My Blogs (olelog) are mainly based on my daily reading of earth science news.
Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.
Posted on Thursday, 3 November 2005, 09:07:30
Desert roses are rosettes of gypsum crystals with sand inclusions. They form when water evaporates under arid sandy conditions like at the edge of Sahara with its large salt lakes in Southern Tunisia. Sand grains are trapped in the gypsum crystals during their growth and give them the same colour as the sand.
In Tunisia you can buy really nice pieces for less than one Euro or Dollar (or 1 Dinar for that matter). To find them yourself usually need some digging, and knowledge of where to dig.
Gypsum is by the way a hydrous calcium sulphite – CaSO4.2H2O. It can be crushed into a white powder and be used in building materials like plaster and sheetrock. Plaster has been used in the Mediterranean region for at least 9000 years – I mean as a building material – I once had my arm in plaster, but that is another story, and of a more recent date.
“Pure” calcium sulphate, CaSO4, is called anhydrite, and gypsum is formed by hydration of anhydrate.
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