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According to the image that you provided, what you are seeing are not spherulites, but rather epoxy bubbles. More on that later. I will first answer what are spherulites: Spherical features observed in thin sections can form by several ways. Devitrification - this is the recrystallisation of glass. I think it is more common in rhyolitic rocks than in ...


4

you might have guessed: There is no such an easy answer as a precise, always valid maximum grain size. Point counting is an upscaling technique. It works when the sample area you inspect is >= to the so-called representative elementary area (REA) of the rock facies. The larger your grains, the larger needs the REA to be. Same holds true for increasing ...


3

It would look almost identical to the image obtained using plane-polarised light. Some minerals would look darker, especially those with strong pleochroism such as biotites and amphiboles, because all light is now transmitted or blocked regardless of orientation. Minerals with large relief differences such as carbonates (e.g. calcite) would cease to have ...


1

There are several options. During decompression, biotite is no longer stable and the hydrogen volatilises, leaving behind fine-grained intergrowth of oxide and silicate minerals. Oxidation of the rims during exposure of the lava to the atmosphere. Higher Fe3+ contents would make the biotite darker. It is impossible to tell without a detailed electron ...


1

Theoretical answer: You need to measure extinction angles. If it is always extinct when aligned to cleavage or crystal faces, then it is orthorhombic. If it sometimes aligns and the colour is consistent, it's monoclinic. Otherwise it's triclinic. You do not need to know how cleavage is oriented with respect to the axes in advance because there very few ...


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