Black Tourmaline Research Articles

Sc- and REE-rich tourmaline replaced by Sc-rich REE-bearing epidote-group mineral from the mixed (NYF+LCT) Kracovice pegmatite (Moldanubian Zone, Czech Republic)

Primary black thick-prismatic Al-rich schorl to rare fluor-schorl (TurP1), locally overgrown by brownish-green Li-rich fluor-schorl to fluor-elbaite (TurP2) from the Kracovice pegmatite (mixed NYF+LCT signature), was partly replaced by secondary Li-rich fluor-schorl to fluor-elbaite (TurS) plus the assemblage REE-bearing epidote-group mineral + chamosite. Primary Al-rich schorl (TurP1) shows high and variable contents of Sc (33-364 ppm) and Y+REE (40-458 ppm) with steep, LREE-enriched REE pattern. Overgrowing (TurP2) and replacing (TurS) Li-rich fluor-schorl to fluor-elbaite is typically depleted in Sc (21-60 ppm) and Y+REE (3-47 ppm) with well-developed tetrad effect in the first (La-Nd) and the second (Sm-Gd) tetrads. Scandium- and REE-rich black tourmaline (TurP1) crystallized earlier from the melt, whereas crystallization of primary Li-rich fluor-schorl to fluor-elbaite (TurP2) most likely took place during late magmatic to early hydrothermal conditions. Both the secondary Li-rich fluor-schorl to fluor-elbaite (TurS) and the unusual assemblage of REE-bearing epidote-group mineral + chamosite are likely coeval products of subsolidus reactions of the magmatic Al-rich schorl (TurP1) with evolved REE-poor, Li,F-rich, alkaline pegmatite-derived fluids. Well-crystalline REE-bearing epidote-group mineral (Y+REE = 0.42-0.60 apfu) confirmed by Raman spectroscopy has a steep, LREE-rich chondrite-normalized REE pattern with significant negative Eu anomaly and shows variable and high contents of Sc (<= 3.3 wt% Sc2O3) and Sn (<= 1.0 wt% SnO2). Substitution ScAl-1, and minor vacancy in the octahedral sites are suggested in the REE-bearing epidote-group mineral.

Effect of Heat Treatment on the Far-Infrared Emission Spectra and Fine Structures of Black Tourmaline

Mineral black tourmaline powders were heat-treated at different temperatures. Their crystal structure was studied by X-ray diffractometer. Their infrared absorption and emission spectra before and after the heat treatment were analyzed by the Fourier transform infrared spectrometer. The corresponding fine structures were discussed in detail. The results showed that the powders possessed higher infrared emissivity at the band where they showed stronger infrared absorption. However, there is no certain correlation between the peak intensity of infrared absorption and emissivity values at the same frequency. Because of the crystal shrinkage of c-axis, the electronic transitions were stimulated between different energy levels, and the abilities of infrared absorption and emission were enhanced with increasing the temperature of heat treatment.

Compositional Trend in the Specks of Tantalite, Tourmaline and Beryl Hosted within Complex Basement Rocks Using Geophysical and Geochemical Methods of Exploration

Vertical Electrical Sounding (VES) using Schlumberger array was employed in data collection. Data acquisition was carried out along Oyan river bank with a total of twenty-three (23) soundings. This location was divided into two zones: VES 01 -09 and VES 10 -23. Rock samples hosting specks of tantalite, tourmaline and beryl were collected from the artisan pits within the study area and taken for geochemical analysis in the laboratory. The result of geochemical analyses revealed the quality of the solid minerals in terms of mineral compositions (major, trace and rare earth elements) while geophysical field measurement revealed the resistivity values, vertical and lateral distributions and depth of the solid minerals. The range of electrical resistivity values of these solid minerals as revealed by laboratory measurement is 19.4 - 31.1Ωm while that of the host rock is 10.0 - 100.0Ωm as obtained from field measurements. Specks of tantalite, tourmaline and beryl were suspected to be sparsely distributed in other VES locations except at VES02 and 09, where vast deposits were suspected. Two litho-facies changes were observed and this revealed the economic values of these solid minerals. Black tourmalines, beryl and tantalite samples collected as specks within the study area were of low quality while pink tourmaline is of moderate quality on the basis of the chemical constituents contained.

Geochemical and geological controls on the genesis of gem-quality "Paraiba Tourmaline" in granitic pegmatites from northeastern Brazil

The electric blue Paraiba Tourmaline (PT) occurs in northeastern Brazil only in four deposits of zoned and highly fractionated granitic pegmatites of the Borborema Pegmatitic Province (BPP). These PT-bearing pegmatites are rich in spodumene or lepidolite, and are emplaced in iron-poor quartzites or metaconglomerates of the Neoproterozoic Equador Formation. Black comb-textured dravitic tourmaline in the border zone indicates crystallization from a boron-bearing granitic magma. The PT occurs at the transition between the albite-rich inner intermediate zone (IIZ) and the quartz core of the pegmatites, in primary wedge-shaped crystals of near-end-member color-zoned elbaite, arranged in fan-like, radial or comb-textured groups of crystals. The PT most commonly forms the intermediate growth-zone of color-zoned elbaite crystals having a pink to red core and a green to dark green rind. The PT-bearing pegmatites are distinguished by intensive late replacement of primary K-feldspar, spodumene, elbaite and Li-phosphates by lepidolite. The lepidolite replacement of zoned elbaite is highly selective; it begins in the core, then affects the light blue intermediate zones, and preferentially leaves intact the green rim. PT crystals or crystal zones preserved from lepidolite replacement are rare, and occur enveloped in quartz of the pegmatite core or in vug fillings. According to electron-microprobe data on color-zoned elbaite crystals, PT crystal-zones are distinguished not only by high Cu contents, but also by higher values of Li and F (mostly fluor-elbaite), Fe concentrations near zero, and lower concentrations of Mn than the contiguous zones with other colors. Increasing Fe + Mg concentrations are responsible for the green color in the rims enveloping PT, even if Cu values remain elevated. Values of Al, X □ and Mn are slightly elevated in the pink zones in comparison with PT in the same crystal. Textural relations indicate that PT formed by direct crystallization from a pegmatite-forming melt rich in B, Li and F, at the early stage of formation of the quartz core, and prior to the appearance of secondary lepidolite and other late hydrothermal minerals. The difference between PT-bearing and other elbaite-rich pegmatites may not necessarily be the failure to produce primary PT, but also the failure to preserve it from late conversion to lepidolite. The formation of PT is linked with low content of mafic minerals in the wallrock and in the pegmatite-forming melt itself.

Synkinematic growth of tourmaline on brittle-ductile normal faults, Despotiko Island, Aegean Sea, Greece

Synkinematic tourmaline and quartz growth on slickensides of brittle–ductile normal fault planes was studied by electron-microprobe analysis and X-ray crystallographic methods. The faults show top-to-the-southwest sense of shear and cross-cut mylonitic gneisses of the Cycladic Blueschist Unit on Despotiko Island, located southwest of Antiparos, in the Cyclades, Greece. The black tourmaline crystals, up to 180 μm in length and 50 μm in diameter, form slickenside fibers on these fault planes and therefore have grown during fault slip. They record a chemical variation from X (Na 0.66 Ca 0.20 □ 0.14 ) Y (Fe 2+ 1.17 Mg 0.92 Al 0.78 Ti 4+ 0.12 Mn 2+ 0.01 ) Z (Al 5.00 Mg 1.00 ) (BO 3 ) 3 T (Si 5.95 Al 0.05 )O 18 V (OH) 3 W [(OH) 0.88 F 0.12 ] to X (□ 0.50 Na 0.48 Ca 0.02 ) Y (Fe 2+ 1.75 Al 1.31 Mn 2+ 0.02 Ti 4+ 0.01 ) Z (Al 5.49 Mg 0.51 ) (BO 3 ) 3 T (Si 5.98 Al 0.02 )O 18 V (OH) 3 W (OH). The tourmaline compositions belong to the foitite – schorl – dravite series. On the basis of X □− X Mg , X □−Ca, Ca– X Mg relationships and significant chemical differences between analogous and antilogous poles, these tourmalines most likely grew at low- to medium-grade metamorphic conditions, corresponding to crystallization temperatures between 300 and 400 ± 50°C. This is consistent with the low amounts of [4] Al and F in these tourmalines, the observation of almost completely chloritized biotite near the slickensides, and the deformation microstructures of quartz in the fault. Together with structural measurements in the field, these data suggest that the tourmaline slickenside fibers crystallized during a late stage of overall NE–SW-oriented extensional exhumation, bringing the rocks from lower-greenschist facies conditions to the brittle–ductile transition zone.

Electric Dipole Characteristics of Mineral Tourmaline Superfine Powders and its Properties of Far Infrared Emission

According to the hypothesis that the tourmaline particle is composed of crystal electric dipoles, the far infrared emission and its vibration frequency of the natural black tourmaline superfine powders were researched. The behavior of the electric dipole in the ground and excitation states was calculated. It was found that there is a relationship between infrared absorption spectra and the microscopic D-value of electric dipole moment. The high far infrared emissivity of tourmaline was ascribed to the coherent light created by stimulated emission between different excitation states of electric dipole.

EMERALD MINERALIZATION ASSOCIATED WITH THE MAVIS LAKE PEGMATITE GROUP, NEAR DRYDEN, ONTARIO

The Taylor 2 (also known as Ghost Lake) emerald showing near Dryden, in northwestern Ontario, is associated with a granitic pegmatite of the Mavis Lake Pegmatite Group proximal to the 2685 Ma Ghost Lake Batholith. The Taylor 2 pegmatite consists of three separate limbs that intruded a wide zone of chlorite schist near the eastern end of an altered ultramafic sill. Most of the beryl and emerald occurs in a “zone of mixing” between the southern and central limbs of the pegmatite. The rock in the “zone of mixing” consists of relict orange K-feldspar crystals (to 30 cm) in a matrix of anhedral bluish plagioclase, quartz, fine-grained black phlogopite, blue apatite crystals (to 1 cm), and black tourmaline crystals (to 2 cm). The beryl occurs as euhedral crystals up to 2.3 × 1.8 cm; most are opaque to translucent and white to pale green in color, but about 10% are emerald. The emerald compositions show average Cr concentrations of 0.25 wt.% Cr2O3 (maximum 0.42 wt.% Cr2O3 or 0.03 Cr apfu ) and maximum V concentrations of 0.05 wt.% V2O3. Primary fluid inclusions in the emerald crystals show variable compositions and high temperatures of homogenization (to 490°C). Oxygen isotope compositions of the emerald crystals, which average 9.6(7)‰, suggest a high degree of homogenization and fluid buffering during fluid–rock interaction. Whole-rock compositions show high concentrations of Be (89 ppm) in the the Taylor 2 pegmatite and elevated Cr in the chlorite schist (2610 ppm) and the altered ultramafic sill (3050 ppm). Geochemical similarities support the hypothesis that the chlorite schist is the faulted analogue of the altered ultramafic sill. The absence of beryl in the latter unit may be due to lower amounts of fluid or F concentrations (~150 ppm versus ~1300 ppm for the chlorite schist). The emerald in the Taylor 2 showing and the associated phlogopite most likely formed through metasomatism of the chlorite schist, which was driven by granitic magmatism.

Backgrounds for the Industrial Use of Black Tourmaline Based on Its Crystal Structure Characteristics

The industrial use of black tourmaline is based on the pyroelectricity, emission of far infrared radiation, adsorption of ions, release of negative air ions, and influence on biologic activity. The crystal structure of schorl was analyzed through a virtual three dimensional hard ball model and related to these properties. Explanations for the pyroelectricity, emission of far infrared radiation, adsorption of ions, and release of negative air ions are given based on the crystal structure characteristics.

Mechanism of Far Infrared Emission from Mineral Tourmaline Fine Powders

The little disturbance theory in quantum mechanics was used to research the frequency and the high emissivity of far infrared emission from natural black tourmaline on basis of the electric dipole model. Comparing the theoritical and the experimental value of the frequency we found that they are in match case. So we testified that the tourmaline partical is really composed of lots of crystal electric dipole. At the same time we explained the curves of infrared absorption analysis of natural black tourmalines at room temperature are simillar despite of their diversification in chemistry component and producing area, and they have high infrared emissivity from the theory point of view. Finally we pointed out the condition under which the high infrared emissivity of tourmaline can be obtained theoritically.

The Ti-influence on the tourmaline color

Titanium was examined in different black tourmalines by spectrophotometric analyses in the region between 12500 and 27000 cm-1 (800-370 nm) using a microscope spectrophotometer MPV-5P, Leitz (Germany), and chemically analyzed on an electron microprobe Camebax Microbeam, Cameca (France) in WDS Mode. Fourteen tourmaline samples with their face oriented parallel to the principal axis, cut and polished down to thin sections, showed polarized spectra of two broad absorption bands at approximately 14000 cm-1 (715 nm) and 24000 cm-1 (417 nm). Precision thickness measurements were used to calculate the absorption coefficients α. For the absorption around 24000 cm-1, a linear correlation was observed between α and the TiO2 content (mass %). Another linear regression was observed between α and the product of TiO2 and FeO (%) contents. Cations located in the Y- and Z-octahedrons of the structure that are connected over edges might contribute to intervalence charge transfer transitions (IVCT) between Ti3+-Ti4+, Fe2+-Ti4+ and Ti3+-Ti4+-Fe3+.

Origin and internal evolution of the Li-F-Be-B-P-bearing Pinilla de Fermoselle pegmatite (Central Iberian Zone, Zamora, Spain)

The Li-F-Be-B-P bearing Pinilla de Fermoselle (PF) pegmatite occurs in the apical part of a leucogranite body. It shows a clear non-symmetrical vertical zoning from the contact with the leucogranite to a contact with the metamorphic country rocks. The pegmatitic facies evolve upward from (1) the undifferentiated Lower Border Zone (LBZ), with quartz, feldspars, muscovite, biotite, and black tourmaline, through (2) the Intermediate Zone (IZ), with quartz, muscovite, zinnwaldite, black tourmaline, and Fe-Mn phosphates, to (3) the highly evolved Upper Border Zone (UBZ), with quartz, albite, lepidolite, zinnwaldite, elbaite, and beryl. The composition of the pegmatite-forming minerals suggests that a residual melt become progressively enriched in F and Li until the crystallization of the apical UBZ, whereas P partitions in the melt only until the intermediate levels of differentiation attained in the IZ. Chemical variations in the mica and tourmaline as well as in the feldspar and Fe- Mn phosphate minerals are consistent with an internal evolution by crystal fractionation processes. A plausible model for the crystallization of the PF pegmatite involves a rapid, in situ, bottom-up crystallization from significantly undercooled liquids. The lack of metasomatic effects in the metamorphic host-rock and the estimated P content of the initial leucogranite melt suggest that the PF pegmatite mainly crystallized under closed system conditions.

Amazonitização em granito resultante da intrusão de pegmatitos

Borrachudos-type granite from Santa Maria do Itabira in Minas Gerais is cut by amazonite-bearing pegmatites, being marginally transformed into a green granite due to amazonitization. This paper deals with the processes and the mineralogical and chemical variations involved in the transformation of the normal granite into a green granite. The meter-wide pegmatite is composed of decimeter-large amazonite crystals of intense green color, cleavelandite, biotite, quartz and small amounts of black tourmaline, galena, fluorite and anatase. The alkaline anorogenic granite is composed of quartz, nearly pure albite, microcline with triclinicities of 0.68 to 0.93, Fe-biotite and accessory minerals. The mean SiO 2 content is 74 weight %, but in quartz-poor samples it decreases to 62%. MgO is very low (< 0.05 weight %), Fe 2 O 3total , reaches 4.4% and alkalis 8.5%. A strong negative Eu anomaly could be due to the retention of Eu in the calcic plagioclases left over in restmc rocks which generated the alkaline magma by partial melting. The intensity of the green color in the granite increases towards the pegmatite contact. Chemically this color variation is accompanied by an increase in the Pb (and to a lesser extent also in Rb) contents, while no difference could be registered for the major and other trace elements. Pb varies from 37 ppm to 302 ppm, reaching 406 ppm in the amazonite. It is possible that the subsolidus introduction of not only Pb but also of water from the pegmatite into the country rock was responsible for the generation of H,O - Pb pairs in the microcline, which are postulated in the current literature as being the cause of the green color of amazonites. Considering the relatively low temperatures of pegmatitic magmas and the massive structure of the granite the metasomatic process of amazonitization was very efficient since the aureoles locally reached a width equal to the width of the pegmatite.

Chemistry, paragenesis and significance of tourmaline in pegmatites of the Southern Tin Belt, central Namibia

Numerous pegmatite bodies occur in several distinct belts within the Damara orogen of central Namibia. Many of the pegmatites have been mined extensively for Sn, Li, Nb, Ta, Cs, mica, feldspar, and gem tourmaline and gem beryl. Tourmaline is widespread as an accessory and locally abundant mineral in different geological environments within the Damara Orogen. In pegmatites from the Southern Tin Belt (STB), it exhibits diverse habits and colours, from black to dark green, light green, and pink, as well as variations in composition depending on pegmatite type and location within the pegmatite. Electron microprobe analyses of the tourmalines show a significant chemical variation from Fe-rich compositions with moderate Al in the Y sites to Li-rich compositions with high YAl , and relatively high F. Fe/(Fe+Mg) ratios for black, green, and pink tourmalines vary from 0.65 to 0.95, 0.66 to 0.99, and 0.50 to 0.87, respectively. Na/(Na+Ca) ratios increase from Li-bearing tourmaline (0.83–0.94) to black tourmaline (0.95–0.99). Tourmaline compositions mostly plot along trends involving proton-loss and alkali-defect substitutions from the schorl additive component. These substitutions account for increases of Al in the Y site, and decreases in the X site and OH occupancies at the expense of the schorl component. Amounts of Al (Y) range from 0.23–0.63 in black tourmalines to 0.82–1.44 in green and pink tourmalines. Within individual pegmatites, the abrupt transition from black tourmaline to green tourmaline, and later pink tourmaline, reflects decreasing Fe and increasing Li and Al via the substitution LiAlFe −2. Enrichment of Li in the tourmaline is accompanied by increases of Be, Sn, Rb, Cs, and Pb. Insignificant amounts of Fe 3+ in the tourmalines suggest a low oxidation state of the pegmatite-forming fluids. As the precipitation of cassiterite requires an increase in fO 2, such conditions may account for the distribution of tourmaline and cassiterite in different pegmatite zones. Field relations, distribution, petrographic features, and tourmaline chemistry provide evidence of two pegmatitic domains in the STB: (1) a western one, characterized by pegmatites with Sn, Nb>Ta, (Li, Be) mineralization and containing Fe–Mn phosphates and black tourmaline, which may be explained in terms of simultaneous crystallization of two petrologenetic units; and (2) an eastern one characterized by pegmatites with Li>Be, (Sn, Nb>Ta) mineralization and elbaite, which formed via fractionation processes and sequential crystallization from the margins inward to the cores.

GRANITIC PEGMATITES OF KORADI-KOLAR SECTOR, NAGPUR DISTRICT, CENTRAL INDIA: FIELD, PETROGRAPHIC AND MINERALOGICAL FEATURES

Several bodies of pegmatites have been located within the rocks of Sausar Group (Precambrian) occurring in the Kolar valley, north of Koradi, in Nagpur district of Central India. The XRD and the chemical data have shown that these pegmatites are mostly potassic (K2O= 15.58 to 16.31 % and Na2O=2.50 to 3.12 %) and are barren of economic mineral deposits. The pegmatites range from a few cm to more than 50 m in width with a maximum extension of about one kilometre. Several mappable pegmatite bodies trend roughly EW and NS, and many good outcrops are encountered in the Kolar Valley and Suradevi Hills. Based on the field and petrographic studies, the pegmatites of this area are characterised in terms of size, shape and mode of emplacement. They are mostly tabular, lenticular, branching and irregular bodies, localized along bedding, foliation, fault planes, tension joints and fold axis. Structurally they can be subdivided into concordant and discordant types on the basis of their relationship with the country rocks. Mineralogically, pegmatites are grouped into (1) Simple Pegmatite, containing quartz, feldspar, biotite, apatite, garnet and (?) monazite, and (2) Complex Pegmatite comprising in addition muscovite, tourmaline and epidote. However, common mineral variation is observed at different places within the same body of pegmatite. In these pegmatites, muscovite, quartz, tourmaline and feldspar seem to have developed mainly in two distinct generations, for example, the muscovite of earlier generation forms tiny books while later crystals occur as small disseminated flakes in the groundmass of less coarser quartz. The earlier quartz is strongly deformed, while the later occurs as veins of variable dimensions. The black tourmaline of earlier generation is represented by schorlite and of later origin has been identified as dravite. It may be concluded that these pegmatites were formed by the influx of fluids rich in silica, Al and K that filled the fractures and other planes of weakness (foliation) in the host rocks. The alkali components while migrating towards the wall of the country rocks, crystallised as muscovite and feldspar. In larger pegmatite bodies of Suradevi Hills, the development and concentration of muscovite along their contact with the mica schist is probably due to the migration of Al and K and volatiles towards the border of the intrusion. This crystallization of muscovite in preference to potash-feldspar may be due to the presence of high contents of volatiles.

GRANITIC PEGMATITES OF KORADI-KOLAR SECTOR, NAGPUR DISTRICT, CENTRAL INDIA: FIELD, PETROGRAPHIC AND MINERALOGICAL FEATURES

Several bodies of pegmatites have been located within the rocks of Sausar Group (Precambrian) occurring in the Kolar valley, north of Koradi, in Nagpur district of Central India. The XRD and the chemical data have shown that these pegmatites are mostly potassic (K 2 O= 15.58 to 16.31 % and Na 2 O=2.50 to 3.12 %) and are barren of economic mineral deposits. The pegmatites range from a few cm to more than 50 m in width with a maximum extension of about one kilometre. Several mappable pegmatite bodies trend roughly EW and NS, and many good outcrops are encountered in the Kolar Valley and Suradevi Hills. Based on the field and petrographic studies, the pegmatites of this area are characterised in terms of size, shape and mode of emplacement. They are mostly tabular, lenticular, branching and irregular bodies, localised along bedding, foliation, fault planes, tension joints and fold axis. Structurally they can be subdivided into concordant and discordant types on the basis of their relationship with the country rocks. Mineralogically, pegmatites are grouped into (1) Simple Pegmatite, containing quartz, feldspar, biotite, apatite, garnet and (?) monazite, and (2) Complex Pegmatite comprising in addition muscovite, tourmaline and epidote. However, common mineral variation is observed at different places within the same body of pegmatite. In these pegmatites, muscovite, quartz, tourmaline and feldspar seem to have developed mainly in two distinct generations, for example, the muscovite of earlier generation forms tiny "books" while later crystals occur as small disseminated flakes in the groundmass of less coarser quartz. The earlier quartz is strongly deformed, while the later occurs as veins of variable dimensions. The black tourmaline of earlier generation is represented by schorlite and of later origin has been identified as dravite. It may be concluded that these pegmatites were formed by the influx of fluids rich in silica, Al and K that filled the fractures and other planes of weakness (foliation) in the host rocks. The alkali components while migrating towards the wall of the country rocks, crystallised as muscovite and feldspar. In larger pegmatite bodies of Suradevi Hills, the development and concentration of muscovite along their contact with the mica schist is probably due to the migration of Al and K and volatiles towards the border of the intrusion. This crystallization of muscovite in preference to potash-feldspar may be due to the presence of high contents of volatiles.

Evidence of Magma Mixing and Hybrid Source on Calc-Alkaline Sarihan (Bayburt) Granitoid, NE Turkey

The study area is in the South Zone of Eastern Pontides (NE Turkey). The basement is Pre-PermoCarboniferous aged Pulur Massif which contains greenschist, micaschist, para-amphibole schist, ortoamphibolite, gneiss, marble, quarzite and intruding Middle-Carboniferous aged Saraycik Granodiyorite. Liassic aged Hamurkesen Formation overlying unconformably the massif begins with Dikmetas Conglomerate and continues with volcano-sedimentary rocks containing tuff, sandstone and andesite, all of which are cut by diabase dykes. Maim-Lower Cretaceous aged Hozbirikyayla Formation overlies conformably Hamurkesen Formation and contains limestone and sandy limestones. Hozbirikyayla Formation is overlied by conformably Otlukbeli Melange containing red-limestone, pyroclastics, serpentinite, radiolarite, limestone olistoliths and breciated basalt. Campanien aged Sarihan Granitoid cuts all these lithologies. Sarihan Granitoid covering an area of km 2 contains quartz-monzodiorite (60%), granodiorite (35%) and quartz-diorite (5%), and is cut by 5 to 10 cm width aplite dykes. These rocks show similar petrographical features. In the center of intrusion, 0.5-1 cm width cracks were filled by black tourmaline. The rocks of the intrusion shows medium-grained, poikilitic, anti-rapakivi and sometimes myrmekitic textures, and contains 43-64% plagioclase, 7-15% orthoclase, 10-29% quartz, 5-20% hornblende, 1-8% biotite and 1-6% opaque oxides. Plagioclase is subhedral, mainly oligoclase and rare andesite in composition (Anls-37 Ab55-79 Or22_9). Large crystals show generally oscillatory zoning and albite twinning. Crystals with oscillatory zoning range in composition from An33 in the core to Anls in the rim. Mirmekitic textm'es developed at the contact of orthoclase. Large plagioclase may contain plagioclase, apatite, magnetite and rare hornblende inclusions.

Black Tourmaline from Little Cahuilla Mountain, Riverside County, California

(1988). Black Tourmaline from Little Cahuilla Mountain, Riverside County, California. Rocks & Minerals: Vol. 63, No. 6, pp. 440-444.

Uranium-bearing pegmatites of the antsirabe-kitsamby district, madagascar

Madagascar, sixth largest island in the world, is about 75% covered by a deep red laterite mantle explaining its sobriquet “L'île rouge” — the red island. In 1907 a Malagasy geologist discovered euxenite-bearing Tertiary lake sediments near Vinaninkarena, south of Antsirabe in Central Madagascar. Interest in radium stimulated search for primary uranium minerals in the adjacent Precambrian plateau known to contain pegmatites. Prospecting in 1908–1909 identified betafite-euxenite-and samiresite-rich pegmatites near Betafo in the Kitsamby hills and other uraniferous pegmatites were found around Ampangabe, Ambatohasana and Ambatofotsikely. 1919–1924 marked the peak of productive activity, when about 25 pegmatites were exploited: Ambatofotsy, Ambatohasana and Ambatofampykely accounting for over 50 tonnes, chiefly betafite, averaging 12–15% of U 3O 8, or about 50% of Madagascar's total uranium production. The major uraniferous pegmatites are identified by a central massive quartz core in deeply lateritized terrain. In simplest form, they consist of microcline and quartz, with or without accessory biotite, muscovite, black tourmaline, beryl, alamandine and apatite. There are two contrasting sub-types. 1. (1)Pegmatites containing columbium-tantalum-complex, rare-element “rest” or pocket minerals carrying uranium and thorium. Biotite zones contain the predominant mica, with little or no beryl or muscovite. 2. (2)Deposits rich in beryl and muscovite, with biotite subordinate or absent. This paper describes the physical setting, geology and mineralogy of four large pegmatites that accounted for a major portion of Madagascar's betafite-euxenite production, namely Ambotofotsy, Ambatohasana, Ambatolampykely and Vorondola in the Kitsamby district. A map or a section of each deposit is provided.

The relationship between optical properties and occurrence of some black tourmalines from Northern Nigeria

SummarySeventy-nine black tourmalines collected over a 12 000 km2 area of Precambrian to lower Palaeozoic rocks in northern Nigeria show a close correlation between maximum R.I., absorption colour, and occurrence in non-feldspathic micaceous schists and schorl rocks on the one hand, and in gneisses, migmatites, and feldspathic pegmatites on the other. Pleochroism is known to depend upon the total iron content and the Fe2+Fe3+ interaction. Colour variations may thus provide useful indicators of the geochemical environment during crystallization.Limited published data, supplemented by additional measurements on four tourmalines from outside the original area, suggest that similar correlations would be obtained for the rest of Nigeria.Selective processes of boron metasomatism may have been important in the association of different coloured tourmalines with feldspathic and non-feldspathic rocks, and the apparent lack of biotite in most tourmaline-rich rocks. The source of the boron probably lay within the upper Proterozoic to lower Palaeozoic metasedimentary belts with which the tourmaline is most strongly associated.

The crystal field spectra and dichroism of tourmaline

Optical absorption spectra of colorless, pink, green, blue, brown and black tourmaline have been taken to determine the origin of the colors and to explain the color changes which occur on heating. Absorption data were taken with polarized radiation parallel and perpendicular to the c axis at 77 and 300°K. Cell dimensions supported by partial chemical analyses were used to characterize the specimens. The data combined to show that generally speaking the color is due to electronic transitions in and the amount of Fe 2+, Fe 3+ and Mn 2+ in the crystal. Specimens on the dravite-sehorl join are characteristically colored shades of brown. With small to moderate concentrations of transition metal ions, colors on the elbaite-schorl join are clear pinks, greens and blues according to the Fe 2+ (Fe 2++ Fe 3+ + Mn 2+) ratio in the tourmaline. Intensity changes of absorption peaks and shifts in the absorption edges, correlated with removal of hydrogen and consequent oxidation of Fe 2+ to Fe 3+ in the structure, account for color changes observed when tourmaline is heated in air. Blue tourmaline becomes green on short-term heating and redish-brown on further heating. Assignments of absorption peaks from 0·3 to 2·0μ to specific transitions in the appropriate cations have been made and the origin of the characteristic dichroism is discussed.