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Saturday, April 30, 2011

Finding Gemstones and Minerals in a Pegmatite

The inside of a pegmatite mine, Ruggles Mine in New Hampshire
Photo by Squelle

Pegmatites are mined not so much for their exotic minerals, but the more mundane feldspar, mica and quartz. The exotic minerals that include gemstones are usually a byproduct of this mining. This is true of most pegmatite mines except for a few like the Maryall mine in Connecticut, and the Himalaya mine in San Diego County California. Because pegmatite it is a very hard rock it is almost impossible to mine without the use of blasting is for this reason the average rockhound is usually excluded. However, from prior mining activities it is possible to go over the mine tailings looking for specimens.

Most miners are more interested in the three primary minerals of feldspar, mica and quartz than they are in the more exotic minerals; therefore these minerals are often thrown onto the waste pile where they remain for the rockhounds to collect later. It is among these tailings where most of the gemstones are found.

By working tailing heaps you are working with stone that has already been mined, and broken up making for much easier collecting. Most of the time you'll encounter stones that are not much more than a foot in diameter, and many of them are usually smaller. You will occasionally encounter a larger stone where it is extremely handy to have in your collection of tools a fairly large sledgehammer weighing more than 8 pounds. Most collecting on a tailings heap can be done with a sledgehammer that weighs 8 pounds, and you can do a considerable amount of damage with an eight pound hammer.

For most collecting in a pegmatite one of the most useless things is a regular geologist’s pick because most of them are too light. What is far more effective is a 4 pound engineer’s hammer and a bullpoint. Bullpoints are a hardened steel rod about three quarters of an inch in diameter having a hexagonal shape about a foot long and coming to a hardened point on one end..

Some of the other things you need in your tool kit for collecting specimens in a pegmatite is a good jeweler's loup is at least 8X magnification on a chain so you can wear around your neck. A stone chisel is another device that should be included. Your kit should also include some old newspapers to wrap your specimens. A small pad and pencil to write down descriptions of the specimens you have found. You should also leave a number on the newspaper holding the specimen so you'll know what it is when you get home.

Friday, April 29, 2011

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.