| MadSci Network: Earth Sciences |
A typical granite is composed of several minerals...
All granites have some quartz which usually appear as smoky-colored glassy grains, potassium feldspar which is often pinkish, sodium plagioclase feldspar which is often white-colored, and the dark-colored ferromagnesian minerals - minerals containing iron (Fe) and magnesium (Mg) - that you're wondering about.
What are some common ferromagnesian minerals in granite? They are typically one or more of the following minerals - biotite mica, the amphibole mineral hornblende, or a pyroxene.
Biotite mica is easy to identify because micas split into very thin sheets. If you can peel off thin flexible flakes of the black mineral with your fingernail or the tip of a knife, then it's probably biotite.
Amphiboles and pyroxenes are more difficult to identify and you have to look for cleavage. Cleavage is how a mineral breaks. In general, if the black minerals look blocky and square-like, then they're probably pyroxenes. If they look needle-like or diamond-shaped in cross-section, then they're probably amphiboles (most likely the amphibole mineral hornblende).
Visit Rob's Granite Page for more information about granites on the world-wide web.
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