Muscovite Composition, Crystallization & Structure

Muscovite - Common Mica

Composition. H₂KAl₃(Si0₄)₃. Contains also frequently small amounts o ferrous and ferric iron, magnesium, calcium, sodium, lithium, fluorine, titanium, etc.

Crystallization. Monoclinic with axial angle nearly 90°. Occurs in tabular crystals with prominent base. The presence of prism faces having angles of 60° and 120° with each other gives the plates a diamond-shaped outline, making them simulate orthorhombic symmetry. If the clinopinacoid faces are also present, the crystals become hexagonal in outline with apparently hexagonal symmetry. The prism faces are roughened by horizontal striations and frequently taper.

Structure. Foliated in large to small sheets; in scales which are sometimes aggregated into plumose or globular forms. Distinct crystals comparatively rare.

Physical Properties. Extremely perfect cleavage parallel to base, allowing the mineral to be split into excessively thin sheets. Folia flexible and elastic. H. = 2-2.5. G. = 2.76-3. Vitreous to silky or pearly luster. Transparent and almost colorless in thin sheets. In thicker blocks, opaque with light shades of brown and green. May be yellow to white. Some crystals are translucent when viewed perpendicular to the prism zone but opaque in a direction perpendicular to the base.

Tests. Fusible at 4.5--5. Unattached by boiling hydrochloric or sulfuric acids. Characterized by its micaceous structure and

Light color. Told from phlogopite by its not beige decomposed in sulfuric acid and from lepidolite by not giving a crimson flame B.B.

Occurrence. A widespread and very common rocks-making mineral. Found in such igneous rocks as granite and found lining cavities in granites, where it has evidently been formed by the action of mineralizing vapors during the last stages of the formation of the rock. Muscovite is chiefly characteristic of the deep-seated igneoscommon in metamorphic rock, as gneiss and schist, forming the chief constituent in certain mic-schists. In some schistose rocks it occurs

in the form of fibrous aggregates of minute seals having a silky luster, but which do not show so plainly the characters of the mineral. This variety is known as sericite, and is usually the produced of alteration of feldspar. Muscovite also originates, as the alteration product of several other minerals, as topaz, cyanide, spodumene, adalusite, scapolite, etc. Pinite is a name given to the micaceous alteration product of various minerals, and which corresponds in composition more or less closely to muscovite.

In the pegmatite veins, muscovite occurs associated with quartz and feldspar, with tourmaline, beryl, gamet, apatite, fluorite, ete. n is found often in these veins in large blocks, which are at times several feet across.

Muscovite is found in the United States in commercial deposits chiefly in the Appalachian and Rocky Mountain regions. The most productive pegmatite veins occur in New Hampshire and in North Carolina, and in the Black Hills of South Dakota. Of less importance are the deposits in Colorado, Alabama and Virginia. Muscovite has been mined in Maine and Connecticut. Large deposits lire found in Canada in the township of Greenville, east of Ottawa, and in a distriet to the east of Quebee. Large and important deposits occur in India.

Name. Muscovite was so called from the popular name of the mineral, Muscovy-glass, because of its use as a substitute for glass in Russia. Mica was probably derived from the Latin micare, meaning to shine.

Use. Used chiefly as an insulating material in the manufacture of electricaI apparatus. Used as a transparent material (isinglass) for stove doors, lanterns, ete. Berap mica, or the waste material in the manufacture of sheet mica, is used in many ways, as in the manufacture of wall papers to give them a shiny luster; as a lubricant when mixed with oils, as a nonconductor of heat and as a fireproofing material.

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