GREISEN (in

See also:

• FRENCH
• FRENCH, DANIEL CHESTER (1850– )
• FRENCH, NICHOLAS (1604-1678)

French, hyalomicte)  , a modification of granite, consisting essentially of quartz and white mica, and distinguished from granite by the absence of felspar and biotite . In the hand specimen the rock has a silvery glittering appearance from the abundance of lamellar crystals of muscovite, but many greisens have much of the appearance of granite, except that they are paler in colour . The commonest accessory minerals are tourmaline, topaz, apatite, fluorspar and iron oxides; a little felspar more or less altered may also be present and a brown mica which is biotite or lithionite . The tourmaline in section is brown, green, blue or colourless, and often the same crystal shows many different tints . The white mica forms mostly large plates with imperfect crystalline outlines . The quartz is rich in fluid enclosures . Apatite and topaz are both colourless and of irregular form . Felspar if present may be orthoclase and oligoclase . Greisen occurs typically in belts or veins intersecting granite . At the centre of each vein there is usually a fissure which may be open or filled with quartz . The greisen bands are from I in. up to 2 ft. or more in thickness . At their outer edges they pars gradually into the granite, for they contain felspar crystals more or less completely altered into aggregates of white mica and quartz .

The transition between the two rocks is perfectly

gradual, a fact which shows that the greisen has been produced by alteration of the granite . Vapours or fluids rising through the fissure have been the agents which effected the transmutation . They must have contained fluorine, boron and probably also lithium, for topaz, mica and tourmaline, the new minerals of the granite, contain these elements . The change is a post-volcanic TI or pneumatolytic one induced by the vapours set free by the granite magma when it cools . Probably the rock was at a relatively high temperature at the time . A similar type of alteration, the development of white mica, quartz and tourmaline, is found sometimes in sedimentary rocks around granite masses . Greisen is closely connected with schorl rock both in its mineralogical composition and in its mode of origin . The latter is a pneumatolytic product consisting of quartz and tourmaline; it often contains white mica and thus passes by all stages into greisen . Both of these rocks carry frequently small percentages of tin oxide (cassiterite) and may be worked as ores of tin . They are common in Cornwall, Saxony, Tasmania and other districts which are centres of tin-mining . Many other greisens occur in which no tin is found . The analyses show the composition of Cornish granite and greisen .

They make it clear that there has been an introduction of fluorine and boron and a diminution in the alkalies during the transformation of the granitic rock into the greisen . (J . S .