Pyrophyllite Composition, Crystallization & Structure
Composition., H2AI2(Si03)4 = Silica 66.7, alumina 28.3, water 5.0.
Crystallization. Monoclinic (?). Not observed in crystals.
Structure. Foliated, sometimes in radiating lamellar aggregates. Also granular to compact Identical with talc in structure and appearance.
Physical Properties. Perfect basal cleavage. Folia somewhat flexible but not elastic. H. = 1-2 (will make a mark on cloth). G. = 2.8-2.9. Pearly to greasy luster. Color white, apple-green, gray, brown. Usually opaque. Greasy feel.
Tests. Infusible. Unattached by acids. Characterized chiefly by its micaceous structure and cleavage, its softness and greasy feel. Only to be easily distinguished from talc by moistening a small fragment with cobalt nitrate and igniting, when it assumes a blue color (aluminum). Talc under the same conditions would become pale violet.
Occurrence. A comparatively rare species. Found in metamorphic rocks; frequently with cyanite. Occurs in considerable amount in Guilford and Orange counties, North Carolina.
Use. Quarried in North Carolina and used for the same purposes as talc. It does not command, however, as high a price as the best grades of talc A considerable part of the so-called agalmatolite,from which the Chinese carve small images, is this species.
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