Features of rocks

Important features to look for when writing descriptions and identifying rocks are:

Texture:Refers to the shape, arrangement and distribution of minerals or grains / clasts within the rock - the texture in a geological sense does NOT refer to the roughness of the surface of the rock;
Structure:Refers to broader features of a rock which may extend beyond the hand specimen into the outcrop; examples are bedding (in sedimentary rocks), foliation (in metamorphic rocks);

Note: Texture and structure, collectively referred to as fabric, are of primary importance in determining which major rock group a particular rock specimen belongs to.
Grain size:Refers to the size of individual mineral crystals or clasts (pieces of pre-existing rock) in a rock. Grain size is useful for determining various rock types within the three major rock groups;
Mineralogy:Refers to the minerals present within the rock, and also their relative proportions (especially important in the case of igneous rocks);
Other features:Features such as colour, hardness / strength and specific gravity can also provide useful information, and should be included in a complete petrographic description. If a rock appears to be weathered this is an important feature to note as weathering can change physical characteristics markedly. The size of the sample should also be noted.

Each of the three major rock groups has specific features which are useful for their determination, as do the rocks within the three groups.

Preparing a petrographic description

To ensure good systematic petrographic descriptions the following steps should be followed when investigating rocks. (Important note: physical characteristics should be determined from a fresh, unweathered surface):

  1. Observe and note the rock's fabric (texture and structure). This should enable you to determine the group into which the sample falls (igneous, sedimentary or metamorphic).
  2. Identify any minerals visible in the sample by their properties. If minerals comprise a significant portion of the sample, try to estimate the relative proportions of each. If the sample contains fragments of other rock types, try to identify these.
  3. Determine the grain size, or range of grain sizes, in the sample. Use estimates of grain size as follows:
    fine: < 0.1mm diameter, medium: 0.1 – 2mm diameter, coarse: > 2mm diameter.
    Take note of whether different minerals are of different sizes, and / or whether there are large variations in grain size.
  4. Note other features including colour, hardness / strength, specific gravity, sample size and any weathering.