Phlogopite Composition, Crystallization & Structure
Composition. A magnesium mica, near biotite, but containing no iron, H2KMg3AL(SiO4)3(?). Usually contains about 3 per cent of fluorine.
Crystallization. Monoclinic. Usually in six- sided plates or in tapering prismatic crystal. Crystals frequently large and coarse.
Structure. In crystals or foliated masses.
Physical Properties. Perfect basal cleavage. Folia flexible and elastic. H. =205-3. G. = 2.86. Luster Vitreous to pearly. Color yellowish brown, green, white, often with copper-liker reflections from the cleavage surface. Transparent in thin sheets to opaque in the mass.
Test. Fusible at 4.5-5. Insoluble in hydrochloric acid. Decomposed by boiling concentrated sulfuric acid, givin a milky solution. Characterized by its micaceous structure, cleavage and yellowish brown color. Told from muscovite by its decomposition in sulfuric acid and from biotite by its lighter color. But it is impossible to draw a sharp distinction between bitter and phlogopite.
Occurrence. Occurs as a product of metamorphism in crystalline magnesium limestones or dolomitic marbles and in also found in serpentine. Rarely found in igneous roks. Notable localities are in Finland; Sweden; Campolungo, Sweden; Ceylon; Madagascar, etc. In North America, found chiefly in Jefferson and St. Lawrence counties, New yorks; at North and South Burgess, Ontario, and in various localities in Ontario, and Quebec, Canada.
Name. Named from a Greek word meaning firelite, in allusion to its color.
Use. Same as for muscovite.
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