£1.25 – £3.35
Greisen metamorphic rock specimens from Caldbeck Fells, Cumbria, composed mainly of quartz and mica with an off yellow colouration. The rock is known as an endoskarn and forms when hydrothermal fluid and heat alter a granitic body, resulting a heavily altered rock which may contain trace amounts of rare elements. These samples are suitable for advanced collectors and higher level education and are available in three sizes, each pieces will come supplied in a card tray with a label.
Greisen are metamorphic rocks formed by hydrothermal alteration of felsic igneous rocks such as granites or pegmatites. As such, they are mostly composed of micas and quartz mienerals but little to no feldspar and can be more clearly defined as an endoskarn. This process occurs when a granitic pluton releases heat and hydrothermal fluids as a later stage which results in a process known as greisenisation, altering the pre-existing granite already in place. Essentially, the granitic intrusion metamorphosed its self. The rock its self has a coarse grained texture with crystals being easily visible with the naked eye, similar to granites naturally, however, they tend to have an overall yellow colouration due to processes that led to the formation which leave the rock not very attractive to look at. But the unique process in which greisen forms does result in rare elements crystallising within the endoskarn which are often of high economic value, these include fluorites, wolframite, cassiterite, lepidolite mica and many more, thus making greisen bodies an excellent source of rare metals in the past and as an indicator to potential ore deposits in a particular area. These particular samples come from the Caldbeck Fells in Cumbria and are ideal for advanced collectors or those interest in rare metals and their associated rock formations, as well as higher level education to describe altered bodies of rock and the potential to find ore deposits in the area. The specimens are available in three sizes and come supplied in a card tray with an information label.