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Author Topic: In the category of rocks, what does quartz, feldspar and mica mean?  (Read 26008 times)

Offline The Scientist

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Please explain as detailed as possible. According to my knowledge, it has something to do with the minerals that can be found in rocks. Thanks!


 

Offline traveler

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Granite
 

Offline Mazurka

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Quartz, feldspar, mica etc. are a few of the minerals that form rocks.
Some rocks are defined by the minerals they contain - as Traveler points out those three minerals are the obvious crystals that make up grantite.  However some rocks like limestone are composed of calcium carbonate, but are not crystaline - pure calcium carbonate is a mineral called calcite.  Sandstone is mainly quartz (which is a grey mineral) but is often some shade of red, brown or yellow... 
 

Offline Bass

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Quartz, feldspar and mica are some of the most abundant minerals in the earth's crust.

Quartz (SiO2) is present in almost all rocks.  It is hard, resistant to chemical weathering and has no cleavage, which means it is also resistant to physical weathering.  It is abundant in silica-rich igneous rocks, like granite and rhyolite.  Because it is so resistant to erosion, it is also found in most sedimentary rocks (even if as very small grains).  It is pervasive in most metamorphic rocks.

Feldspar (K,Na,Ca)(AlSiO4) is probably the most common mineral in the earth's crust.  They are present in abundance in all and in many igneous rocks.  Feldspars are susceptible to both chemical and physical weathering, breaking down into clays. 

Mica is really a whole host of minerals, muscovite, biotite and chlorite being the most common.  Mica is soft and can be distinghed by it perfect cleavage, which means the mineral breaks into very thin sheets.  It is easily eroded to form clay minerals.  Mica is abundant in silicic igneous rocks and is the main component of many metamorphic rocks (add enough heat and pressure to clay and it becomes mica).  Schist is a metamorphic rock composed almost entirely of mica.
 

Offline nicholasmarshall

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Quartz, feldspar and mica are some of the most abundant minerals in the earth's crust.

Quartz (SiO2) is present in almost all rocks.  It is hard, resistant to chemical weathering and has no cleavage, which means it is also resistant to physical weathering.  It is abundant in silica-rich igneous rocks, like granite and rhyolite.  Because it is so resistant to erosion, it is also found in most sedimentary rocks (even if as very small grains).  It is pervasive in most metamorphic rocks.

Feldspar (K,Na,Ca)(AlSiO4) is probably the most common mineral in the earth's crust.  They are present in abundance in all and in many igneous rocks.  Feldspars are susceptible to both chemical and physical weathering, breaking down into clays. 

Mica is really a whole host of minerals, muscovite, biotite and chlorite being the most common.  Mica is soft and can be distinghed by it perfect cleavage, which means the mineral breaks into very thin sheets.  It is easily eroded to form clay minerals.  Mica is abundant in silicic igneous rocks and is the main component of many metamorphic rocks (add enough heat and pressure to clay and it becomes mica).  Schist is a metamorphic rock composed almost entirely of mica.


Really Nice Post

Thanks For Information
« Last Edit: 28/09/2010 12:38:55 by peppercorn »
 

Offline geo driver

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so would mica be abundent in like slate? for example, but have little feldspar, where clay if mainly feldspar with alittle quartz, and granite would be quartz and mica, with little feldspar?
 

Offline Ophiolite

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so would mica be abundent in like slate? for example, but have little feldspar, where clay if mainly feldspar with alittle quartz, and granite would be quartz and mica, with little feldspar?
Yes, mica -typically muscovite - is a major constitutent of slates. Clay is mainly clay minerals which are weathered feldspars. They are no longer feldspar, but complex phylosilicates - sheet like minerals. Granite can easily contain 50% feldspar, along with the quartz and the ferromagnesian minerals - often mica, but it could be amphibole.

This link is probably much to complex for what you are looking for, but give it a once over. It may be useful to you.
It is the British Geological Survey rock classification scheme.
http://www.bgs.ac.uk/bgsrcs/
 

Offline Bass

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Feldspars are found in virtually all igneous rocks, most metamorphic rocks and in some sedimentary rocks.  Feldspars weather chemically in humid environments, breaking down to form various clay minerals (in arid environments they tend to weather mechanically).  Clays are the most abundant sediments, forming shales.  When subject to further heat and pressure, shales change to slates- which are primarily composed of very fine-grained illite and muscovite (+biotite and chlorite)- micaceous minerals formed from the original clay minerals.  With further heat and pressure, these minerals change to coarser micas, quartz and feldspars and the rock becomes schist or gneiss.
Granitic rocks are composed of a mixture of quartz, feldspar, micas and ferromagnesium minerals.  The feldspar content of granitic rocks is commonly in the 30% to 80% range.  Feldspars can be further broken into two groups- plagioclase and alkali feldspars.  Plagioclase is a solid solution series of calcic to sodic feldspars (anorthite is the calcic end-member, albite is the sodic end-member).  Orthoclase (potassic feldspar) is the most common alkali feldspar.  As igneous rocks become more silicic, the feldspars tend to be more sodic and alkalic- so low-silica basalts contain mostly calcic plagioclase while high-silica granites are dominated by albite and orthoclase. 
« Last Edit: 04/10/2010 16:45:20 by Bass »
 

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