File:Anyolite (corundum-amphibole zoisitite) (Neoproterozoic; Mundarara Mine, about 27 km west of Longido, Tanzania) 4 (49143983001).jpg

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Anyolite from the Precambrian of Tanzania. (public display, Field Museum of Natural History, Chicago, Illinois, USA)

Green = Cr-zoisite Black = amphibole Reddish = ruby (corundum)

This attractive rock is called anyolite, or corundum-amphibole zoisitite (or corundum-amphibole zoisite metamorphite). Anyolite is a metamorphic rock consisting of finely-crystalline chromian zoisite (green, Ca2Al3(Si2O7)(SiO4)O(OH) - calcium aluminum hydroxy-oxysilicate with Cr impurity) with minor chromian amphibole (black, apparently either chromian tschermakite and/or chromian edenite) and some large porphyroblasts of red corundum (ruby) (Al2O3 - aluminum oxide with Cr impurity). There’s also minor Ca-rich plagioclase feldspar (anorthite, CaAl2Si2O8) in this rock.

Origin: Published mineralogy studies indicate that this chromian zoisite-ruby combination is the result of very high-grade metamorphism of anorthosite, an intrusive igneous rock dominated by Ca-rich plagioclase feldspar. The chromium (Cr) in the zoisite and the corundum (ruby = corundum with chromium impurity) is derived from metamorphic alteration of chromite crystals (FeCr2O4 - iron chromium oxide) in the original anorthosite unit. Chromite and chromitite (= chromite-dominated igneous rock) are commonly associated with anorthosites in LLIs (= large layered igneous intrusions, such as Montana’s Stillwater Complex).

Geologic Context & Age: This Tanzanian anyolite is hosted in gneisses exposed in the Mozambique Collision Belt, an ancient, north-south trending, tectonic collision zone in eastern Africa. It dates to the Pan-African Orogeny (Neoproterozoic), during which the ancient continents of West Gondwana (~modern-day South America & Africa) and East Gondwana (~modern-day India-Australia-Antarctica) collided, forming the long-lived, small supercontinent Gondwana.

Locality: Mundarara Mine, ~27 km west of Longido, northeastern Tanzania, southeastern Africa (2° 37.876’ South latitude, 36° 28.421’ East longitude)

Mostly synthesized from:

Game, P.M. 1954. Zoisite-amphibolite with corundum from Tanganyika. Mineralogical Magazine 30: 458-466.

Mercier, A., P. Debat & J.M. Paul. 1999. Exotic origin of the ruby deposits of the Mangari area in SE Kenya. Ore Geology Reviews 14: 83-104.
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Source Anyolite (corundum-amphibole zoisitite) (Neoproterozoic; Mundarara Mine, about 27 km west of Longido, Tanzania) 4
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/49143983001 (archive). It was reviewed on 30 November 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

30 November 2019

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current16:45, 30 November 2019Thumbnail for version as of 16:45, 30 November 20192,823 × 1,823 (4.99 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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