Canadian Rockhound: Andalusite

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Andalusite
By Doug Miller

Andalusite. Photo courtesy of theImage.

Andalusite, Al₂O(SiO₄), has been found along the Hanson Lake Road (Highway 106) in Saskatchewan. The andalusite occurs as pink and white to grey transparent to opaque prismatic crystals. Crystals are typically 1 cm in diameter. The crystals are often fractured and are not suitable for faceting. The andalusite is associated with pink orthoclase and white to deep greyish blue quartz. All three minerals are harder than the enclosing schist and therefore are prominently exposed on weathered surfaces.

The andalusite occurrences have very easy access for collectors, being immediately on the south side of the Hanson Lake Road from 101 to 103 km east of the intersection of Highway 165 (from La Ronge) with the Hanson Lake Road.

A variety of andalusite called chiastolite may also be occasionally found at these sites. Chiastolite is a highly impure cigar-shaped crystal that forms with distinctive cross-like inclusions. Polished cross sections of chiastolite are distinctive and attractive.

Andalusite is brittle but tough, has a hardness of 7.5 (3.5 to 4.5 for chiastolite), and conchoidal fracture. Specific gravity is 3.1 to 3.2. Index of refraction is approximately 1.64. Andalusite is slightly birefringent (0.007 to 0.011). Pleochroism visible without a dichroscope sets andalusite apart from most gems. Brazilian material appears green through the prism faces and deep brownish-red through the c axes.

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The preceding article was first published in the November 1997 issue of the Saskatoon Lapidary & Mineral Club newsletter. Reprinted in the Canadian Rockhound with permission from the author.