Pegmatites are intrusive or plutonic rocks with unusually large mineral crystals. Typical grains are > 1cm (0.4 inches). Some are several feet to tens of feet and weigh tons. However, common sizes are 8-10 cm (3-4 inches).
They form from the water-rich last portion or residual magma melt to crystallize. These melts are high in other volatiles. Furthermore, some can have elements that are uncommon in igneous rocks.
Most pegmatites have a granite-like composition. However, nepheline syenite pegmatites are common in alkaline igneous complexes. Also, diorite and gabbro occur but are less often.
Usually, when loosely used without a qualifying rock name, pegmatite refers to granite pegmatite.
However, strictly speaking, pegmatite is a textural or fabric term, not a compositional or rock name. It refers to rocks with exceptionally coarse grains. Rocks with this texture are said to be pegmatitic.
Therefore, when naming these rocks, always start with the rock name, such as granite, gabbro, diorite, or nepheline syenite. It will give clarity.
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Where do they occur?
Pegmatites mostly form lenticular bodies (lenses) and sometimes as dikes, veins, or pods within and around the margins of large plutons like stocks and batholiths. They are most common towards the upper roof.
However, they can extend into country rocks as irregular dikes or sills.
Usually, they are genetically related in space and time to the batholiths and socks they are associated with.
Pegmatite composition
Most pegmatites have a granite-like composition with mostly quartz, feldspars, and muscovite (mica). Also, they have lesser biotite, phlogopite, tourmaline, apatite, garnet, and Fe-Ti oxides. These are the simple pegmatites.
On the other hand, occasionally, some pegmatites are enriched with rare elements like fluorine, lithium, beryllium, boron, cesium, tantalum, tungsten, niobium, tin, thorium, rubidium, uranium, and rare earth elements (REE).
Also, they may have scandium, Yttrium, wolframite, zirconium, gold, silver, zinc, chlorine, phosphorus, sulfur, and copper. These are often called complex pegmatites.
Complex pegmatites often have valuable minerals like beryl, spodumene, topaz, pyrochlore, cassiterite, fluorite, amblygonite, lepidolite, columbite, monazite, and molybdenite.
Description and characteristics
Pegmatites are unusually coarse-grained rocks that occur as small masses ranging from a meter to several hundreds of meters. They are often white, gray, cream, silvery, and reddish brown.
The outer margin is compositionally like surrounding rocks and has no chilled margins with fine grains. Instead, there is an abrupt increase in crystal size.
Some of the characteristics of pegmatites include
1. Pegmatites grow largest crystals
Pegmatite crystals are >1cm, most 8-10 cm, and some tens of feet in size. Some of the notably large crystals include:
- A giant microcline crystal measuring 49.4 × 36 × 13.7 m weighs about 15,000 tons. It came from Devils Hole Beryl Mine, Colorado, USA
- Phlogopite plate measuring 10 by 4.3 meters from Ontario, Canada
- The hugest spodumene crystal is over 14.3 meters long from Etta Mine in South Dakota, USA.
- The largest beryl crystal, measuring 18 m long, with a crosscut of 3.5m and weighing 380 tons, is from Malakialina in Madagascar.
- 2.7 m tourmaline crystals from Black Hills, South Dakota.
2. Crystals grow inward
Large crystal nucleates on the pegmatite wall and grow inward. Therefore, elongated ones lie perpendicular to the contact.
As they grow inward, they become larger, developing a cone shape, a characteristic of alkali feldspar. Also, some branches grow and thicken or flare inward, giving rise to a comb layering fabric.
In some younger, inner minerals can replace or grow between older ones. However, the reverse doesn’t happen.
3. Pegmatites show zonation
Pegmatites show mineral and texture zonation, especially the large dikes and stocks.
For instance, crystals grow larger inward. Also, potassium-bearing minerals may concentrate on the top, while their core has quartz and some unusual minerals.
4. Graphic granite texture is common
As crystals grow inward from their margins, quartz and feldspar (microcline) can intimately intergrow, creating a granite graphic texture.
This texture resembles ancient cuneiform writing and has large feldspar with quartz inclusions.
5. can have single mineral zones
Pegmatites may have single mineral zones, especially at their centers. For instance, pure quartz forms euhedral crystals into the remaining miarolitic cavities at the center.
6. They lack feeders
They don’t have feeders like most igneous bodies. Instead, they occur in lenses and pods without a root.
7. They are never porphyritic
They never show porphyritic texture. This texture has large crystals called phenocrysts in a finer-grained matrix or groundmass.
Although they can have varying crystal sizes, there is no single population you could regard as phenocrysts.
8. Can have xenoliths
Xenoliths, i.e., foreign rock pieces trapped in igneous bodies, can occur in pegmatites.
However, quartz or alkali feldspar usually replaces their original minerals. This makes it hard to distinguish xenoliths.
9. Often have aplite
Pegmatites often have aplite, a fine-grained rock. It can cut across or appear in irregular zones or patches within pegmatites, not just on their contacts.
Most show deformation evidence and grains are oriented perpendicular to the walls of pegmatites.
Aplite seems to have grown from a static environment, and its smaller grain size isn’t from quick quenching rates.
10. Don’t show magma flow evidence
Crystals like tourmaline can elongate, but they aren’t aligned to show evidence of magma flow.
11. May have cavities at their core
Their core may have cavities or vugs. These voids have some of the best-formed minerals. Others may show a druzy texture. The druzy texture is characterized by tiny, projecting mineral crystals.
How do pegmatites form?
Pegmatites form from water-rich, and the last dregs of melt are rich in water to crystallize in the plutons. These melts are also unusually high in volatiles like carbon dioxide, chlorine, and fluorine and incompatible elements.
These volatiles and incompatible elements aren’t incorporated into most rock-forming mineral structures. Thus, they will progressively concentrate in the last portion of residual aqueous melt that separates from mostly crystallized magma mushes.
The high incompatible element concentration proves that pegmatites form from the last melt portions to crystallize.
For such large crystals to grow, crystal nucleation must be low. Nucleation is the process that leads to the formation of an embryo upon which crystals can grow. Low nucleation will give space for crystals to grow large unimpeded.
Also, diffusion rates must be high to allow quick movement of ions, molecules, or atoms to crystal growth sides.
Usually, the high water, other volatiles, boron, and phosphorus depolymerize or break long silica chains. Also, it lowers viscosity.
Depolymerization delays nucleation, i.e., prevents feldspar or quartz from forming nuclei until the melt has a high degree of super-saturation.
When nucleation eventually happens, crystals grow quickly since low viscosity allows elements to move fast. Thus, large crystals in pegmatites form.
In extreme cases, a phase separation can happen in highly water-rich melts. This supercritical, hydrous fluid is rich in alkali, completely polymerized silica, and incompatible elements. Usually, incompatible elements will prefer a depolymerized vapor phase.
A vapor phase will favor faster diffusion by magnitude order through the fluid phase, forming large crystals.
The size of pegmatites depends on the interconnection of the vapor phase. Also, those that form early result in large pegmatites.
Lastly, the vapor phase can replace or alter early-formed minerals. This forms unusual assemblages in plutons that don’t occur when magma crystallizes.
How crystallization happens
The formation of pegmatites is directly connected to larger crystallizing associated plutons.
It all starts when water-rich melts, or fluid phases segregate from the bulk crystallizing magma. These will form lenses, veins, or pods inside magma, squeeze to pluton margin, or flow into surrounding country rock, forming sills and dikes.
Usually, pegmatites crystallize from the outer edges inward. As this happens, volatiles, other fluxing elements, and incompatible elements will progressively concentrate inward, too.
The inward crystallization makes crystals progressively larger as viscosity decreases and diffusion rates increase. Also, it will cause mineral segregation.
During pegmatite crystallization, potassium feldspar, plagioclase, quartz, and mica can simultaneously form at the edge.
As you move inward, only potassium and feldspar crystallize. Sometimes, undercooling can result in quartz and feldspar intergrowth forming graphic granite texture.
At the center, only quartz will crystallize. At this center, layers from all sides meet, and the largest crystals grow. Also, volatiles, fluxing elements, and incompatible elements are highest. Thus, some will form exotic minerals.
Lastly, water concentration becomes too high at the core, forming bubbles. Gem-filled miarolitic cavities may form if pressure isn’t so high, such as in shallow depths.
How do aplite in pegmatites form?
Pegmatites often have fine-grained aplite rocks. They form because water and elements like phosphorus, beryllium, boron, rubidium, and cesium act as flux components, i.e., reducing melting point. This causes supercooling that hinders nucleation.
If some water escapes from the residual melt, solidus temperature will rise. This will cause subcooling that will trigger sudden nucleation, forming fine-grained aplite rock.
Solidus is the temperature below which all the melt will be solid, while subcooling is when the temperature is below the normal melting point.
Similarly, a rapture in fluid causes a pressure-lowering melting point. This will also cause rapid crystal nucleation.
Why are pegmatites important?
Pegmatites host economically valuable deposits like lithium, beryllium, wolframite, cesium, tungsten, tantalum, tin, thorium, niobium, rubidium, boron and fluorine, and REE and other metals.
Also, they are a source of industrial minerals like feldspar, quartz, and mica sheets called books.
That is not all. Compared to other rock types, pegmatites produce well-formed, largest, and most beautiful gem-quality crystals. Such occur at the core or center.
These gemstones include beryl (aquamarine, morganite, heliodor, and goshenite), chrysoberyl, spodumene, topaz, zircon, cryolite, and brazilianite.
Others are quartz gemstones like smoky quartz, morion, rose quartz, and alkali or potassium feldspar minerals like sunstone, moonstone, and amazonite.
Frequently Asked Questions?
No. Some late-stage become unusually enriched with copper and iron. An example is Kiruna, Sweden, with 60% magnetite solidified to form one of the largest iron deposits in the world.
No. The large crystals don’t form from very slow cooling histories but from a fluid-rich environment that favors quick crystal growth and low nucleation.
Pegmatites are small bodies that cool at the same rate as surrounding plutons, as they cannot retain heat for long. Sometimes, they cool faster for those that form as veins in country rocks. Therefore, large crystals aren’t from slower cooling. Most only take a few weeks to months to crystallize.
They occur in several states, including New England, California, Montana, Colorado, Arizona, South Dakota, etc. Black Hills district of South Dakota has about 24,000 pegmatite bodies in over 700 km2 area.
Minas Gerais in Brazil, Madagascar, DR Congo, Ural in Russia, Sri Lanka, Kenya, Namibia, India, Afghanistan, Zimbabwe, Tanzania, Nigeria, USA (California, New England and Colorado), India, Pakistan, etc.
No. Some, the so-called abyssal pegmatites, form from a low degree of partial melting during high-grade metamorphism. They occur in migmatites, which are partially melted gneiss rocks.