Tuesday, 20 September 2011

Anhydrite, gypsum and selenite ~

Anhydrite, gypsum and selenite are mineral variants of the same thing, plus or minus the presence of water.  The common factor is calcium sulphate (also spelt calcium sulfate).  How very different they look!

Small anhydrite pebble

Anhydrite is anhydrous calcium sulphate, which has the chemical formula CaSO4.  The term 'anhydrous' means it's without water.

The crystal below is shown from both sides so that detail can be seen.

These two have the identical chemical formula CaSO4.2H2O, which when compared with the one above for anhydrite shows the addition of water into the equation.  The Wikipedia article linked to here states that 
This is a very soft mineral composed of calcium sulphate dihydrate.
The article goes on to say that
When gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses it is called selenite.
Here is a rather special chunk of selenite, termed 'fishtail selenite' on account of its shape:
It's about 6 inches from what appears here to be nose to tail, and feels and looks glassy.

Here is the side view:


A breathtaking spectacle of giant selenite crystals can be found in the National Geographic article 'Cavern of Crystal Giants' published in November 2008.  This article linked to here features those found deep in the Naica mine of Mexico.  

Here is a YouTube clip of views within the same set of caves:

At the other end of the gypsum spectrum we have the softer forms such as 'desert rose' structures and Plaster of Paris! 

Here is an image of a 'desert rose'.  It's about an inch and a half across: 

The simplest explanation I have found of the relationship between gypsum and anhydrite is in the Wikipedia article on Plaster.  In the section headed Gypsum plaster / Plaster of Paris it is stated that:
When the dry plaster powder is mixed with water, it re-forms into gypsum. If plaster or gypsum is heated above 200°C anhydrite is formed, which will also re-form as gypsum if mixed with water.[2]
The Wikipedia article 'Hydrate' is helpful in giving greater insight into this watery subject!  

In general I have avoided such technical detail in this chronicle, but the three are so inextricably linked that this was the only way I could both make sense of them and write about them together, thus avoiding confusing repetition.

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