Geology And Geophysics

Mining a Pegmatite



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Pegmatites come in two flavors simple and complex, or dirty. The complex pegmatites are the source of many rare minerals used both as gemstones, and rare metal ores. Pegmatites being relatively small in comparison to other mineral deposits lend themselves to small mining operations. Some of them can even be mined by one man, although I can't see why one man would want to work one of them. It takes a certain amount of patience to mine one of them.

Pegmatites are an igneous rock similar in chemical composition to granite the only real difference is expressed in size of crystals. Some of the pegmatite crystals can be absolutely monstrous. There is a report of a pegmatite quarry in the Urals of Russia that occupies a single crystal of feldspar. Some of the other common minerals found in a simple pegmatite are quartz, feldspar and muscovite mica. It is when a pegmatite is complex that it becomes interesting, and the more complex it is the more interesting it becomes.

Although a pegmatite shows itself commonly as a light colored vein in a darker rock it is really tabular of roughly oval in shape. Most pegmatites are also zoned in structure with their center usually composed of quartz, which is surrounded by a layer of quartz and feldspar. Even further out the next layer is usually feldspar and muscovite. There are pegmatites where as many as seven different layers of distinctive rock types have been observed. Although all pegmatites have large crystals a similar type of deposit called an aplite is found often alongside pegmatites. Aplites are composed of very fine crystals, and usually are of no interest to miners.

It is believed by geologists that pegmatites are small intrusions of granitic material injected into another rock type usually a metamorphic rock. They feel that these are usually what is left over from a larger intrusion of granite that hasn't been unroofed. An unroofed deposit is one that hasn't been exposed at the surface by weathering.

Mining a pegmatite is like butchering a hog these is a demand for practically everything you find. The most important mineral is feldspar that has many uses un both industry as a mild abrasive, and the home as a cleaning product such as scrubbing powder. Mica is probably the most important. In the early days of electronics it was extensively used because of its high dielectric strength. As suck it was used in condensers and radio tubes. Today its most extensive use is as scrap mica that is ground extremely fine as a filler for many products and coatings.

Dirty pegmatites are also the home of several other rare minerals, and gemstones of the beryl, topaz and quartz families. In rare occurrences they may also contain sapphires. One of the most valuable ores found in pegmatites is tantalite the ore for the metal tantalum. Although rare tantalite sometimes tantalite can be found in large masses, and can be worth more then $100 per pound. A road contractor in Maine had to blast through a pegmatite and discovered a crystal of columbite/tantalite the size of a locomotive boiler. This single crystal paid for the road job, and the amount of money he received from the state was pure profit.

In practice mining a pegmatite is simply a blast and dig proposition. Special care must be taken with any potential gemstones however. The finest ones are usually found in cavities in the rock called "Vugs." A vug is a cavity left in the rock caused by just not enough to fill it in when the pegmatite is formed. They are often the home of the most spectacular crystals one will find. A vug in Newry, Maine yielded over eight million dollars of tourmaline and other gemstones in the late 1970's. It is not unusual to find vugs in pegmatites, but this particular vug was huge; the size of your bedroom completely lined with crystals. There were even doubly terminated crystals laying loose on the floor of the cavity.

Other then gemstones for the most part today pegmatites are passed through a series of crushers and reduced to the size of sand. They are then put through a process known as flotation that separates the minerals present by a series of chemical processes. The heavy components are removed first by simple gravity. The quartz is the next to go; this is first etched by hydrofluoric acid then the particles are passed into a flotation cell filled with a chemical reagent that will cause the quarts grains to float out the top of the cell when air is bubbled up through the cell. A similar reaction in a flotation cell removes the mica from the feldspar in a similar way. All that is left then is the feldspar.

Each mineral found in a pegmatite has a series of special uses, and the miner bags up the dried minerals, and sells them to various industries for further processing into finished products,

 

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