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Pressures beyond rubies

 

Alan Wright investigates the biggest ruby porphyroblasts you’re ever likely to see…


Geoscientist 19.11 November 2009


The “Star of Tanzania”Anyone keeping up with the financial pages of the national press recently (and who isn’t these days?) may have been intrigued to read a story about a very large ruby, said to be worth £11million, which was a major “asset” of Wrekin Construction when it went into administration earlier this year. Given an exotic name the “Star of Tanzania” and the huge valuation, it was nonetheless pictured as a large, undistinguished red lump, 13cm across. The valuation was said to be an estimate of the value of gems that might be cut from it. If you wondered how it comes about that large rubies the size and appearance of house-bricks come to be lying about in Tanzania, I will try to explain.

When I was working in the Petrology Lab. of the Tanganyika Geological Survey (1957-60) one of the most interesting investigations John Harpum and I had to make was on a zoisite- corundum rock from the Merkerstein area of Northern Tanganyika. Two very large fragments of ruby are in the collections (fragments of single crystals approx. 13cm x 12cm x 11cm) found in the same localities as the zoisite–corundum rock. It seems highly likely that the “Star of Tanzania” comes from the same deposit.

So how do such rubies come to be formed? The zoisite-corundum rock is a remarkable deposit, possibly unique to the Merkerstein area (about 50miles WNW of Kilimanjaro) of Tanganyika. Outcrops of the rock are spread over an area of roughly 400sq.miles set in granitic gneisses. Only reconnaissance mapping of the area had been done at that time, but they seem to occur in a series of metamorphosed basic igneous plutonic rocks.

The ruby occurs as euhedral porphyroblasts in a rock made up largely of apple green chromiferous zoisite with a dark green edenitic hornblende. The red colour of the ruby and the green of the zoisite are due to the presence of chromium. The rock had already attracted the attention of mineralogists at the British Museum and the zoisite had been described by P M Game (in 1954). He also noted that the small amounts of plagioclase present were unusually anorthite rich (An97, from its optical properties). I subsequently confirmed this by XRF analysis at Birmingham University, when I analysed the individual minerals in order to determine their trace elements. The chromium contents of the three major minerals, were found to be: ruby 4500ppm; zoisite 2200ppm and hornblende 5500ppm.

We were able to make thin sections of a wide variety of these rocks, and the ruby shows several interesting properties. It quite often had concentric colour zoning and lamellar twinning, gave a red fluorescence under ultraviolet light and, most unusually, was strongly triboluminescent, which is to say it gave a bright red flash when hit with a hammer. Surely not many mineralogists ever come across rubies large enough to try this experiment! (We were forced to hit it with a hammer, to get a fragment for chemical analysis, since corundum is so hard that our normal cutting wheels made very little impression on it). The normal porphyroblasts are still of substantial size and are platy rather than the normal barrel shape. The platy form, though, has nothing to do with the metamorphic foliation as the rubies lie across it at almost any angle.

The zoisite-corundum rock was given the name “Anyolite” from the Masai word for green. It is a most unusual and attractive rock, sometimes called Tanganyika Artstone in an attempt to create a market for it as decorative stone. The rubies in it, although a very good, deep red colour, are not of gem quality, as all the corundum shatters into small fragments. However some gem-quality rubies were won from the area, so it seems that there are pockets of rock where the ruby is unshattered. The rubies are always at least a few centimetres in size. We know, from the two hand specimens in the Tanzanian Geological Survey, that very large crystals are found in that area; so it is possible that the “Star of Tanzania “ is of gem quality through and through.


Anyolite from the Merkerstein area of Northern Tanganyika. Apple green zoisite, red ruby corundum and very dark green hornblende. The cut side of the specimen is 11cm long.

The origin of Anyolite


In order for corundum to grow in a metamorphic rock, it has to be very rich in alumina. The commonest such rocks are clays, but they will not normally have chromium in them. The presence of chromium in all the major minerals suggests a basic igneous parent, and the basic igneous rock with the highest alumina is anorthosite (which is composed entirely of calcic plagioclase). Large layered basic intrusives with abundant anorthosite and chromitite layers were commonly developed in Archaean times in most ancient shields. The rocks most closely similar to anyolite occur in Madagascar2 and Madras3 although not with quite such extreme chemistry. Both areas have corundum and other alumina rich minerals in metamorphosed anorthosites from layered basic intrusions.

Readers who remember their first-year mineralogy may recall that zoisite is a metamorphic proxy for anorthite, the calcic end-member of plagioclase. Game3 recognised that the plagioclase remaining in the rock is one of the most anorthite rich examples found, until that time, in nature. Zoisite is, however much denser than plagioclase, and Game’s determination was 3.36. (It is of interest that the original zoisite from Zoissa, Carinthia comes from an eclogite). The other two major minerals ruby (3.85) and the hornblende (3.13) are also very dense, suggesting that this rock was metamorphosed at very high pressure. To develop an excess of alumina, from an original anorthosite, the rocks must have been desilicated. It is also likely that they have lost soda, as the only soda bearing mineral now is the hornblende with only 2.38%. It is therefore postulated that the formation of migmatitic granites, by partial melting of the country rocks, resulted in the abstraction of silica and soda from anorthosite layers in the basic igneous complexes, giving rise to the zoisite and ruby-rich rocks.


This view shows the foliation of the zoisite and hornblende and the cross-cutting nature of the ruby porphyroblasts; also an unusual cross-twin.

So igneous differentiation of a basic igneous magma, followed by metamorphic differentiation, then elevation to eclogite facies finally resulted in the crystallisation of this very pretty, and perhaps very valuable, rock. The reason for the enormous size of the ruby porphyroblasts, however, remains as much a mystery as the source of its £11m valuation…



References

  1. Lacroix, A. 1941 Les gisements de phlogopite de Madagascar et les pyroxenites qui les renferment. Ann. Geol. Serv. Min. Madagascar No.11
  2. Subramaniam, A.P., 1956 Mineralogy and petrology of the Sittampundi Complex, Salem District, Madras, India. Bull geol. Soc. Amer., 67, 317-390
  3. Game, P.M. 1954 Zoisite-amphibolite with corundum from Tanganyika. Min Mag. , 30, 458-466

* Alan Wright retired some aeons ago but is still Honorary (i.e. unpaid) Research Fellow at Birmingham University. E: aewright@onetel.com