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Andrew Alden

The Rock-Forming Minerals

By April 3, 2009

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Geologists often refer to the rock-forming minerals—they're minerals so common and widespread that they predominate in most rocks. Almost every rock has at least one of them, and the very definitions of many rock types depend on them.

olivineThe usual lists of rock-forming minerals contain anywhere from seven to eleven names. The seven-name version is amphibole, carbonate, feldspar, mica, olivine, pyroxene and quartz. Some of you are ready to argue, "Hey, most of those are mineral groups, not individual minerals." That's true. But at a glance, in the field, those seven are the names you assign to a rock's ingredients. The more you learn about rocks, the more you unpack each of the seven names until at the professional level they amount to maybe 20 minerals. For my new gallery of the rock-forming minerals, I've unpacked "mica" into biotite and muscovite and "carbonate" into calcite and dolomite. To go beyond those, you need to become a bit of a rockhound, tackling feldspar, amphibole and pyroxene in that order.

There's one more mineral that ought to be listed but never is: clay.

Olivine — Geology Guide photo


August 17, 2012 at 7:02 pm
(1) Just Starting says:

Very helpful, thanks!

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