How a Pegmatite Works

A pegmatite vein in a roadcut.
Photo by Woodlouper

The next several articles that are going to be published in this blog concern a very special type of rock formation that is called pegmatites. Another name that is sometimes used for this rock is giant granite because the large sized crystals of different minerals that are found in them. The most common minerals in a pegmatite are feldspar, mica and quartz with quartz in many pegmatites making up almost 50% of the total minerals present.

Although they are described as veins of pegmatite most of them are tabular shape masses of rock that are injected as a granitic magma into pre-existing cracks in the country rock, or sometimes they also form large ovals that are more or less deformed by earth movements that occur at the same time they are being deposited deep in the earth.

A pegmatite mine.
Photo by Pfuss

The crystals are found in a pegmatite are the result of very slow cooling that occurs in the depths of the earth's crust several miles beneath the surface. The pegmatites that are now exposed at the surface have all had several miles of rock eroded away so we can now see them at the surface.

Although most pegmatites are at home in gneiss or schist some of them are actually found in granite intrusions that have taken a long time to cool so that the pegmatite grades from granite. That only pertains to some of pegmatites, it is theorized that changes in the chemical composition can cause some pegmatites to be formed in a very short time; sometimes a matter of days and still have large sized crystals.

Closeup of a pegmatite
Photo by Mark A. Wilson

Unless a pegmatite is within a granitic intrusion they are all outliers of such an intrusion. The further a pegmatite is from the parent intrusion the more likely it is to have rare minerals in its composition. There are three main types of pegmatites of granitic composition and the fourth that is found in more mafic rocks such as gabbro. The further they are from the granitic or gabbro like intrusion the more apt they are to contain the proper chemical ingredients to form gemstones.

There are generally three major minerals and under the right conditions can form gemstones, they are beryl, topaz and tourmaline. There are also more than 100 other minerals that under the right conditions can also form gemstones. Sometimes even the body of the pegmatite itself is code is cabochons if it is especially attractive. Graphic granite that is found in some pegmatite makes especially attractive 

cabochons.

The rare mineral "bazzite" found in a pegmatite cavity. This is an analog of beryl containing the rare earth element scandium.
Photo by Rob Lavinsky 

Many pegmatites display different zones that are apt to contain different suites of minerals. The outer portion of a pegmatite displays very fine crystals and is often termed an aplite. There are several different shells that a pegmatite can display numbering up to seven. The inner core of all zoned pegmatites is usually quartz and in the inner core is where you usually find cavities that are filled with flawless crystals of various gemstones. Although it is unusual there are many instances where a single large cavity can produce several million dollars worth of gemstones.

Hunting Gems in a Pegmatite

A pegmatite is the host rock for many gemstones that are not found any where else along with a host of other rare minerals.  The stone itself is an intrusive igneous rock that chemically is the same as granite in fact one of its nicknames is “Giant Granite” because of its large sized crystals.  Most pegmatites are what are termed simple pegmatites composed of quarts, feldspar and mica that are composed of grains that exceed 2.5 centimeters in size. There are however some other classes of pegmatites that are known as “Dirty Pegmatites” that contain a host of other minerals contaminating them with one of then being the lithium minerals lepidolite, a form of lithium mica that is purple, and spodumene, lithium silicate.  It is in these pegmatites where most gemstones are found.

An "X" shaped cross of pegmatite in a darker colored gneiss.
Photo by Arlette 1

Most gemstones in a pegmatite are found in cavities that are formed inside the pegmatite where there is not a sufficient amount of material to completely close the inside of the vain leading an open space where clean crystals are formed. There are also other areas within the pegmatite where it is possible for gemstones to form the subsequent deposits usually cause these particular stones to be imperfect.

A set of rubbelite crystals from the Pala District of San Diego County, California.
Photo by Rob Lavinsky

Most of these pockets or vugs are found in association a particular rock texture called graphic granite where the vug is found to beneath a layer of graphic granite where the individual crystals displayed keep getting larger. Graphic granite resembles cuneiform writing like that used in ancient Babylon.

There are many different gemstones that are associated with pegmatites including: amblygonite, beryl, corundum, fluorite, topaz and tourmaline among others. Pegmatites are the home of many other exotic minerals that are not classified as gemstones, however some of these minerals are used for ores for lesser-known metals like beryllium, niobium and tantalum. 

Beryl Crystals on dolomite.
Photo by Rob Lavinsky

Several years ago a roadbuilding contractor was working in Oxford County Maine when in the course of blasting a rough cut discovered a crystal of tantalite weighing several tons. Rather than just dumping it with the rest of the rock he sold the tantalite for a not-for-profit that it paid for the entire roadbuilding project. He simply pocketed the profit.

A crystal of tantalite in a pegmatite matrix.
Photo by Rob Lavinsky

Pegmatites are found all over the world wherever you have crystalline rocks. Some of the notable pigmented to the United States are found in the Appalachian Mountains, the Black Hills, the Rocky Mountains into the Pala district in San Diego County, California.