"Rock of the Month # 105, posted March 2010" --- Sample of twinned microcline (amazonite) feldspar, obtained from Grenville Minerals of Kingston, Ontario. The sample is displayed here on a bed of roasted coffee beans from southern Ethiopia. Fine coffees are grown in Sidamo province, which includes the Oromia district, and the Yirgacheffe valley. Indigenous exporters include the Oromia Coffee Farmers' Co-operative. A less well-known export from the region is fine mineral specimens.

Amazonite, occasionally dubbed amazonstone, is a green variant of microcline K-feldspar. This specimen is a beautiful simple twin, displaying perthitic exsolution of sodic plagioclase feldspar. The crystal is embedded in a pale matrix which appears to be largely granular smoky quartz and white albitic feldspar. The Archean bedrock of the Sidamo region includes a range of igneous and metamorphic rocks, with some overprinting by the geologically youthful (Cenozoic) magmatism of the East African Rift. The amazonite occurs in pegmatites in the Precambrian basement (see, e.g., Larson, 2004; London, 2008).

References

Bedini,RM and Bodinier,J-L (1999) Distribution of incompatible trace elements between the constituents of spinel peridotite xenoliths: ICP-MS data from the East African rift. Geochim.Cosmochim.Acta 63, 3883-3900.

Larson,B (2004) Sainte-Marie aux Mines show 2003. Mineral.Record 35, 145-148.

London,D (2008) Pegmatites. Canadian Mineralogist Spec.Publ. 10, 347pp.+CD-ROM.

Peccerillo,A, Mandefro,B, Solomon,G, Hambisa,G, Bedru,H and Tesfaye,K (1998) The Precambrian rocks from southern Ethiopia: petrology, geochemistry and their interaction with the recent volcanism from the Ethiopian Rift Valley. Neues Jahr.Min.Abh. 173, 237-262.