Pegmatitic texture refers to crystalline plutonic or intrusive igneous rocks with exceptionally large crystals. Rocks of this texture have crystals at least 0.4 inches (1 cm) in diameter, with larger ones more than 1.2 Inches (3 cm) to as large as several feet.
Most pegmatitic textured rocks have feldspar, quartz, mica, and silicic compositions like granites. But can sometimes have rare elements and minerals uncommon in igneous rocks.
These ‘uncommon’ elements and minerals include beryl, emerald, lepidolite, tourmaline, lithium, and spodumene. Others are apatite, tin, columbite-tantalite, and boron, among many others.
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However, some rocks may have a mafic like a gabbro or intermediate composition like diorite. Also, some may be of nepheline syenite that is like granitic but has feldspathoid (nepheline) instead of quartz.
Rocks with pegmatitic texture are said to be pegmatites. Please note that this term doesn’t indicate these rocks’ mineralogy or chemical composition.
To give composition, we use rock type as a prefix, e.g., gabbro pegmatite or granite pegmatite. Also, some people will use rare elements or minerals as a prefix. A good example is beryl or emerald pegmatite. Unfortunately, this way of naming doesn’t tell you the rock composition.
What is the origin of the word pegmatite?
The word pegmatite begs its origin from the Homeric Greek word πήγνυμι (pēgnymi), which means to bind together. It refers to the interlocking feldspar and quartz crystals typical in graphic granite (pegmatitic granite rock) that resemble writings.
René Just Haüy first used pegmatite as a graphic granite synonym in 1822. However, in 1845, Wilhelm Karl Ritter von Haidinger referred to pegmatite rocks with abnormally large crystals.
Characteristics of pegmatite texture
Rocks with a pegmatite texture show some peculiar characteristics. Common ones include the following:
1. They have wholly crystalline, abnormally large crystals
Pegmatitic texture has holocrystalline (wholly crystalline) extremely large mineral crystals. These crystals form from a low-viscosity fluid phase when the last portion of magma melt crystallizes.
Also, these crystals often vary in size. However, some rocks have equigranular (nearly equal grain sizes).
Lastly, the smallest mineral crystals in pegmatitic texture are at least 1 cm in diameter, with larger ones more than 1-1.2 inches (2.5-3cm). Some pegmatites may have giant crystals tens of feet in size. However, most are typically 3 to 4 inches (8-10 cm) in diameter.
2. Pegmatites form some of the largest crystals ever found.
Pegmatites have some of the world’s largest microcline, mica, spodumene, beryl, tourmaline, and quartz crystals. Some of these crystals measure dozens of feet and weigh thousands of tonnes.
Examples of the largest documented crystals from pegmatitic mines include the following:
- Largest feldspar crystals (Karelia, Russia) – This feldspar crystal weighed thousands of tons.
- Largest quartz crystal (Itapore, Goiaz, Brazil) – Measured 20ft by 5.9ft by 5.9ft (6.1m by 1.5m by 1.5m) and weighed 39,916 kilograms
- Largest spodumene crystal (Black Hills, South Dakota, USA) – Measured 42 feet (13m) long and weighed about 33.5 tonnes (37 tons).
- Largest beryl crystal (Malakialina, Madagascar) –weighing 344 tonnes (380 tons), measuring 59ft (18 m) with 11 feet (3.5 m) crosscut.
- Largest Mica crystal (Lacey Mine, Ontario, Canada) – 33 ft by 14ft by 14 ft and weighed 330 tonnes (360 tons)
- Largest beryl crystal in the US (Albany, Maine, USA) – Measured 27 feet long by 6 feet in diameter (8.2m long and 1.8 m diameter).
3. They may have intergrown or interlocking crystals.
These rocks may show graphic, skeletal, or radial intergrown, intertwined, or interlocked crystals. For instance, you may have quartz-feldspar intergrowths that form graphic granite rocks.
Granite rocks with interlocking feldspar and quartz crystals are said to be graphic. Why? Because they resemble writings.
4. Most pegmatitic textured rocks show mineral zoning.
Most pegmatites show a strong internal zonation, mostly symmetric with some asymmetric. Zonation occurs both in mineral crystals (minerals are spatially zoned) and in fabric (rock texture or mineral habits).
For instance, mineral zoning will occur from outside inward, with some dominating certain concentric zones when the hydrous fluid is injected into fractures in country rock.
A good example is that you will have quartz, microcline, and some small amounts of garnet or schorl in the first zone, then albite, tourmaline gem, beryl, spodumene, topaz, lepidolite, apatite, fluorite, etc. The center of the pegmatitic body may have gemstone crystals.
Lastly, the zoning can be simple to complex depending on the minerals in the hydrous or fluid phase.
5. Often have directionally oriented crystals.
Pegmatites may have directionally oriented or dependent crystals (from the margin inward). For instance, various sizes of crystals and constituent minerals may have a parallel orientation to the wall rock (country rock) and some concentric in the case of pegmatitic lenses.
How do pegmatitic rocks form?
Pegmatites are plutonic or intrusive igneous rocks, not volcanic or extrusive. They form from the slow cooling of a low-viscosity fluid phase rich in water and volatiles like carbon dioxide, fluorine, and chlorine, which separate from the last portion of magma to crystallize. This fluid phase will also have some incompatible rare elements or minerals.
When a magma body intrudes, minerals with higher melting points will begin crystallizing, leaving behind those of lower melting points, water with any dissolved incompatible elements, and other volatiles.
When the last portion of magma crystallizes, the high amount and volatiles of water cause phase separation, so you will have the remnant melt and fluid containing water, incompatible elements, and other volatiles.
The fluid phase is what forms the pegmatite. It is low in viscosity due to water and other volatiles. This low viscosity allows faster movement of elements to grow larger crystals.
Significance
Today, pegmatites are a source of valuable gemstones and rare elements. Some of these minerals and elements have many industrial uses. Also, some, like granitic pegmatites, may have few uses as dimensional stones (tiles, countertops, slabs, etc.) if pretty.
Frequently asked questions?
No. A pegmatitic texture isn’t the same as a porphyritic texture. The latter has larger phenocrysts or crystals embedded in a finer-grained matrix or groundmass. There is no groundmass associated with pegmatite, and crystals are unusually large.
No. While both are coarse-grained and occur in intrusive or plutonic igneous rocks, they differ. Phaneritic texture has coarse grains formed by the slow cooling of magma inside the Earth’s crust, and the crystals are not as unusually large. Also, they don’t show mineral zoning or directional orientation.
In contrast, pegmatitic texture from a fluid phase is formed when the final magma crystallizes. Also, the crystals are very large and show mineral zoning, directional orientation, etc.
Pegmatite rocks are plutonic or intrusive igneous rocks, i.e., they form deep inside the Earth’s crust, especially on or near margins of large magma intrusions or batholiths.
Pegmatites are associated with cratons and greenschist-facies metamorphic belts. They occur as individual pockets in magma bodies or as pegmatite dikes, sills, and lenses, especially on margins of batholiths or large magma intrusions.
References
- Pegmatite. (2022, July 29). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Pegmatite&oldid=1101203092
- Snelling, A. A. (2020, November 3). Large Gem Crystals Grew within Hours: Consistent with Rapid Granite Formation on a Young Earth. Answers in Genesis. Retrieved September 12, 2022, from https://answersingenesis.org/geology/large-gem-crystals-grew-within-hours/
- Quartz. (2022, September 8). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Quartz&oldid=1109103494
- Stemprok, M. (1989). Pegmatitic ore deposits. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30845-8_174
- London, D. (2021). Pegmatites. Encyclopedia of Geology, 184–195. https://doi.org/10.1016/b978-0-12-409548-9.12489-3