What is about Tourmaline Rock
Tourmaline, a boro-silicate of singularly complex composition, is an extremely important mineral whose significance goes well beyond being another attractive mineral. Tourmaline gives information on the thermal and fluid history of rocks in which it develops, is intimately associated with some of the world's premier metallic ore deposits, retains chemical signatures of the sources of tourmaline detritus in clastic rocks, yields isotopic evidence for the environmental sources of the boron that makes up tourmaline, is an extremely critical link in the boron cycle on the Earth and has many other useful petrogenetic features
Tourmaline rocks associated with Stratabound Scheelite Mineralization. The occurrences investigated are located in parts of the Austroalpine Crystalline Complex consisting of metavolcanoclastic and metacalcareous sequences of probable Lower Paleozoic age. Tourmalinites composed of tourmaline, quartz, plagioclase, ± almandine-rich garnet, ± muscovite, ± biotite, and minor ilmenite, rutile, graphite,- ± pyrite and, rarely, scheelite are of pre- to synmetamorphic origin. Tourmalines from the tourmalinites have been identified as intermediate members of the dravite-schorl solid solution series with minor amounts of other tourmaline end members. They can be compared to tourmalines from massive sulphide and stratabound tungsten deposits. Tourmalines from pegmatoids, on the contrary, plot close to the schorl end member. Tourmalinites are interpreted as metamorphosed products of elastic sedimentary material which has reacted with boronrich solutions of probable exhalative-hydrothermal origin. These exhalative processes are genetically connected to the transport of B and W and to the formation of syngenetic/syndiagenetic tungsten mineralization. Metamorphic mobilization of these primary concentrations led, during the Variscan and the Alpine metamorphic events, to the formation of scheelite-bearing quartz-tourmaline-, quartz-plagioclase-tourmaline mobilizates and pegmatoids.
Tourmaline-rich rocks are common in the lowgrade, interior portions of the Barberton greenstone belt, where shallow-marine sediments and underlying altered basaltic and komatiitic lavas contain up to 50% tourmaline. The presence of tourmaline-bearing rip-up clasts, intraformational tourmalinite pebbles, and tourmaline-coated grains indicates that boron mineralization was a low-temperature, surficial process. The association of these lithologies with stromatolites, evaporites, and shallow-water sedimentary structures and the virtual absence of tourmaline in correlative deep-water facies rocks in the greenstone bels strengthens this model. Five tourmaline-bearing lithologic groups (basalts, komatiites, evaporite-bearing sediments, stromatolitic sediments, and quartz veins) are distinguished based on field, petrographic, and geochemical criteria. Individual tourmaline crystals within these lithologies show internal chemical and textural variations that reflect continued growth through intervals of change in bulk-rock and fluid composition accompanying one or more metasomatic events. Large single-crystal variations exist in Fe/Mg, Al/Fe, and alkali-site vacancies. A wide range in tourmaline composition exists in rocks altered from similar protoliths, but tourmalines in sediments and lavas have similar compositional variations. Boron-isotope analysis of the tourmalines suggest that the boron enrichment in these rocks has a major marine evaporitic component. Sediments with gypsum pseudomorphs and lavas altered at low temperatures by shallow-level brines have the highest delta11B values (+2.2 to-1.9permil); lower delta11B values of late quartz veins (-3.7 to-5.7permil) reflect intermediate temperature, hydrothermal remobilization of evaporitic boron. The delta11B values of tourmaline-rich stromatolitic sediments (-9.8 and-10.5permil) are consistent with two-stage boron enrichment, in which earlier marine evaporitic boron was hydrothermally remobilized and vented in shallow-marine or subaerial sites, mineralizing algal stromatolites. The stromatolite-forming algae preferentially may have lived near the sites of hydrothermal discharge in Archean times.
Something about tourmaline
1. Infrared and electron microprobe analysis of tourmalines
Infrared and electron microprobe analysis of natural tourmalines from the dravite-schorl and elbaite-schorl series were carried out. The infrared study differentiates between OH groups located at the centre of hexagonal rings and those which are placed between hexagonal pillars and are coordinated to two Al ions. The correlation of infrared spectra with chemical composition of tourmalines made possible the assignment of different OH stretching bands to the more frequent octahedral cation associations. The study of the thermal dehydroxylation of tourmalines in air indentified the IR bands corresponding to OH bonded to Fe+2 ions in AlAlFe, AlFeLi or FeFeFe environments. The change in intensity of the OH absorption lines with the sample orientation has permitted the identification of several orientations of the OH bond axes. Electron microprobe analysis of zoned coloured samples has shown that the Fe, Mn distribution is partially ordered in some samples of the elbaite-schorl series.
2. Chemical compositions of tourmaline in the vein-type tungsten deposits of the kaneuchi mine, Japan
Chemical compositions of tourmaline from the vein-type tungsten deposits of the Kaneuchi mine in Japan are obtained with an electron probe microanalyzer. Tourmaline occurring at this mine belongs to the dravite-schorl series. Magnesium contents are high compared with tourmaline from granitic zones. This suggests that ore-forming solutions are not formed directly from magmatic fluid and reacted with sedimentary rocks. Compositions of tourmaline in quartz veins are similar to those in host rocks, and the bulk compositions of host rocks do not indicate magnesium metasomatic reactions. It can therefore be concluded that water/rock interactions occurred at a deeper level which remains unexplored.
3. Tourmaline cutting for Gemstone industry:
Tourmaline is also piezoelectric, that is, it becomes electric by pressure. If a crystal be subjected to pressure along the optic axis, it behaves as though it were contracting by reduction of temperature.The tourmaline should be cut with the table parallel to the optic axis. It was in tourmaline that the phenomenon of pyroelectricity was first observed.
From "tourmaline today" of Darrell Henry, and one artical of Y. Shibue1 in Geological Institute,
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