Crystal Formation & Metamorphic Rock
Metamorphic Rock
Metamorphic rock is associated with the 'mountain-building process,' and is created by a combination of heat, pressure, and time. There are four basic types of metamorphic rock:
- Contact (thermal) Metamorphism Altered by heating from igneous intrusions (dykes)
- Dynamic Metamorphism Altered by pressure (mountain building - fold rock, fault rock)
- Metasomatic Metamorphism Altered by chemical replacement from gas or liquid intrusion
- Regional (thermal) Metamorphism Altered by heating from subduction, creating plutons
Fold rock is created when differential force is applied to a material, causing it to change its form [3]. Lower quantities of pressure will cause a material to 'fail' by fracturing, while higher stress levels will cause it to flow (ductile behavior).
Fault rock is created by friction caused by the enormous force of two plates fracturing and/or gliding against each other during an earthquake or land slide, causing a sheering motion (thrust fault or strike-slip fault).
Common varieties of metamorphic rock include gneiss, marble, schist, slate, and quartzite. Metamorphic rock can change into either igneous, and sedimentary rock, depending on the external geological forces that are applied to it.
Banded hornfels caused by contact metamorphism |
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Platy plagioclase feldspar caused by foliation |
Hydrothermal activity can cause chemical alteration of rock in a process called metasomatism. Metasomatic rock is created by mass transfer (high-concentration to low-concentration) of a volume of metamorphic rock that is under higher stress and temperature, into a zone with lower stress and temperature, with hydrothermal solutions acting as a solvent.
Contact Metamorphism
Contact metamorphic rock is created when when magma intrudes into the surrounding solid "country rock," making contact and creating a metamorphosed zone called a "contact metamorphism aureole." The temperatures are highest at the boundary of contact, and decrease with distance from it, and the formation of ore minerals may occur by the process of metasomatism which takes place at or near the contact zone.
Foliation
Layering within metamorphic rocks is referred to as banding or "foliation," and is the product of differential stress which occurs when rock is compressed along one crystallographic axis during recrystallization. Slate is an example of a fine-grained foliated metamorphic rock, which shows distinct planes of cleavage caused by differential stress.
Foliation can create elongated or "platy" crystals in minerals such as plagioclase feldspar (sheet mica in moonstone, sunstone), which grow with their long axes perpendicular to the orientation of the force. Varying textures are separated into foliated and non-foliated categories.
Regions of Gemstone-Bearing Metamorphic Rock
Myanmar, Sri Lanka and Thailand are regions associated with the formation of the mineral corundum (ruby and sapphire), which 'metamorphically' crystallized under the heat and pressure created as the Eurasian Plate and the Australian/Indian Plate collided. When these gemstones remain in their original primary location (in situ), and are not transported to a new location by erosion, they are excavated by hard-rock or open-pit mining.
Minerals that are not the result of metamorphism may be found within metamorphic rock, and these include amphiboles, feldspars, micas, olivines, pyroxenes, and quartz, all of which are associated with igneous rock.
Gemstones Associated with Metamorphic Rock
Pseudomorphic Replacement
A pseudomorph is a mineral that takes form of different mineral. When the replacement mineral fills the shape and volume of a parent mineral, and that parent mineral completely dissolves away, the replacement mineral can take on its exact appearance. Although pseudomorphic replacement can occur in all types of rock, it is a function of metamorphism. The mineral known as tiger's eye was though to be a seudomorph.
Bibliography and Reference on Gem & Rock Formation
1. Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals  . Martin's Press
2. UC Berkeley, Pegmatites . ist-socrates.berkeley.edu
3. Merguerian, Geologic Structure - A Primer . people.hofstra.edu
4. D. Nahon, Y. Wang, Pseudomorphic Replacement . www.minersoc.org
5. Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition
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