Tuesday, 4 June 2013

Garnets from the BGS collections

SILEX granatus. Garnet. Plate no. 44. From: Sowerby, James. 1802-1817. British Mineralogy: Or Coloured figures intended to elucidate the mineralogy of Great Britain. Plate from vol: 1. page no.92. Modern name: Garnet.
BGS image ID: P704670
SILEX granatus. Garnet. Plate no. 44. From: Sowerby, James. 1802-1817. British Mineralogy: Or Coloured figures intended to elucidate the mineralogy of Great Britain. Plate from vol: 1. page no.92. Modern name: Garnet.


Rock specimen of garnet pegmatite. Loch Garve, Garve, Rosshire, Scotland. The photograph is a detail of the surface of a garnet pegmatite sample, characterized by ruby-red spherical garnets of varying sizes, within a matrix of pale quartz and white mica. British Geological Survey Petrology Collection sample number EMC589. Garnets of this colour are generally rich in iron and manganese, termed 'almandine' garnets. The pegmatites in the Garve area have been dated using radiometric dating techniques, which indicate they are Precambrian in age.
BGS image ID: P521247
Rock specimen of garnet pegmatite. Loch Garve, Garve, Rosshire, Scotland. The photograph is a detail of the surface of a garnet pegmatite sample, characterized by ruby-red spherical garnets of varying sizes, within a matrix of pale quartz and white mica. British Geological Survey Petrology Collection sample number EMC589. Garnets of this colour are generally rich in iron and manganese, termed 'almandine' garnets. The pegmatites in the Garve area have been dated using radiometric dating techniques, which indicate they are Precambrian in age.


Photomicrograph of garnet-mica schist. Light: XPL, Magnification x 2. Glen Roy half a mile north-east of Stone and east of Beinn a' Monicag, Invernessshire.
BGS image ID: P524411
Photomicrograph of garnet-mica schist. Light: XPL, Magnification x 2. Glen Roy half a mile north-east of Stone and east of Beinn a' Monicag, Invernessshire. The image is a photomicrograph taken under crossed-polarizing lenses of a thin section of garnet mica-schist. British Geological Survey Petrology Collection sample number S 82420. Light: XPL, Magnification x 2. Light: XPL, Magnification x 2. The sample comes from the Monadhliath Mountains of the Grampian Highlands of Scotland. It is a semipelitic rock (a metamorphosed siltstone or impure sandstone) of Precambrian age which has become completely recrystallized with the development of an assemblage of metamorphic minerals. As the original sedimentary rocks become deeply buried in the Earth's crust they undergo a series of profound changes which, under the effects of elevated temperature and pressure, result in the breakdown of the original sedimentary minerals and the growth of new metamorphic minerals. This specimen is from the Leven Schist Formation, Appin Group, Dalradian Supergroup (Precambrian) from Glen Roy, half a mile north-east of Stone and east of Beinn a' Monicag. This formation consists of laminated, pale grey, pelitic schists which are locally flaggy and variably garnetiferous with thin carbonate beds towards the top of the formation. Following the progressive shallowing associated with the Grampian Group basin there was slow subsidence resulting in a marine transgression which led to the deposition of the Lochaber Subgroup. This was followed by a significant increase in water depth which led to the deposition of the Leven Schist Formation.

Bob McIntosh

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