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SCHORL

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Originally appearing in Volume V24, Page 377 of the 1911 Encyclopedia Britannica.
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SCHORL , in See also:

mineralogy, the name given to coarse See also:black varieties of See also:tourmaline (q.v.). The schorl rocks are crystalline aggregates of See also:quartz and tourmaline. They are granular and massive, not banded or foliated as a See also:rule, See also:grey of various shades, the darkest coloured being most See also:rich in schorl. Some are very See also:fine grained, but in most cases the individual crystals are easily discernible with the unaided See also:eye. They are hard, splintery, and very resistant to weathering. Veined, brecciated, porous and banded varieties occur, but are less See also:common than the granular massive rocks. Schorl rocks occur practically always in association with tourmaline-bearing granites. Most of them are of igneous origin and, though there may be a few which are See also:direct products of consolidation' from a plutonic magma, in the vast See also:majority of cases they originate by the See also:action of gases and vapours on granites, porphyries and other rocks. All magmas contain vapours in See also:solution and give them off more or less readily as they crystallize. See also:Water, carbonic See also:acid and hydrochloric acid (or chlorides) are the commonest dissolved substances, but See also:fluorine, See also:boron, See also:lithium and phosphoric acid occur also, and as they pass outwards these last may See also:act on the surrounding rocks, probably still at a high temperature and produce minerals of a See also:special See also:kind. This action is said to be pneumatolytic. Tourmaline contains boron and flourine, hence the presence of these elements in the emanations from the See also:granite may be assumed.

Schorl rocks often also contain varieties of See also:

white See also:mica which are rich in fluorine and lithium; in addition See also:apatite is usually See also:present. Lastly, many of the rocks of this See also:group contain tinstone or are associated with See also:tin-bearing See also:veins, and it is probable that the ores of this See also:metal were brought up in solution as fluorides or chlorides and deposited in the situations where now they are found. Along the sides of fissures, through which, no doubt, the gases ascended; the granite is converted into schorl See also:rock for a distance ranging from a fraction of an See also:inch to several feet, and vein-like masses of grey schorl rock branching and uniting are thus produced. In other places considerable areas of granite are changed in this way, principally near the margin of the granite, and an interrupted See also:belt of this kind of rock encircles some of the larger outcrops of granite in See also:Cornwall. A similar origin must be ascribed to See also:greisen (q.v.), the aggregate of quartz and white mica commonly found in association with tin-bearing granites; there are See also:complete gradations between schorl rock and greisen, according to the varying See also:pro-portions of white mica and tourmaline which may be present in each specimen. Another See also:mineral which is produced by the pneumatolytic alteration of granite is See also:topaz (a silicate and fluoride of alu-minium) ; an aggregate of quartz and topaz is called topaz-fels or topaz rock, and is largely See also:developed in some of the tin-See also:mining districts of See also:Germany, though not found in Cornwall. As might be expected every See also:stage of the See also:conversion of granite into schorl rock can be found. Tourmaline may have been to some extent an See also:original constituent of the granite, but most of it is of new formation and must have resulted from the alteration of the See also:biotite and the See also:felspar of the original rock, both of these minerals having disappeared when the See also:metamorphosis was complete. It is commonly found that the schorl is of a See also:brown See also:colour in the interior of the crystals but See also:blue at the edges; probably the brown is See also:primary or has been derived from biotite, but the blue principally from the replacement of felspar. The rock known as luxullianite, obtained near Luxullian See also:village in Cornwall and used as an ornamental See also:stone for the See also:sarcophagus of the See also:duke of See also:Wellington's See also:monument in St See also:Paul's See also:Cathedral, is a tourmaline granite in which the replacement of biotite and felspar by quartz and tourmaline can be seen in progress. The new tourmaline is in fine pointed needles which have a stellate or divergent arrangement, and is embedded in quartz: often these needles are planted on the See also:surface of corroded crystals of primary brown schorl. This rock still contains a See also:good See also:deal of flesh-coloured felspar in large porphyritic crystals which contrast well with the dark See also:matrix and give polished specimens a very handsome See also:appearance.

In the completely altered schorl rocks there are rarely needles of tourmaline, but this mineral occurs as irregular grains mingled in varying proportions with small crystals of quartz. In nearly all cases the structure of the granite has vanished, but at Trevalgan, St Austell, and other places in Cornwall there are schorl rocks which contain white pseudomorphs of quartz after porphyritic crystals of See also:

orthoclase. In porphyries of " elvans " tourmalinization also is frequent, though not so common as greisening. Veins of quartz with stellate schorl needles may be seen spreading through the groundmass or when this has been previously converted into an aggregate of quartz and fine scaly white mica, the porphyritic crystals of felspar alone may be replaced by bunches of tourmaline embedded in quartz. Tinstone often makes its appearance in these rocks either in small crystals enclosed in quartz or lining fissures and cavities See also:left by the removal of a portion of the rock in solution. The same See also:process goes on also in sedimentary rocks; a felspathic See also:sandstone may yield a schorl rock which can hardly be distinguished from one derived from a fine-grained granite. In shales brown tourmaline is often deposited in the vicinity of fissures, and the whole See also:mass may be converted into a hard splintery aggregate of quartz and schorl (often containing also See also:rutile and tinstone). But these rocks are always banded, like the original See also:slate; their original structures (bedding and cleavage) are probably never completely effaced and the ultimate product has been called schorl-schist (tourmaline See also:hornfels, cornubianite). The stanniferous veins which in large See also:numbers intersect the granites of See also:Devon and Cornwall and the slates around them, and have yielded a large See also:part of the See also:world's See also:supply of tin consist mostly of quartz, tourmaline and See also:chlorite (with varying proportions of cassiterite). The veinstones are typically very fine grained, hard and dark blue or dark See also:green in colour. The green varieties contain much chlorite, the blue are richer in tourmaline, and both kinds are known to the miners as " See also:peach." Essentially aqueous deposits in lines of fissure, these rocks show that quartz and tourmaline were carried up in hot solutions at a See also:late See also:period in the cooling of the granite, and the changes above described are due to the operation of these solutions as they spread outwards through the surrounding rocks. Their tourmaline crystals are very small and usually of dark-blue shades, but owing to repeated movements of the walls of the veins the ore deposits have sometimes an intricate See also:history, as microscopic studies show that the first infillings of the fissures have been broken up and cemented together again by a later material of slightly different See also:character.

(J. S.

End of Article: SCHORL

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