Andalusite - Chiastoline Composition, Crystallization & Structure

Composition. Aluminum silicate, Al₂SIO₅ = silica 36.8, alumina 63.2.

Crystallization. Orthorhombic. Usually in coarse, nearly square prisms. Closely related crytallographically o topaz.

Structure. In crystals; massive.

Physical properties. H. = 7.5. G. = 3.16-3.20. Vitreous luster. Flesh-red, reddish brown, olive green. Often with dark colored carbonaceous inclusion forming a cruciform design, lying parallel to the axial directions (variety chiastolite or macle). Transparent to opaque. At times strongly diachronic, appearing, instrumented light, green in one direction and red in another.

Test. Infusible. Insoluble. When fine powder is made into a paste with cobalt nitrate and intensely ignited it turns blue (aluminum).

Occurrence. Found as a contact mineral in clay slates andschits, especially in connection with granite intrusion. Often impure and commonly, at least partly altered. Notable localities are in Andalusia, Spain; the Tyrol; in water-worn pebbles from Minas Geraes, Brazil. In the United States at Standish, Maine; Westford, Lancaster and Sterling, Massachusetts; Delaware County, Pennsylvania. Chiastoline is found at Bimbowrie, South Australia; and Massachusetts.

Use. When clear and transparent may serve as a gem stone.

Sillimanite - Fibrolite
An aluminum silicate like andalucite, Al₂SiO₅. An orthorhombic mineral, occurring in long slender crystals without distinct termination; often in parallel groups; frequently fibrous. Perfect pinacoidal cleavage. H. = 6-7. G. = 3.23. color hair-brown to pale green. Transparent to translucent. Infusible. Insoluble. A comparatively rare mineral, found as an accessory constituent of metamorphic rocks; gneiss, mica-schist, etc.

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