Abrasive Minerals

pumice stones
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Abrasives today are largely precision-manufactured substances, but natural mineral abrasives are often still used. A good abrasive mineral is not just hard, but also tough and sharp. It must be plentiful -- or at least widespread -- and pure.

Not many minerals share all these attributes, so the list of abrasive minerals is short but interesting. 

Sanding Abrasives 

Sanding was originally done with (surprise!) sand -- fine-grained quartz. Quartz sand is hard enough for woodworking (Mohs hardness 7), but it's not very tough or sharp. The virtue of sand sandpaper is its cheapness. Fine woodworkers do occasionally use flint sandpaper or glass paper. Flint, a form of chert, is a rock made of microcrystalline quartz. It's no harder than quartz but it's tougher so its sharp edges last longer. Garnet paper is still widely available. The garnet mineral almandine is harder than quartz (Mohs 7.5), but its real virtue is its sharpness, giving it cutting power without scratching wood too deeply.

Corundum is the workhorse abrasive of sandpaper. Extremely hard (Mohs 9) and sharp, corundum is also usefully brittle, breaking into sharp fragments that keep on cutting. It's great for wood, metal, paint, and plastic. All sanding products today use artificial corundum -- aluminum oxide. If you find an old stash of emery cloth or paper, it probably uses the real mineral. Emery is a natural mix of fine-grained corundum and magnetite.

Polishing Abrasives 

Three natural abrasives are commonly used for polishing and cleaning metal: enamel finishes, plastic, and tile. Pumice is a stone, not a mineral, a volcanic product with a very fine grain. Its hardest mineral is quartz, so it has a gentler action than sanding abrasives. Softer still is feldspar (Mohs 6), which is most famously used in the Bon Ami brand household cleaner. For the most delicate polishing and cleaning work, such as with jewelry and fine crafts, the gold standard is tripoli, also called rottenstone. Tripoli is microscopic, microcrystalline quartz mined from beds of decomposed limestone.

Sandblasting and Waterjet Cutting

Applications of these industrial processes range from scrubbing rust off of steel girders to inscribing gravestones, and a wide range of blasting abrasives is in use today. Sand is one, of course, but airborne dust from crystalline silica is a health hazard. Safer alternatives include garnet, olivine (Mohs 6.5) and staurolite (Mohs 7.5). Which to choose depends on many factors other than mineralogical considerations, including cost, availability, the material being worked, and the experience of the worker. Many artificial abrasives are in use in these applications, too, as well as in exotic things like ground walnut shells and solid carbon dioxide.

Diamond Grit

The hardest mineral of all is diamond (Mohs 10), and diamond abrasive is a large part of the world diamond market. Diamond paste is available in many grades for sharpening hand tools, and you can even buy nail files impregnated with diamond grit for the ultimate grooming aid. Diamond is best suited for cutting and grinding tools, however, and the drilling industry uses lots of diamond for drill bits. The material used is worthless as jewelry, being black or included - full of inclusions - or too fine-grained. This grade of diamond is called bort.

Diatomaceous Earth

The powdery substance composed of the microscopic shells of diatoms is known as diatomaceous earth or DE. Diatoms are a kind of algae that form exquisite skeletons of amorphous silica. DE is not abrasive to humans, metals, or anything else in our everyday world, but at the microscopic scale, it's very damaging to insects. The broken edges of crushed diatom shells scratch holes in their hard outer skins, causing their internal fluids to dry out. It's safe enough to strew in the garden or to mix with food, such as stored grain, to prevent infestations. When they aren't calling it diatomite, geologists have another name for DE, borrowed from German: kieselguhr.

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Alden, Andrew. "Abrasive Minerals." ThoughtCo, Apr. 5, 2023, thoughtco.com/what-are-abrasive-minerals-1439101. Alden, Andrew. (2023, April 5). Abrasive Minerals. Retrieved from https://www.thoughtco.com/what-are-abrasive-minerals-1439101 Alden, Andrew. "Abrasive Minerals." ThoughtCo. https://www.thoughtco.com/what-are-abrasive-minerals-1439101 (accessed June 4, 2023).