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To promote a
wider interest in the science of geology through organised lectures,
field excursions and social activities.
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By Ian Donaldson |
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Amongst the usual bits of rose quartz, malachite and galena on the French market stall, one specimen caught my eye. Made of small cafe au lait cubes, it looked familiar, but the name was strange - ANKERITE. However it wasn't from Turkey, as the box was stamped MAROC. The stall owner wasn't any wiser about its composition. But it had aroused my curiosity, and as it wasn't expensive, I bought it.
Back home, to my surprise, none of the popular books on minerals listed it, nor did some more detailed mineralogy textbooks, and I was beginning to think it a French name for something else when my eyes lit up on finding it in the Encyclopaedia Britannica. But the crystals were described as rhombic. Surely mine were cubes? Looking again at my specimen, but more closely, I saw my error - they were rhombs. So what is the connection with Ankara?
Well, there isn't any, just as Ilmenite doesn't come from Ilminster. Anker was the man after whom the mineral is named. It is a derivative of dolomite, in which some of the magnesium has been replaced by iron. Which is why it looked familiar - Dave Green had shown us a face of dolomite crystals in the Forest of Dean. So it is part of a series of carbonate minerals, ranging from Calcite, which is pure calcium carbonate, and fizzes with vinegar, through Dolomite, a calcium-magnesium carbonate, which needs to be heated with strong hydrochloric acid before it fizzes significantly, then Ankerite, a calcium, magnesium and iron carbonate, which reacts with cold hydrochloric acid. As the iron concentration increases, Siderite, or pure iron carbonate is the last member of this branch of the series. (The other branch terminates in pure magnesium carbonate as the mineral Magnesite, used in fire bricks and face powder.) .More rigorously, Ankerite is defined as dolomite in which more than 20% of the magnesium atoms have been replaced by iron. After heavy firing it becomes magnetic, calcite and dolomite do not. It varies from white, and cream, to brown or red and has a white streak. It has a hardness of 4 on Mohs' scale, a Specific Gravity of about 3, and occurs in sedimentary rocks as a result of "low temperature metasomatism" - whatever that means - as well as commonly containing some manganese. And " it is found in deposits of iron ore with chalybite". What is that? Searching for chalybite yielded "see siderite" which turns out to be just another name for it. But hasn't chalybite something to do with mineral waters? Paraphrasing Bloxham, "chalybeate" waters contain "iron carbonate dissolved in carbonic acid, and hence the rusty deposits when they are exposed to air". He also mentions other sorts of mineral waters described as "hepatic" or "sulphureous" . - "Harrogate water is eminently sulphureous and its nauseous odour is due to sulphuretted hydrogen". He hadn't tried the water from the Pump Room in Bath, or at Tunbridge Wells, both of which are also eminently sulphureous.
Chalybite, or siderite is an important constituent of "Spathic" iron ore. And spathic comes from the German "spath" or spar - which refers to any translucent non-metallic mineral, such as felspar and fluorspar, that can be easily split into layers, or else a fragment of such a mineral. Iron rich dolomites are also loosely known as Brown-Spar, Pearl-Spar or Bitter-Spar. Siderite comes from sideros, the Greek for iron, but it is also the name of a type of iron meteorite.
Professor Mathias Anker (1771 - 1843) was a mineralogist from Arnold Schwarzenegger's province of Styria in south - east Austria , but he didn't discover it. It was Wilhelm von Haidinger, a colleague of Mohs, who recognised it as a distinct mineral species , in 1825, and named it in his honour.
Sources include :- 1) Bloxham's
Chemistry 9th Edition, Churchill, London 1904
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