ABRASIVE MINERALS - Sanding was originally done with 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. 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.

..................................................................................................................................

Abrasive Minerals

by Andrew Alden 

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.

Andrew Alden

Professional geologist, writer, photographer, and geological tour guide

Thirty-seven years of experience writing about geological subjects

Six years as a research guide with U.S. Geological Survey (USGS)

Member of Geological Society of America (GSA), and American Geophysical Union (AGU)

Experience

Andrew Alden is a former writer for ThoughtCo who contributed hundreds of articles for more than 17 years. Andrew works as a geologist, writer, editor, and photographer. He has written on geological subjects since 1981 and participates actively in his field. For example, Andrew spent six years as a research guide with the U.S. Geological Survey, leading excursions on both land land and at sea. And since 1992, he has hosted the earthquakes conference for the online discussion platform, The Well, which began as a dialogue between the writers and readers of the Whole Earth Review. 

In addition, Andrew is a longtime member of the member of the Geological Society of America — an international society that serves members in academia, government, and industry; and the American Geophysical Union — a community of earth and space scientists that advances the power of science to ensure a sustainable future.

Andrew lives in Oakland, California; and though he writes about the whole planet and beyond, Andrew finds his own city full of interest too and blogs about its geology. 

Education

Andrew Alden holds a bachelor's (B.A.) degree in Earth Science from the University of New Hampshire, College of Engineering and Physical Sciences, in Durham, N.H.

Awards and Publications

Andrew Alden on Earthquakes ​(The Well Group, Inc., 2011)

Assessment of River — Floodplain Aquifer Interactions (Environmental and Engineering Geoscience, 1997)

Andrew Alden on Hosting (The Well Group, Inc., 1995)

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/what-are-abrasive-minerals-1439101