One, replacing seven...
Mineral hardness is measured in a variety of ways, the best known and the most rough and ready being Mohs hardness scale, which compares the relative hardness of ten minerals by the simple expedient of playing the game of who scratches who. It is not a linear scale, with the steps between 8 and 9 (topaz and corundum) and 9/10 (corundum/diamond) each being orders of magnitude greater than the previous ones, but it does serve for basic field tests. In this beautiful 5.7 x 3.8 x 3.1 cm specimen from Bavaria, the softest mineral has replaced one of the hardest, providing a nice excuse for a digression into the clay mineral talc.
We are all familiar with this clay, most of us were liberally sprayed with it in powder form as babies, and many of us continue to do so on a regular basis throughout life. Its colour ranges from the well known white to grey or green, and in massive form its metamorphic variant has a greasy feel and is known as the carving material called soapstone. The name dates from antiquity, and comes from ancient Persian.
It is formed by the chemical weathering of magnesium rich volcanic minerals such as pyroxene and olivine in the presence of water and CO2 or by a metamorphic reaction between the magnesium rich limestone known as dolomite and silica rich rocks, often mediated by the hot mineralised fluids of intruding granites spewing into the surrounding limestones, forming complex mineral assemblages known as skarns. It can also form at the deep pressures encountered at the keel of continents and in rocks that have been dragged deep into subduction zones and spat back out.
As well as talcum powder it has many uses, including whitening paper, a filler in plastics, ceramics (in the body and in fluxes or glazes) and paints, as a lubricant, i rubber, and varied industrial and laboratory uses that take advantage of its resistance to heat, electricity and chemicals. The top mines are in France, China, South Korea, India, United States, Finland and Brazil. Here talc has replaced a crystal of quartz in a process known as pseudomorphism (literally translating as faking the shape).
Loz
Image credit: Rob Lavinsky/iRocks.com http://www.minerals.net/mineral/talc.aspx https://www.mindat.org/min-3875.html http://www.galleries.com/talc http://bit.ly/2kKgFth http://geology.com/minerals/talc.shtml