Definition of Minerals and Rocks

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Definition of Minerals and Rocks

Minerals differ from rocks in that they are a naturally occurring, homogeneous solid with an ordered internal crystalline structure which results in characteristic morphological properties.

Individual minerals have a consistent mineral formula but can vary somewhat in chemical composition due to substitution of elements with similar atomic radius and charge. In rocks, mineral grains are commonly visible with the naked eye or with the aid of a magnifying hand lens. They vary in size and can range from large centimetre sized crystals to small sub-millimetric grains visible only under a microscope. They often appear as shiny reflective surfaces, which is due to the reflection of light from mineral cleavage planes and can be a diagnostic feature for the identification of some minerals. Quite commonly, where visible, these minerals can have other diagnostic characteristics that make them easily recognisable to even inexperienced students. The more common diagnostic features are colour, hardness, lustre, crystal habit or shape, and cleavage. Characterisation of several of these attributes in a single mineral grain is usually enough to result in the positive identification of a mineral. In addition to the type of mineral present in a rock, other conditions to be recorded include:

  • the shape of the mineral (rounded vs. crystalline)
  • the relationship of the mineral to the surrounding groundmass/matrix
  • ore neighbouring minerals

Since most of the major rock-forming minerals can be found in a wide range of environments, further description of their setting within the rock sample can tell much about how they came to be formed.

Minerals are not Rocks! But they are the basic building blocks of all rocks; therefore, the identification of minerals is important for the characterization of rocks.

Rocks, a Definition

 

Rocks are naturally occurring solid materials comprising an aggregate of one or more minerals and compose the Earth’s crust. They are characterised by the assemblage of minerals contained within, relationships between the individual minerals and the surrounding matrix or groundmass. Rocks can be composed entirely of one or different types of mineral grains, or various fragments of different rocks and/or fossilised biological material. Due to differences in rock forming environments and mineral composition, rocks exhibit different diagnostic properties, these include colour, hardness, and grain size.