Calcite, or Iceland Spar, is a strongly birefringent material and is used for polarisers and retardation plates. Calcite is often mined naturally. Crystran Ltd has a stock of small calcite "rhombs" of good clear optical quality. Calcite cuts and polishes well.
CLEAVEAGE PLANE:
There can be confusion in the definition of the cleavage plane in calcite. Conventionally this has always been referred to as {10-11} but recent papers on AFM studies use {10-14}. Calcite cleaves between the bonds of the CO3 groups (in the CO3 layer). The CO3 group are offset relative to each other and inclined to the c axis giving 3 cleavage directions defining a rhomb. Following the {10-11} nomenclature the unit cell requires ¼ the length of the c axis as measured from XRD (on a dimension 4 times longer). The correct Miller indices are {10-14} but the conventional {10-11} is often used in order not to confuse and for easier comparison. (1)
REFERENCES:
1. Private Communication. J.A.Elliott. Dept. Materials Science, University of Cambridge. 2011
2. Crystallography - LINK to:
Practical Crystal Measurement. A.E.H.Tutton, Vol2, page 13293. Linear Thermal Expansion of Calcite - Link to:
Mineral Society of America, Linear Thermal Expansion of Calcite.
WEBLINKS:
http://webmineral.com/data/Calcite.shtml - Has further information regarding Calcite.
Optical data are for ordinary ray (o).