Once a crystal of Tourmaline, now an exotic rarity
Quite often during the vagaries that rocks and minerals are subject to as geological history unfolds they change in nature, their minerals literally turning into others in a diverse panoply of ways and due to any one of many possible causes. Metamorphism is one common one, when granites bake their surrounding rocks on intrusion or mountain ranges bake and squish rocks by stacking them atop each other in nappes (see https://bit.ly/2zgW1b3). Here minerals that are stable at one set of temperature and pressure conditions turn into others more appropriate for their new environment. Various kinds of alteration by passing fluids of varying acidity, oxidation state (see (https://bit.ly/1I4XWKt for an explanation) and chemistry is another common cause, turning minerals deposited in one condition into others due to the different chemical conditions.
The original crystal here would have formed in a boron rich pegmatite, the last remnants of a cooling granite that concentrate rare and unusual minerals distilled out of the crust and focus them into these dregs since these elements don't fit into the crystal structure of the usual granitic minerals. Whether it contained strontium as well or whether it arrived with the sulphur rich fluids is unknown, but the Tourmaline was transformed into the strontium phosphate mineral we see here. The phosphorous may have come with the fluids, or from alteration of Apatite (see https://bit.ly/2akYWkg). For those who like this kind of mouthful the actual formula is SrAl3(SO4)(PO4)(OH)6 .
Called Svanbergite, its common habit is to crystallise in near cubic rhombohedra, a bit like calcite, rather than the hexagonal cross sectioned crystal we have here. When one mineral replaces another while retaining the initial shape of a crystal structure not its own the result is called a pseudomorph, from the Greek for fake shape. This doesn't tend to happen in metamorphism, where the recrystallisation into new minerals is more complete.
The setting in which this piece was formed is very unusual since it is more typically metamorphic, forming during the transformation of aluminium rich rocks, usually at medium grade temperatures and pressures and in some bauxite deposits. Here a particular type of sulphur rich fluid passed through a pegmatite in what is now Brazil's Bahia state, producing a kind of alteration known as argillic.
It was first discovered in Sweden in 1854, and named after a local chemist and mineralogist. As well as the pale yellowish material here it can also be colourless or reddish to orange or brown. The Mohs hardness is 5. It is hard to classify since it is both a phosphate and a sulphate simultaneously. Alongside Sweden and Brazil, other localities include Arizona and the Champion Andalusite Mine in California. This unusual specimen measures 2.8 x 1.4 x 1.2 cm, and while aesthetically less impressive than a colourful gemmy tourmaline it has both rarity value and an unusual geological history to compensate.
Loz
Image credit: Joe Budd/Rob Lavinsky/iRocks.com
https://www.mindat.org/min-3837.html http://www.galleries.com/Svanbergite https://bit.ly/2rKVHMR
