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
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).
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.
- 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).
- Distorted layering with crystals (Photo 10) – possibly due to preferential growth in a certain direction or deflection due to obstructions in the glacial till.
- Ripple/wave-like surface features (Photo 11) – possibly due to dissolution by flowing water over the crystal surface.
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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