Pegmatites, the Home of Gemstones.

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The most common gemstones found in pegmatites are beryl, garnet, topaz, tourmaline.

Over a hundred other gemstones are also found in them, including:
albite feldspar, amazonite feldspar, apatite, aquamarine beryl, brazilionite, cassiterite (tin), chrysoberyl, corundum, danburite, elbaite tourmaline, euclase, fluoapatite, goshenite beryl, heliodor beryl, hiddenite spodumene, indicolite tourmaline, kunzite beryl, labradorite feldspar, lazulite, lepidolite mica, morganite beryl, quartz, rhodozite , rubelite tourmaline, ruby, sanadine feldspar, sapphire, spessertine garnet, spodumine, topaz, verdelite tourmaline, and zircon.

Actually “pegmatite” is more of a textural term, but they are well worth looking for along mineral belts.

The single feature that is diagnostic of all pegmatites is their large size crystals.

That is moreover the main evidence that they are formed by volatiles, also evident in their mineral compositions as in:
mica - water, tourmaline - boron, calcite - carbon dioxide, chloro-apatite - chlorine, fluoro-apatite and topaz - fluorine.

Pegmatites are found worldwide within the major cratons and usually within greenschist metamorphic belts. Within the metamorphic belts, pegmatites tend to concentrate around granitic bodies in zones of extension and low strain.
Some skarns associated with granites also tend to host pegmatites.



Most pegmatites are composed primarily of quartz, feldspar and mica, having a basic composition similar to granite. “Graphic texture” with feldspar and quartz intergrown is common, but pegmatites also occur in neptheline syenite (Syenite pegmatites are quartz depleted and contain large feldspathoid - silica-poor variants of the feldspar group - crystals instead) and even in some gabbros.
When a pegmatite dike is found in association with granitic plutons it is likely that it will have a different trace element composition with greater enrichment in large-ion lithophile (incompatible) elements such as boron, beryllium, aluminum, potassium and lithium, uranium, thorium, cesium, or some mixture of rare earths.

Pegmatite bodies are usually of minor size compared to typical intrusive rock bodies, body size is only on the order of one to a few hundred meters, but there may be many bodies in an area.
Compared to typical igneous rocks they are rather mixed and may show zones with different mineral assemblages. Crystal size and mineral groups are usually oriented parallel to the wall rock or even concentric in pegmatite lenses.






Pegmatites are thought to form predominantly in granites during their final stage of solidification. As crystals are formed, incompatible elements including rare earths are enriched. The final fraction of mineral material is high in water, CO2, and often also in elements such as boron, chlorine, fluorine, or lithium. This fluid is forced to the edge of the granite pluton and forms pockets, vugs, thick veins, lenses, pods, or copulas. The fluid apparently solidifies rapidly at relatively high temperatures, under conditions that favor a few very large crystals rather than many small ones.
Crystal diameters in excess of a meter are not unusual. The largest crystal ever found was in a pegmatite, a spodumene about 14 meters long.

Take a field trip through the Stewart Tourmaline Mine

In the Mine

There are three main theories for pegmatite formation;
1. Metamorphic : pegmatite fluids are created by de-watering of metamorphic rocks, particularly felsic gneiss, to liberate the right constituents and water, at the right temperature
2. Magmatic : pegmatites are formed as exsolved granitic material crystallizes in the surrounding country rocks.
3. Metasomatic : pegmatite, in a few cases, could be explained by the action of hot alteration fluids upon a rock mass, with bulk chemical and textural change.

There is a fourth, that late stage magma pulses drive the remaining fluids through the crystallizing mass, collecting and enriching incompatible minerals along the way.

Three Systems of Granitic Pegmatite Formation have been recognized:

I. Low-Pressure Pegmatites
1. Crystal-bearing Formation
a. Fluorite-rock crystal-bearing Subformation
b. Subrare-metal (with precious stones) Subformation
Miarolitic Facies Evolution Sequences: Topaz-Beryl and Tourmaline
2. Rare-metal – Rare-earth Formation
Evolutionary Sequences: Nb-Y, F-Ta-Y and Be-REE
Miarolitic Facies Evolution Sequences: Amazonite

II. Moderate-Pressure Pegmatites
1. Rare-metal Formation
a. Petalite Subformation
Evolutionary Sequences: Be, Li, P-Ta-Li, F-Ta-Li and Cs-Ta-Li
b. Spodumene Subformation
Evolutionary Sequences: Ta-Be, Li, Ta-Sn-Li, P-Ta-Li and Cs-Ta-Li
Miarolitic Facies Evolutionary Sequences for the Formation as a whole: Beryl
(morganite)-Tourmaline, Tourmaline-Kunzite, and Phosphate-Tourmaline

III. High-Pressure Pegmatites
1. Mica-bearing Formation
a. Rare-metal – Muscovite Subformation
Evolutionary Sequences: Columbite-Muscovite and Beryl-Muscovite
Miarolitic Facies Evolutionary Sequence: Beryl-Tourmaline
b. Muscovite Subformation
Evolutionary Sequences: Quartz-Muscovite and A-shape Muscovite
2. Feldspar Formation
Evolutionary Sequences: U-REE and Non-specialized

At present, Cerny’s classification (1991 revision) is the most widely used classification of pegmatites. It uses a combination of depth of emplacement, metamorphic grade and minor element content in 4 main Classes. They are:
1. Abyssal - high pressure, high temperature (6-10 kbar, 700-800 °C)
2. Muscovite 1 - - high pressure, lower temperature, (5-8 kbar 580-650 °C),
Muscovite 2 -- lower pressure, lower temperature(3-7 kb, 540-650 °C)
3. Rare-Element -(low temperature and pressure). (2-4kb, 500-650 °C)
The Rare-Element Classes are subdivided based on composition into LCT and NYF types:
LCT for Lithium, Cesium, and Tantalum enrichment and
NYF for Niobium, Yttrium, and Fluorine enrichment.
The Rare-Element Class is further subdivided into types and subtypes according to various mineralogical / geochemical characteristics.
4. Miarolitic - (shallow level, low pressure). (1-1.5 kbar)

As they are dependent on whatever elements impacts left in the area, I suspect there will be many additions.