PPL Staurolite garnet muscovite schist. Glen Lethnot Grit Formation, Water of Saughs, Glen Lethnot, Tayside Region. NO4339 7355. Collected B. Young, 2008. Staurolite in pleochroic yellows (upper part of field of view) and a garnet (lower right). The garnet is 1mm in length (left to right).

XPL Crossed polars show staurolite and isotropic garnet in a matrix of mainly quartz (grey and white interference colours) and muscovite (tabular crystals with bright interference colours). The BGS lexicon describes the Glen Lethnot Grit Formation (Dalradian) as follows: mixed psammite, semipelite and pelite. Psammites typically gritty. Pelites include types rich in staurolite and muscovite, and some are also kyanite-bearing. Schists rich in iron-ore minerals form mappable units in places. Beds of quartzite with specks of green mica occur rarely near the base of the Formation.

The hand specimen shows porphyroblasts of staurolite (reddish brown) with its occasional characteristic cruciform twinning (seen left centre in the hand specimen). The mineral is named after the Greek word stauros meaning cross. Staurolite occurs in regional metamorphic rocks rich in alumina and iron.

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