Photographic feature: Gypsum crystal mineralogy

Gypsum Crystal Mineralogy

This photographic article considers the habit and mineralogy of gypsum crystals from a glacial till deposit and Jurassic rock strata in Northamptonshire (UK). The crystals display excellent examples of fish-tail and swallow-tail twinning as well as unusual features such as deformed twin planes, clay inclusions and ripple-like textures.

Gypsum in the Oadby Member Glacial Till

General features

Coarse selenite (prismatic/well-formed gypsum) crystals, up to 100mm in length, are present within the Oadby Member glacial till exposed within a quarry local to the Weldon area of Northamptonshire, UK. Gypsum crystals from this area range in size from 5mm to 150mm (typically 30-50mm) and are generally colourless to light grey. Most crystals are isolated prisms found at or near the surface of the glacial till (Photos 1 to 3).

Figure X.

Photo 1. Semi-transparent single selenite prism. Long axis approx. 60mm.

 

Figure 3.

Photo 2. Single selenite prism with inclusions of clay. Long axis approx. 45mm.

 

Figure X.

Photo 3. Semi-transparent single selenite prism with inclusions of clay along internal planes. Long axis approx. 60mm.

 

Unusual features

A number of less common morphological appearances and habits can also be found and are described below.

    • Inclusions of clay and/or rock (Photo 5) – likely encapsulated during crystal growth.
Figure X. with rock material.

Photo 5. Inter-grown selenite crystals with inclusions of clay and external particles of sandstone and mudstone. Long axis approx. 60mm.

 

    • A variety of crystal-group and crystal-twinning styles (Photos 6-9), including penetration twinning (intersections at a common point, with many sub-types) and contact twinning (fish-tail).
Figure X. Multiple penetration twins - rosette.

Photo 6. Multiple penetration twins. Here intergrown symmetrical and asymmetrical crossing twins have produced a “rosette”-like appearance. Long axis approx. 90mm.

 

Figure X. with multiple penetration twins forming a rosette-like appearance.

Photo 7. As described in Photo 6, with fragments of red-brown mudstone at crystal intersections. Long axis approx. 100mm.

 

Figure X.

Photo 8. V-shaped twinning (RHS) known as “fishtail twinning”, where two halves of the twinned crystal meet at a twin plane. Long axis approx. 90mm.

 

Figure X

Photo 9. Penetration (LHS) and fishtail (RHS) twinning. Long axis approx. 120mm.

 

  • Distorted layering with crystals (Photo 10) – possibly due to preferential growth in a certain direction or deflection due to obstructions in the glacial till.
IMG_0225

Photo 10. Distorted layers within a gypsum crystal. Long axis approx. 90mm.

 

  • Ripple/wave-like surface features (Photo 11) – possibly due to dissolution by flowing water over the crystal surface.
Figure X. Single selenite prism with ripple-surface effect. Possibly a result of partial dissolution.

Photo 11. Single selenite prism with ripple-surface effect. Possibly a result of partial dissolution by water.

 

Gypsum within Jurassic rock strata

Gypsum is locally abundant within an orange-brown sandstone layer (possibly the basal Rutland Formation or upper Lincolnshire Limestone Formation) in the Northamptonshire area. This layer is approximately 5-10cm in thickness and comprises both prismatic and fibrous forms of gypsum (Photo 12). This layer likely represents an evaporite deposit – formed from the chemical precipitation of calcium and sulfate due to evaporation of water containing these ions.

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Photo 12. Prismatic and fibrous gypsum crystals within a sandstone rock unit in the Jurassic strata at Weldon, Northamptonshire.

 

Copyright

All photographs copyright and credit to PyriteProblem.com.

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