ANYOLITE
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.
Locality:
Mundarara Mine, ~27 km west of Longido, northeastern Tanzania, southeastern
Africa.
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.
Anyolite
(= corundum-amphibole zoisitite) (4.7 cm across at its widest) from the
Neoproterozoic of the Mundarara Mine, northeastern Tanzania. Green =
Cr-zoisite; black = amphibole; reddish = ruby (corundum).
Mostly synthesized from Game (1954), Mercier et al.
(1999), and info. provided by Alan Wright.
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.