- What is a pegmatite?
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- A pegmatite is a coarse grained igneous rock (formed deep within the earth) having a grain size of 3cm or more. Mostly pegmatites are granitic in origin, that is they are composed of granite and its constituents like quartz, feldspar and mica. In addition to these basic minerals there are also generally rare earth elements. Pegmatites are known to contain aquamarine, tourmaline, beryl, topaz, cassiterite, fluorite, apatite, tin and tungsten plus a host of other minerals. Minerals formed in pegmatites literally had the time to enjoy the benefits of a slow crystallization process from a rich chemical stew. One of the most common indicators of a potential pegmatite is the presence of an aplite dike. The pegmatite bodies occur as lenses within the aplite dike structures. Where other minerals such as garnet occur they are the result of a secondary metamorphic process, i.e. one where temperature and pressure have altered the original deposit. Pegmatites are most famous for their large, high quality mineral specimens. The first gem tourmaline discovered in California goes back to 1872 when Henry Hamilton found and recognized the mineral during a collecting trip to the southeast slope of Thomas Mountain in Riverside County. This fact was unrecorded for several years since the occurrence was not made public even to the California State Mining Bureau. The major pegmatite districts of California are the Mesa Grande District, Pala District, Ramona District, Coahuilla District, Rincon District, Ramona District, and Jacumba District. To find out more specifics about what was found in these areas to start a collection or to possibly collect you might want to sign up for the Minerals of California Online Database. Click here to sign up.
In tourmaline mines when finding pockets there are some key indicators that will aid you in the event that you ever get the opportunity to collect inside. Generally speaking the country rock does not contain tourmaline or the other minerals one typically seeks. there is a relatively lesser known phenomenon known as a chimney. These chimneys are effectively veins of fluids that contain all the require elements to form albite, feldspar, lepidolite, mica, montmorrillonite, stibiotantalite, purpurite, spessartite, tourmaline, etc. the chimneys had pulsating fluids that formed small to large pods bounded by an external shell composed of albite and schorl crystals and garnet crystals; going inward the next mineralization encountered is lepidolite and purple montmorrillonite clay in which gemmy fragments known as nodules may be found; further in are the better crystals including tourmaline and quartz.
In typical quartz/feldspar pegmatites one of the key indicators is to look for float crystal shards with good euhedral crystal faces. Trace them up to the source and start digging to find the hidden crystals. Another indicator is when you are collecting at deposits with large quartz seams look for any plants or vegetation growing out of the rock. Generally vugs and cavities fill with water and allow plants to survive and thrive in their own little water supply especially if some earth has entered the cavity upon collapse in the past. You need to verify the geology around you to determine what type if minerals are available to you nearby.
For you die hards there are phosphate granitic pegmatites, boron granitic pegmatites, lithia pegmatites, and finally boron/lithia pegmatites. The composition of each pegmatite determines to a large extent the types of minerals that you can find there. There are three basic theories as to the formation of the formation of granite pegmatites. The first is that they occur in carbonate rich areas, the second that granitic magma intrudes subterranean aquifers and sedimentary deposits resulting in a phreatic reaction, the third theory is ours that granitic magma is basically kept in a "loose" state disallowing any covalent bonding to occur due to the presence on depolymerizers such as fluorine, boron, lithium etc which basically keep the silica slurry from forming until these depolymerizers have been depleted. They basically dont let the solution "stick" together until the deploymerizers are depleted. Crystallization of large quartz crystals usually occurs at 900 degrees C whereas in granitic pegmatites the temperature of crystallization is dropped to 550 degrees C or so resulting in large megascopic crystals as typically seen at the classic mines like the Stewart Lithia Mine in Pala, CA and the Himalaya Mine in Mesa Grande, CA. To see some interesting data on tourmaline go to the Cal Tech Spectroscopy Server by clicking here.
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- Some interesting sites with pictures of minerals from pegmatites......
- Oceanview Mine, Pala, California
- Audrey Lynn Pegmatite, Anza, California
- The Isle of Elba, Italy (Special Contribution)
- San Diego Mine, Mesa Grande, California
- Himalaya Mine, Mesa Grande, California
- Pikes Peak Region, Colorado
- Lolo Pass, Montana/Idaho Border
- Blue Lady Mine, Riverside County, California
- The Little Three Mine, Ramona, California
- Hallelujah Junction, California/Nevada
- Mont St. Hilaire, Quebec, Canada
- Stewart Lithia Mine Field Trip 8/98 ,California
- Minerals of the Stewart Lithia Mine, California
- Coahuilla Mtn Area Pegmatites, Riverside County, California
- Nuevo, Lakeview Mtns, Riverside County (California)
- The Sacatone Springs Pegmatite, San Diego County (California)
- Big Horn Crags, Lemhi County, (Idaho)
- Pegmatite Field Trip in Finland
- Godk”rra Pegmatite, Sweden Field Trip
- Byrud Pegmatite, Norway
- Kaatiala Pegmatite, Finland
- Haapaluoma Pegmatite, Finland
- Crabtree Mine Pegmatite, Spruce Pine, NC Field Trip
- Selsvitberget Pegmatite, Sweden
- Tourmaline Queen Mine, Pala, San Diego County, California
- Dayton Road District, South Glastonbury, Connecticut
- Barringer Hills, Texas Pegmatite
- Orchard and GE Quarries, Oxford County, Maine
- Bikita Pegmatite, Zimbabwe
- The Harding Pegmatite, Dixon, New Mexico
- The Bernic Lake Pegmatite, Manitoba, Canada
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