How do you tell if a rock is made up of one or more minerals?

Answer 1

You know that all rocks are made up of minerals.

Next you must determine the rock type. Each rock type has specific mineralogy. An orthoquartzite (common sandstone) is always composed of Quartz grains, so it has only one. Metaquartzite has the same composition, but has undergone slight recrystallisation. A shale is also likely to contain similar minerals: mostly silicates.

Therefore, to determine mineralogy, you must first know the rock type. To do this, you must try the classification tables, but it helps if you can do the first steps intuitively:

Rock type? Sedimentary, Igneous or Metamorphic (the latter two are more difficult to distinguish, so if it is crystalline, this will do for the first stage)

Grain/crystal size?

...other classification tests...

Finally, when you have narrowed it down to the extent that you can say 'sandstone', 'limestone', 'extrusive' etc. you can go to the specific tables or triangles of classification.

Take Orthoquartzite as an example. You must determine it by its grain size (medium) sorting (all grains same size) angularity (smooth) and appearance (golden-yellow). With this rock type, there is little need to go to great depth as it is immediately obvious what it is. Even a non-geologist would immediately know it as sandstone (as orthoquartzite is the classic sandstone).

Because of the uniformity of a rack's appearance, you can be almost certain that it contains only one mineral. However if it does not have one colour, this implies that the rock comtains more than one mineral, like Granite, which has Pink (Feldspar), White (Quartz), Black (Mica) and sometimes other colours within it.