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Questions tagged [petrography]

Petrography is the science of observing and interpreting textural relationships of minerals and other features in rocks as observed through thin sections using an optical microscope.

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What do opaque rims on biotite in a volcanic rock mean?

In a volcanic region, i have examined two samples petrographically. One of them has biotites with extensive opacitic rims. The other samples biotites hasn't got this property. They have the same modal ...
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What is the maximum grain size of a sediment that can be point counted using a standard thin section?

Point counting is an extremely useful tool for determining the composition of a clastic sedimentary rock. It is also very useful in provenance studies to determine the sediment source of a sedimentary ...
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Identifying trichroic crystal systems

An unknown mineral shows three distinct pleochroic colours. Explain what observations you need to make in order to decide which crystal system it belongs to. If it has three pleochroic colours this ...
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How are spherulitic structures formed in basalt rock?

After having studied the thin sections of basalt under the microscope, I have come across many spherulitic structures occurring in clusters. How can this happen? Is it due to devitrification or is ...