sillimanite

n.
A usually white, hard mineral, Al₂SiO₅, occurring in highly metamorphosed rock as long, slender, fibrous crystals. Also called fibrolite.

[After Benjamin SILLIMAN.]

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A nesosilicate mineral of composition Al₂O[SiO₄], crystallizing in the orthorhombic system. Sillimanite commonly occurs in slender crystals or parallel groups, and is frequently fibrous, hence the synonym fibrolite. There is one perfect cleavage, luster is vitreous, color is brown, pale green, or white, hardness is 6–7 on Mohs scale, and the specific gravity is 3.23.

Sillimanite, andalusite, and kyanite are polymorphs of Al₂O[SiO₄]. The three Al₂O[SiO₄] polymorphs are important in assessing the metamorphic grade of the rocks in which they crystallized. See also Andalusite; Kyanite; Silicate minerals.

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Al
Orthorhombic -- Rhombic bipyramidal

Environment

Mica schists and gneisses, and in contact metamorphic deposits.

Crystal description

Usually in finely fibrous gray or brownish masses, embedded in rock. Very fine needles can form a compact rock (Brazil and New Mexico) that streams wear into interesting shapes (Brazil). Distinct pale blue prismatic crystals, clear and transparent, with good side pinacoidal cleavage in the rare gemmy variety.

Physical properties

Usually white, sometimes brownish or greenish (gem variety light blue). Luster satiny (glassy when gemmy); hardness 6-7 (but splinters and is difficult to determine; gemmy examples are 7Ɖ); specific gravity 3.2-3.3; fracture splinters lengthwise, but conchoidal breaks across elongation; cleavage perfect pinacoid, but usually fibrous. Translucent to transparent.

Composition

Aluminum silicate (63.2% Al ₂ O ₃ , 36.8% SiO ₂ ).

Tests

Infusible and insoluble, but the crushed mineral, or a bundle of fibers, turns blue when heated with cobalt nitrate solution (aluminum test).

Distinguishing characteristics

Infusibility distinguishes it from fibrous anthophyllite, which it may resemble (anthophyllite fuses to a black magnetic bead). Hardness and brittleness of the fibers distinguish them from those of asbestos.

Occurrence

Relatively rare, in fibrous, more or less parallel masses in schist, and often altered in part to mica; therefore lacking in normal hardness. Interesting because its Myanmar and Sri Lanka occurrences (where it is found in waterworn, clear, blue, gemmy pebbles) are so unlike the fibrous embedded masses of New England at Worcester, Massachusetts, and Norwich and Willimantic, Connecticut. Also in New York and Pennsylvania. Sometimes, as in North Carolina and Brazil, it is compact enough to form tough waterworn pebbles that may be cut to resemble a cat's-eye. Lately in some abundance in Orissa, India. Pebbles of the compact fibers can water-wear into smooth and shiny surfaces, even more lustrous than waterworn jade masses. Another (historical) name, fibrolite, comes from its appearance; the official name sillimanite was given in honor of Yale's first professor of mineralogy, Benjamin Silliman. New Mexican Indians fashioned many tools from sillimanite masses.

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