Minerals

You don't have a rock - you've got a mineral.

Minerals are the building blocks of rocks. They can be present in rocks as grains or crystals. Rocks may be made up of just one kind of mineral, like limestone which is made out of the mineral calcite, and quartzite which is made out of the mineral quartz, or they can be obvious collections of different coloured minerals, like granite which is made up of grey/white quartz, pink feldspar and black mica. Here are some of the common minerals found in rocks:

Quartz
Feldspar
Mica
Amphibole
Pyroxene
Calcite

Quartz is a hard grey/white mineral made of silica. In igneous rocks quartz is usually grey and because it is the last mineral to crystallize it often does not show good crystal shapes. However, quartz can also occur as veins where it has crystallized within cracks in the rock, here it can often show nice crystal shapes.

Quartz can be tinted with impurities. In sedimentary rocks it can be yellowish or reddish, in metamorphic rocks it is usually grey or white. It can look glassy or waxy. Sometimes quartz can be of gemstone quality e.g. Rose Quartz and Amethyst [which is violet coloured quartz containing iron impurities].

Feldspar is mineral containing silica, but also potassium, sodium and calcium. Feldspars are pink, tan or white. In igneous and metamorphic rocks the crystals are usually blocky and nearly rectangular and they show flat shiny faces on broken surfaces of rocks. Feldspar is harder to recognise in sedimentary rocks.

Mica is a mineral containing potassium, aluminium and silica (pale coloured mica) and also magnesium and iron (dark coloured mica). Mica has thin layers that you can peel off with your finger nail. In rocks mica is usually present as flakes or layers of flakes. The colour can range from silvery to brown, to shiny black. Mica is found in igneous, metamorphic and sedimentary rocks.

Amphibole is a mafic (dark) mineral containing lots of magnesium and iron, but also sodium, potassium and aluminium. It is usually dark green to black. It shows flat shiny crystal faces and the crystals are often rectangular or long thin needles. It is found in igneous rocks and dark coloured metamorphic rocks.

Pyroxene is a mafic (dark) mineral containing calcium, magnesium, iron and some silica. It is dark green, dark brown or black and is easy to confuse with amphibole. It is found in igneous rocks and dark coloured metamorphic rocks.

Calcite is usually white, but can be coloured red/brown or black by impurities. It can form good crystals, with flat shiny faces shaped like parallelograms. It is soft and can easily be scratched with a steel point. The test for calcite is that it will fizz in white vinegar or dilute acid. It is principally found in sedimentary rocks and is the main mineral making up Limestone.

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