rocks


To the casual observer, rocks may look grey and uniform, but to the geologist there are many types of rock on Dartmoor, with their own characteristics, texture and mineralogy. Here are some of the different types to be found on Dartmoor, if you care to look a little more carefully.
Igneous Rocks, Hand specimen descriptions. visable with hand lens * may contain plagioclase in thin section.
	Tor or Giant Granite, Felsic, granite. Porphyritic in texture, overall colour leucocratic, speckled buff, cream, black. Groundmass granular with coarse grain size. Crystallinty holocrystalline, sub-hedral. Phenocrysts Present, euhedral to sub-hedral, of cream coloured orthoclase feldspar which may show Carlsbad twining. phenocrysts may be corroded. Essential minerals quartz 30%, alkali feldspars* 60%, mica (biotite) 9% Accessory minerals include, secondary muscovite mica, ilmenite 1% Occurrence, widespread upper layers of granite batholith.
	Quarry Granite (Blue granite), Felsic, granite. Texture Granular Overall colour leucocratic, speckled buff, cream, black Groundmass Granular with coarse grain size. Crystallinty holocrystalline, sub-hedral. Phenocrysts None. Essential minerals, quartz 30%, alkali feldspars* 60%, mica (biotite) 9% Accessory mineral include muscovite mica ilmenite 1% Occurrence in association with Tor granite, see Hay Tor.
	Aplite, Felsic, Aplite. Saccharoidal in texture, overall colour leucocratic white, pink, buff. Groundmass fine grained, microgranitic. Crystallinty holocrystalline, sub-hedral. Phenocrysts None. Essential Minerals, quartz 30%-40%, alkali feldspar 50%-60%, mafic minerals subordinate. Accessory Tourmaline needles 1% Occurrence Dykes cutting granite
	Quartz Tourmaline Schorl Felsic, schorl. Texture Grain size Coarse Colour Leucocratic. Crystallinty holocrystalline, sub-hedral. Mineralogy Massive Quartz and vain tourmaline. Occurrence, Late stage pneumatolytic vain injection
	Pink Granite , Felsic, granite. Texture Granular Overall colour leucocratic, speckled pink, grey, cream, black Groundmass Granular with coarse grain size. Crystallinty holocrystalline, euhedral to sub-hedral. Phenocrysts occasional white euhedral feldspar showing carlsbad twin and flow banding, but equal in size to groundmass. Essential minerals, quartz 30%, alkali feldspars* 60% 50:50 Pink/white, mica (biotite) 4% Secondary muscovite 5% Tourmaline 1% Occurrence, Trowlesworthy Tors
	Blue Granite (localised varient of Tor Granite), Felsic, granite. Porphyritic in texture, overall colour leucocratic, grey, blue, cream Groundmass granular with coarse grain size. Crystallinty holocrystalline, sub-hedral. Phenocrysts Present, euhedral to sub-hedral, alkali (orthoclase) K- feldspar* (carlsbad twin) Essential Minerals, quartz 30%, alkali feldspar 60%, mica (biotite) 9%. Accessory mineral cordierite Occurrence, south part of the Moor, around Ivybridge.
	Luxullianite (Roof rock) Felsic, granite. Porphyritic in texture, overall colour leucocratic, Pink/black Groundmass Granular with coarse grain size . Crystallinty holocrystalline, sub-hedral. Phenocrysts Present, sub-hedral, corroded brick red alkali feldspar. Essential minerals, quartz 10 -30%, alkali feldspar 60% biotite and feldspars replaced byTourmaline upto about 20% Occurrence, Pneumatolytic replacement of granite. margins of batholith.