Answer:
What Minerals Form Rocks?
The list of minerals that commonly form rocks is short. With a little practice you will recognize most of them when you see them. Descriptions of some of the minerals, as they look in rocks, follow: Quartz: Quartz is the last mineral to crystallize, so in igneous rocks it never has any definite shape. In rocks, it does not show flat faces. It is usually gray in igneous rocks; gray, white, yellow, or red in sedimentary rocks; and gray or white in metamorphic rocks. It has a glassy, or sometimes waxy, look to it.
Potassic Feldspars*: (microcline, orthoclase) Potassic feldspars are pink or tan, sometimes white. They show flat, shiny faces in igneous rocks. The crystal grains are usually blocky and nearly rectangular. They look like good china.
Plagioclase Feldspars*: (albite, labradorite) Look like the potassic feldspars, except they are white to dark gray, sometimes black. They may show flashes of blue or green.
Micas*: (muscovite, biotite, phlogopite) Micas have very thin layers that peel off (or cleave) very easily. In rocks they are usually flakes or layers of flakes. Muscovite is silvery to brown; biotite is black; phlogopite is a reddish brown. Phlogopite may be found in marble.
Chlorite*: Like mica, but the flakes are usually not as thin and do not peel apart as easily. The color is medium to dark green, sometimes almost black but with a greenish tint.
Hornblende: Hornblende is dark green to black. It shows nearly flat, shiny faces in almost rectangular or long thin needle like crystals in rock. Hornblende is usually found in dark colored metamorphic rocks; sometimes in igneous rocks.
Actinolite and Tremolite: Actinolite and tremolite are usually in long thin blades or needle like crystals. Actinolite is dark green; tremolite is white to gray. The crystals may be parallel to each other, or spread from a point. Actinolite is usually found in schists or gneisses. Tremolite may be found in marble.
Olivine*: Olivine in rocks is an olive green to greenish yellow. In rocks it is in rounded grains. If there is much of it, it is almost sugary. It is found mostly in dark colored igneous rocks.
Calcite and Dolomite: The color is usually white, but can be other colors when impure. Crystal grains show flat shiny faces, often shaped like parallelograms. Calcite and dolomite are both soft. They are easily scratched with a steel point. Powdered calcite will fizz in white vinegar; dolomite will not. The minerals are found in limestone or dolostone ( the rock is dolostone, the mineral is dolomite) and marble.
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