Two questions relating to mineralogy?

Question

1) What is meant by mineral endmembers? Please include an example.
2) What is a basic definition for basal cleavage?

Answer 1

1) Many minerals are in fact a group of minerals which have a variable formula and form a continuous series between two extremes. These extremes are called the end-members. For example the mineral Olivine is a series of silicates which range from Fosterite - Mg2SiO4 to - Fayalite - Fe2SiO4 - the end members. Most Olivine specimens however contain a mixture of Mg and Fe in varying proportions.
2) Basal cleavage is the property of some crystals to break along a plane within their structure that runs parallel to the base of the crystal. Examples of this are Mica which splits into very thin sheets and Topaz which when used as a gemstone has to be treated with care as a slight knock can cause the stone to split along its basal plane.

Answer 2

Mineral end members are the pure minerals of a solid solution. A good example is in the Plagioclase Feldspars. The end members are Albite (sodium plagioclase) and Anorthite (calcium plagioclase). Any percentage of albite and anorthite can be present in plagioclase, which is why it is a solid solution. True, there are areas where plagioclase will separate into alternating bands of albite and anorthite, such as the gemstone labordite, but that is due to other factors as certain percentages within the solid solution. I think you are confusing basal parting for basal cleavage. Basal parting is evident in certain minerals like (I think) beryl. There is no cleavage in the mineral, but weak, cleavage-like seperation occurs in these minerals. This is as opposed to a mineral like quartz that has no cleavage or basal parting. U K Explorer's answer was good until he said micas have basal parting. Micas typically have perfect cleavage in one direction, and the sheets can be divided further and further until (theoritically) you reach the molecular level. Perfect cleavage would preclude basal parting.

Answer 3

The field guide for Rocks and Mineral of North America is a very good resource for this topic!