Stephenson, D., Bevins, R.E., Millward, D., Highton, A.J., Parsons, I., Stone, P. & Wadsworth, W.J. 1999. Caledonian Igneous Rocks of Great Britain. Geological Conservation Review Series No. 17, JNCC, Peterborough, ISBN 1 86107 471 9. The original source material for these web pages has been made available by the JNCC under the Open Government Licence 3.0. Full details in the JNCC Open Data Policy
1. 'Grorudite' (peralkaline rhyolite, comendite)
Introduction
The 'grorudites' are fine-grained acid (SiO2 = 72–76%) peralkaline rocks for which aegirine rhyolite or comendite would be appropriate modern names. They are compositionally the equivalent of peralkaline granites and have no counterpart among the Caledonian plutonic rocks. They are therefore important in that they extend the compositional range of the Scottish Caledonian magmatism to include both strongly silica under- and oversaturated alkaline rocks, a dichotomy found in many much larger alkaline provinces. Thompson and Fowler (1986) pointed out the chemical similarity between an Assynt grorudite' and a comendite from the shoshonitic volcano of Lipari in the Aeolian Arc. The freshest examples may be pale-green in colour but in general they are pink or reddish-brown. The rocks are usually very fine grained and the matrix is aphanitic, although small pink alkali feldspar phenocrysts are sometimes visible. In section the rocks are characterized by phenocrysts of alkali feldspar and the sodium-iron pyroxene, aegirine, set in a quartz-feldspar matrix crowded with tiny aegirine needles. Various sub-varieties were recognized by Phemister (1926) and Sabine (1953). The reader is referred to these works for petrographical detail. 'Grorudites' are common in Assynt, particularly to the east and north of Inchnadamph (where they may be seen on the Cnoc an Droighinn GCR site