The Earth Story

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

Mohs Scale of Mineral Hardness.

The Mohs scale of mineral hardness (or the Mohs hardness scale) is used by geologists all over the world as an identifying tool for minerals. The basic principal is, that it characterises the scratch resistance of a softer mineral being scratched a harder mineral.  Developed in 1822 by Friedrich Mohs a German Geologist and mineralogist, it is one of several methods used to describe the hardness of minerals and materials in science, however it is the one most commonly used by geologists. Although it has been 200 years since the first implementation of the Mohs hardness scale, the method of comparing one mineral to another to ascertain its hardness is much older. The technique was mentioned in 300BCE by Theophrastus in his treatise On Stones, and then again in 77CE by Pliny the Elder in his Naturalis Historia. The scale each uses a different mineral, and is ordinal in nature. The hardest known substance to man at the time of the scales creation was diamond, and as such diamond is at the top of the scale. The complete scale is as follows; TALC- Mohs hardness 1- Absolute hardness 1 GYPSUM- Mohs hardness 2- Absolute hardness 3 CALCITE- Mohs hardness 3- Absolute hardness 9 FLOURITE- Mohs hardness 4- Absolute hardness 21 APATITE- Mohs hardness 5- Absolute hardness 48 ORTHOCLASE FELDSPAR- Mohs hardness 6- Absolute hardness 72 QUARTZ- Mohs hardness 7- Absolute hardness 100 TOPAZ- Mohs hardness 8- Absolute hardness 200 CORRUNDUM- Mohs hardness 9- Absolute hardness 400 DIAMOND- Mohs hardness 10- Absolute hardness 1600 Whilst it is possible to buy a set of “Mohs mineral pencils” a set of pencils each mounted on the tip with a small piece of each of the minerals above, these for geologists are often more show pieces, and in the field it is important to know the hardness of ordinary materials to make mineral identification easier. A steel knife (very good for many applications in the field) has a hardness of about 6-6.5 whist a fingernail has a hardness of between 2 and 2.5. Just these two tools can make mineral identification so much easier!  To read more, and to discover the hardness of more everyday items on the Mohs scale, head to any of the following links. -LL Links;  http://www.amfed.org/t_mohs.htm http://geology.about.com/od/scales/a/mohsscale.htm http://chemistry.about.com/od/geochemistry/a/mohsscale.htm Image; Gemstone Universe

Talc. Talc is a metamorphic mineral comprising of silica and hydrated magnesium, it has the chemical formula; H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. The crystal habit of Talc varies from elongate fibrous to massive to foliated, and it is incredibly soft, sitting in the lowest spot of 1 on the Mohs hardness scale. It results from the metamorphism of magnesium rich rocks containing minerals such as pyroxene, olivine, serpentine and amphibole, as long as there is the presence of water and carbon dioxide, and this sort of metamorphism produces a suite of rocks known as "talc carbonates". Although this is the primary formation of talc it can also occur as a product from the reaction between dolomite and silica, or as a result of eclogite and blueschist metamorphism.  Talc is a very common mineral terranes that contain high amounts of ultramafic rocks. Talc-Carbonate rocks are incredibly common in Archean rocks, notably the Yilgarn Craton of Western Australia, where they occur in the Komatiite belts. They are also common in the ophiolite belts of Turkey. Talc as a mineral has many uses in industry, including; paper making, plastics, paint making, electric cable, pharmaceuticals, cosmetics, rubber, soap, electrical switch boards and crayons. It is also widely used as an anti-caking agent in table salt. And of course perhaps most famously it is the main ingredient of Talcum Powder. -LL Links; http://webmineral.com/data/Talc.shtml http://www.mindat.org/min-3875.html http://geology.com/minerals/talc.shtml http://csmres.jmu.edu/geollab/fichter/MetaRx/MetaKind.html http://geology.about.com/od/scales/a/mohsscale.htm Image; Thanks to the University of Waterloo Earth Sciences Museum