Mineral Parting

Parting resembles cleavage - it produces smooth, planar breakage surfaces along specific orientations in a crystal. The distinction lies in the cause: cleavage exploits continuous crystallographic planes of weak bonding that run uninterrupted through the crystal, whereas parting results from discrete discontinuities that are spaced up to a millimetre or so apart. Because the surfaces of weakness are discontinuous, a mineral with parting cannot be broken at arbitrary fine spacings the way a good cleavage mineral can - it will only break at the existing discontinuities. Not all samples of a given mineral necessarily display the parting, even when the parting is well documented for that mineral species. ^[1]

Describing Parting

Partings are described in the same way as cleavages. A Miller index designates the orientation, and quality terms such as perfect and good indicate how easily the parting occurs. If the parting lacks strict crystallographic control and is only approximately parallel to a crystal form, it is labelled with a tilde - for example, ~{100}. ^[1]

Exsolution Lamellae as the Primary Cause

The most common source of parting discontinuities is exsolution lamellae - thin planar intergrowths produced when a crystal that was homogeneous at high temperature unmixes during slow cooling. The boundary between an exsolved lamella and its host can act as a plane of weakness, particularly if the lamellae have begun to alter. The pyroxenes are the classic example: their partings, which follow exsolution lamella orientations, are well developed in many specimens. Exsolution lamellae may lie along strictly crystallographic planes or in non-rational orientations, depending on the system. ^[1]

The Twin Plane Debate

It was historically assumed that twin composition planes - the surfaces along which two crystal individuals are joined in a twin - were also planes of weakness and therefore the cause of many partings. Current understanding contradicts this. Twin composition planes are typically structurally strong, and are not likely parting surfaces. Many of the classic associations between parting and twin planes were established in the 18th and 19th centuries using methods that could not distinguish between twinning and other microstructural features. These early observations deserve critical re-examination rather than uncritical acceptance. ^[1]

The minerals in which partings have been attributed to twin planes include calcite {102}, corundum {101} and {001}, hematite {001} and {101}, magnetite {111}, ilmenite {010} and {001}, rutile {092} and {100}, spinel {111}, cassiterite {101}, chromite {111}, marcasite {101}, pyrite {011}, and titanite {221}. Most of these minerals also have exsolution lamellae or other intergrowths that could independently produce planes of weakness.

The Corundum Case Study

Corundum is the most thoroughly investigated example. The old literature confidently reported partings on {101} and attributed them to rhombohedral twinning on that plane. However, modern study showed that the {101} partings in corundum are produced by thin lamellae of boehmite (AlOOH) that most likely exsolved from the corundum during cooling - not by twin planes at all. The twin composition planes turned out to be strong, as expected. The corundum case illustrates why all historical twin-parting associations should be treated as hypotheses requiring modern verification, not accepted facts. ^[1]

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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