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About Pegmatites

via The Mountain Gem & Lithosphere

What is pegmatite?

It is an igneous rock variety with extremely course grain size. To elaborate, a pegmatite has the same base constituents as granite (quartz, feldspar, mica) except the crystals are larger in size. In basic granite, the rock forming minerals usually crystallize in sizes between 0.4 and 1 inch. In pegmatites, the minerals can crystallize into larger sizes. It is not uncommon to find crystals over a meter in length in larger pegmatites. Large pegmatites may extend from 5 to 100 feet thick and 100 to 1000 feet in length. Crystals can grow to tremendous sizes, such as quartz crystals 17 feet long and 8 feet in diameter, orthoclase crystals 33 feet by 33 feet, beryl 19 or more feet in length, tourmaline crystals 10 feet long and mica sheets with up to 68 square feet of surface area.

Pegmatites begin as a concentrated residual rock, rich in water, chlorine, boron and other elements deep within the earth under tremendous pressure. As the surrounding rock begins to solidify, the minerals become more concentrated. Eventually the concentrated material cools off and if the cooling is slow enough, large crystals begin to form. After eons of uplift and erosion, the pegmatites are exposed to the surface. Pegmatites are basically dike shaped bodies but because of their form they are better thought of as veins. The shape of pegmatite is influenced by the type of rock that surrounds it. Pegmatites may be spherical, bowed or curved, pipe-like, tear shaped or irregularly branched. Most often they are lens shaped or table like.

Pegmatites can be classified as either simple of complex. Simple pegamtites are those that contain the common minerals of quartz, potassium feldspars (orthoclase) and mica (muscovite/biotite). Complex pegmatites result from the crystallization of the last gaseous portion of magma which contains a higher concentration of rarer elements. Naturally, the complex pegmatites contain a greater variety of minerals, including rarer ones, and some extremely beautiful specimens can occur. A smokey quartz crystal sitting on a white microcline base can be the prize specimen of someone's collection. In the Pikes Peak region, the feldspar may be the variety amazonite, a beautiful bluish-green microcline valued by man.

Large pegmatites usually have three or four basic distinct zones with different assemblages of minerals in each zone. The zones are:

  1. the outermost zone
  2. the wall zone
  3. the intermediate zone-which may be absent in some pegmatites
  4. the core zone

Most of the pegmatites in Colorado seem to have graphic granite surrounding a cavity associated with igneous rocks, but some are found in sedimentary and metamorphic. There are three types of crystal seams or pockets found in pegmatites:

  1. in situ - open cavities
  2. collapsed cavities or pockets, and
  3. float pockets.

The majority of pockets found in Colorado pegmaties are collapsed variety and are filled with red maroon or brown clay-like soil. Indications of a pegmatite pocket nearby would be crystals or pieces of quartz, feldspar or mica. Sometimes a depression or roll on the surface of the ground might indicate a pegmatite. A change in soil color to red, maroon or brown might indicate a pegmatite nearby.

The San Francisco Gem & Mineral Society, Inc.

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San Francisco, CA 94122

415-564-4230

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