Earlham College - Geology 211

Physical Geology 2004

Pegmatites

Pegmatites are coarse grained, generally igneous mineral deposits which result from disparate rates of cooling within a trapped pocket of magma, possibly due to volitiles which remain after the rest of the pluton has crystallized. Pegmatites are difficult economic targets because their erratic location, structure, and content make them an overly difficult resource to mine. Knowledge of pegmatites is limited, and research is proceeding slowly, largely pursued by industries that want to extract saleable resources. The scarcity of pegmatites also makes study difficult, and much of what we think we know is up in the air.They are found worldwide, but are seen most often in mountainous areas where granitic upwellings are not yet eroded away.
Pegamtites can occur in mafic magmas as well as felsic, but are uncommon and commercially unproductive. They range in age from as early as the Archean to the Cenozoic, but the majority are from the Precambrian, which ended some 545 million years ago.

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There are two main types of pegmatites, zoned and unzoned. The unzoned type is far less complex and is of lesser economic significance because they contain only quartz, feldspar, and mica crystals.Often unzoned pegmatites are created by a pocket of partial melting during metamorphism.. Unzoned pegmatites are generally formed through replacement or recrystallization of the wall rock rather than crystallization out of a fluid and therefore lack the diversity of minerals or the complexity of structure seen in zoned pegmatites. Because unzoned formation usually depends on partial melting due to metamorphism, it generally occurs very deep, where pressure and temperature are greater. Unzoned pegmatites are generally less influenced by hydrothermal factors than the zoned are, which explains in part the lesser mobility of ions which inhibits their ability to crystallize into distinct zones.

The zoned pegmatites have distinct layers which surround a central core, and have a consistent pattern of composition and relative crystal size within each layer. The image above shows a dike shaped pegmatite, which is a common form, with distinct zoning. The outer wall is made of fine grained quartz mixed with other predominant minerals present in the parent material. Inside that is a wall zone which has more coarse grained feldspars and quartz. There are then any number of intermediate zones which may contain many less common minerals such as beryl, zinnwaldite, or pollucite, to name a few examples of the many types of minerals found in the intermediate zones. The unifying factor that causes these minerals to crystallize in a pegmatite is that they all have a crystal radius which is too large to fit in with the parent pluton, and often have an ionic charge which prevents bonding. In the center of the pegmatite is the core, which consists of the biggest crystals, sometimes kilometers in length, often made up of quartz. The thickness of the dike or sill of pegmatite material may fluctuate from a few inches to meters thick, and in the places where the pegmatite swells, there are often open cavities. In these cavities is where the most well formed and large crystals are found, and can be of spectacular quality. The largest crystal yet found in a pegmatite was found in South Dakota, and was 42 feet long.

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Mine of Alakamisy Itenina, Madagascar
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There are two stages to zoning: primary and secondary. The primary results from differentiation resulting from the reactivity series. The secondary results from replacement by hydrothermal activity. Oftentimes, especially in very old pegmatites, zoning will be obscured by hydrothermal replacement of certain minerals with other minerals dissolved in water. The water dissolves minerals in the pegmatite and deposits the ions it was carrying in their place, which is how the hematite finds its way into the surrounding soils, turning it red as seen in the picture to the right. This coloration is characteristic of soil surrounding a pegmatite and can be used to locate them. Hydrothermal ions sometimes are the source of material which forms core crystals over time.

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Today, pegmatic minerals are used for a wide range of applications such as lithium batteries, jet engines, light emitting diodes, nuclear engineering, medical applications, and prosthetic devices. Besides the rare elements, the micas, feldspar and quartz are used for ceramics and insulation purposes. Though there are alternate sources, in some cases, pegmatites are the easiest source of the rarer elements used in high tech industries. It is almost impossible to predict the number of undiscovered pegmatites remaining, but the time involved for development effectively makes them a non-renewable resource. Pegmatites are an extremely valuable resource to jewelry makers or gemstone collecters, because such rare and precious gems can be found. Many pegmatites today are mined exclusively for the gems they can produce.

Sources
Link to other Student Webpages for 2004 Earlham Physical Geology

This website was prepared as an assignment for Geosciences 211 (Physical Geology) taught in the spring of 2004 at Earlham College, Richmond, Indiana.

Earlham College Geosciences Department Earlham Geosciences 211: Physical Geology