Large mineral crystals
Granite is the result of slow solidification of magma that is rich in silica within the earth’s crust.
Given that solidification takes places slowly, the minerals that form granite are large crystals.
If we look at rocks under a microscope, a crystal of one millimetre is considered to be large.
Granite crystals are a few millimetres in size, and can reach up to 10 centimetres.
This gives granite – and especially white granite – a “salt and pepper” appearance, as some crystals are white or transparent (quartz, feldspar) while others are black (usually biotite mica). Granite that has been prepared for the microscope, on the other hand, is sliced so thinly (3 hundredths of a centimetre) that even black minerals can appear transparent. In these images, each of the coloured fields is a different crystal.
Some of these are altered over the period between the rock’s release and its being deposited into the soil. In these cases, the crystal has been substituted by smaller and more elongated minerals. Here, they can be recognised because they form comb or ray shapes.
|Type of rock:||Intrusive magmatic rock|
Quartz, biotite, plagioclase, alkaline feldspar, epidote. Chlorite, sericite, calcite, epidote, titanite as alteration minerals
|Location:||Lazise (N 45° 30′ 39.3″ E 010° 46′ 25.6″)|
pebbles of the glacial deposits of the sarca (morainic amphitheater of the Garda)
Pleistocene (less than a million years)
How the pebble was formed:
How the rock was formed:
on the front of a lowland glacier