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):
- 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).
- 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.
- 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.
- Note other features including colour, hardness / strength, specific gravity, sample size and any weathering.