GREISEN (in French, hyalomicte), a modification of granite, consisting essentially of quartz and white mica, and characterized 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 a paler granite. 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.
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 'in. up to 2f t. or more in thickness. At their outer edges they pass 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 con tained fluorine, boron and probably also lithium, for topaz, mica and tourmaline, the new minerals of the granite, contain these elements. The alumina for these minerals is supplied by the biotite and felspar of the granite, but it is noteworthy that albite is not replaced by the soda white mica, paragonite. The change is pneumatolytic, 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 de velopment of white mica, quartz and tourmaline, is found some times 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, and often contains white mica, thus passing by all stages into greisen. Both of these rocks frequently carry small percentages of tin oxide (cassiterite) and may be worked as ores of tin, and the central filling of the fissure often contains much wolfram, as in Cornwall, where they are common—as also in Saxony, Tasmania and other 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 alkalis during the transformation of the granitic rock into the greisen. (J. S. F.)