- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
The mineral cordierite occurs in Pre-Cambrian gneisses, granulites, and granites of the Guyana shield in British Guiana. In the acid biotite gneisses and acid granulites forming the Kanuku group, the mineral was formed during regional metamorphism. Considerable enrichment of cordierite occurs in the contact aureole around the South Savanna granite. Cordierite of xenocrystal origin also occurs in the South Savanna granite and its apophyses. The 2V and optic sign of the mineral varies widely, and no definite pattern in relation to the paragenesis of the mineral is recognized. Untwinned cordierites are predominant in regionally metamorphosed rocks while lamellar, interpenetration and sector twins are characteristic of cordierites in contact hornfelsed aureole rocks.