GREISEN (in French, hyalomicte)

This article appears in Volume V12, Page 578 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: GRA-GUI

GREISEN (in 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

APPEARANCE (from Lat. apparere, to appear)

from the abundance of lamellar crystals of muscovite, but many greisens have much of the

appearance

APPEARANCE (from Lat. apparere, to appear)

of granite, except that they are paler in colour. The commonest accessory minerals are tourmaline,

topaz

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

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

GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

, 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

BORON (symbol B, atomic
weight
- WEIGHT
I I)

and probably also lithium, for topaz, mica and tourmaline, the new minerals of the granite, contain these elements. The

change

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

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

OXIDE

(cassiterite) and may be worked as ores of tin. They are common in

Cornwall

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

BORON (symbol B, atomic
weight
- WEIGHT
I I)

and a diminution in the alkalies during the transformation of the granitic rock into the greisen. (J. S. F.)

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