These two examples show the shape of individual quartz crystals.
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Quartz is the second most common mineral in Earth’s crust. It is a member of the quartz group, which includes less common minerals such as opal, crystobalite, and coesite.
Silica (Si) and Oxygen (O) are the only elements within pure quartz. If a cooling magma has silica left after feldspars have formed, quartz is likely to form. Because of its low density, even lower than many feldspars, quartz is also like to float in the magma and accumulate in surface rocks.
Quartz can be found in all sorts of rocks. Intrusive igneous rocks sometimes contain large quartz crystals. Metamorphic rocks such as gneiss also have large quartz crystals. Sedimentary rocks such as sandstone are often made of tons of little pieces of quartz crystals. In fact, most sand is made of quartz because it is hard and does not weather away easily. Some pieces of quartz are white like milk but most are clear like glass, sometimes with a little pink or grey tinge of color.
- Shape: Trigonal (Perfect crystals are usually 6-sided prisms with a pyramid shape at the end. However, it is much more common to find many crystals that have grown in a mass or broken crystals.)
- Luster: Glassy or greasy
- Color: Colorless or white. Some varieties are pink or smoky.
- Streak: White
- Hardness: 7 on Mohs Hardness Scale
- Cleavage: None
- Fracture: Conchoidal
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The Fall 2010 issue of The Earth Scientist
, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!
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