cleavage, but all other sections show many sharp, parallel cleavage cracks. [Illustration: FIG. 60.—Biotite, showing basal cleavage, in biotite-granite. (From Cohen). ] For percussion and pressure figures, see reference given below.[93] _Inclusions._—May be arranged parallel to lines of pressure figure. Rutile needles, tourmaline, apatite, etc., common in magnesium mica. Zircon inclusions often surrounded by pleochroic halos. ▄Polarized Light▄: _Pleochroism._—Varies with the color, being very marked in the colored varieties (from pale yellow to chestnut-brown or black). The strong absorption, about parallel to the cleavage lines, is very characteristic of the colored mica, Fig. 59 B. Strong absorption is also noticed in hornblende, tourmaline and allanite. Absorption may even be noticed around inclusions (pleochroic halos) in colorless, non-pleochroic micas. Cleavage plates of biotite are not pleochroic unless the axial angle is large. _▄Crossed Nicols▄_: _Double Refraction._—Very strong (γ − α = 0.034 to 0.058). _Interference Colors._—High order (third). May be very bright in thin sections of the colorless micas, and at times be so high in order as not to show any marked color tints. May not be noticeable in sections of the colored varieties, due to absorption of parts of the light. _Extinction._—About parallel to cleavage lines. Very small extinction angles may be noticed in biotites. Basal sections of biotite (the approximately hexagonal mica) usually appear isotropic. Mottled appearance (like “Birds-eye” maple) characteristic, caused by distortion of the flexible laminæ during grinding. Most noticeable in sections inclined to cleavage and near position of extinction. _▄Convergent Light▄_: Axial plane[94] and Bx_{_a_}, practically at right angles to basal cleavage; therefore cleavage plates always show well defined interference figures, generally biaxial in character. The axial angles vary greatly, being usually small for biotite and phlogopite (may appear uniaxial) and large for muscovite (2_E_ = 55° to 90°). Optical character for all micas (−). ▄Alteration▄: Biotites decompose quite easily, lose color and may become completely bleached, which appears to be due to a leaching out of the iron. May also alter to green chlorite, with a fraying out of the mica and a change to chloritic structure. Phlogopites may alter to fibrous, scaly masses, apparently chiefly talc. “Sagenite” webs of rutile may accompany the alteration. Muscovites are characterized by their freshness, and do not seem to suffer from weathering. ▄Distinguished from▄: