Witwatersrand gold particle chemistry matches model of metamorphosed, hydrothermally altered placer deposits

Analyses of gold particles from the Ventersdorp Contact Reef in the Klerksdorp and Carletonville goldfields of the Late Archaean Witwatersrand Basin reveal a wide range in Au, Ag and Hg concentrations of 80.9 to 92.9, 6.0 to 17.6, and 0.6 to 5.8 wt.%, respectively. However, individual gold particles are generally homogeneous. This observation is predicted by modelling of Ag and Hg diffusion through gold at the peak metamorphic temperature of about 300 °C. The only exception is gold in hydrothermal quartz veins, the formation of which can be ascribed to the mobilisation of originally detrital gold particles during a chloritisation event that was triggered by tectonically expelled fluids, as a consequence of the 2023 Ma Vredefort meteorite impact event. Inter-particle homogenisation on a hand-sample scale was generally not achieved. The mean gold composition for the same reef in different mines (kilometre scale) can vary drastically. These results confirm the model of a metamorphosed and hydrothermally altered placer deposit. Furthermore they indicate that, in spite of a significant post-depositional modification of the composition of individual gold particles, the mean gold composition at a given reef locality may be used to distinguish different source areas of the originally detrital particles. Received: 3 December 1996 / Accepted: 19 May 1997     

Crystal chemistry, surface morphology and X-ray photoelectron spectroscopy of Fe-rich osumilite from Mt. Arci, Sardinia (Italy)

Microprobe analysis, single crystal X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray absorption spectroscopy were applied on Fe-rich osumilite from the volcanic massif of Mt. Arci, Sardinia, Italy. Osumilite belongs to the space group P6/mcc with unit cell parameters a = 10.1550(6), c = 14.306(1) Å and chemical formula (K_0.729)_C (Na_0.029)_B (Si_10.498 Al_1.502)_T1 (Al_2.706 Fe _0.294 ²⁺ )_T2 (Mg_0.735 Mn_0.091 Fe _1.184 ²⁺ )_AO₃₀. Structure refinement converged at R = 0.0201. Unit cell parameter a is related to octahedral edge length as well as to Fe²⁺ content, unlike the c parameter which does not seem to be affected by chemical composition. The determination of the amount of each element on the mineral surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si_2p, Al_2p, Mg_1s and Fe_2p core levels, suggests that Fe presents Fe²⁺ oxidation state and octahedral coordination. Two peaks at 103.1 and 100.6 eV can be related to Si⁴⁺ and Si¹⁺ components, respectively, both in tetrahedral coordination. The binding energy of Al_2p, at 74.5 eV, indicates that Al is mostly present in the distorted T2 site, whereas the Mg peak at 1,305.2 eV suggests that this cation is located at the octahedral site. X-ray absorption at the Fe L_2,3-edges confirms that iron is present in the mineral structure, prevalently in the divalent state and at the A octahedral site.     

U-Th-Pb systematics in hydrothermally altered granites from the Granite Mountains,Wyoming

U-Th-Pb systematics were investigated in 15 samples representing two types of deuterically altered Archean granite (albitized and silicified-epidotized granite) from the Granite Mountains, Wyoming. The loss of K-feldspar during both types of deuteric alteration was accompanied by an extreme reduction of Pb content from roughly 40 ppm to less than 12 ppm in the most altered samples. Nine of the 15 samples yield anomalously young whole-rock Pb-Pb and Th-Pb ages compared to concordia ages for zircons and to whole-rock Pb-Pb and Th-Pb ages for samples of unaltered granite. The young ages are interpreted to be the result of radiogenic Pb loss during a middle Proterozoic metamorphism that disturbed several isotopic systems in the unaltered granite. The loss of radiogenic Pb from the whole-rock systems of the deuterically altered granites is most likely due to the absence of microcline. In many granitic rocks, potassium feldspar tends to act as a receptor for Pb that has been mobilized, and this effect may account for the closed-system behavior of Pb in whole-rock samples of the unaltered granite. It may also account for the apparent gain of radiogenic Pb during the Proterozoic by one sample which was collected at the edge of an alteration zone. The deuterically altered granites may have also lost U during the Proterozoic, but, except for two samples, the dominant U loss occurred relatively recently, probably during Tertiary uplift and erosion.The low common-Pb contents and rather high U and Th contents of the hydrothermally altered granites might seem to indicate that these rocks are well suited for geochronologic studies in the U-Th-Pb system because such rocks should have a high percentage of radiogenic Pb. Unfortunately, open-system behavior in response to post-crystallization metamorphism shows that apparent ages for these types of rocks must be interpreted with caution.     

The mineral chemistry of hydrothermally altered and metamorphosed wall-rocks at the Stollberg Fe-Pb-Zn-Mn(-Ag) deposit,Bergslagen, Sweden

The c. 1.9 Ga old Stollberg sulphide and Mnrich skarn iron ores and sulphide ores in Bergslagen, south-central Sweden are hosted by hydrothermally altered and metamorphosed felsic volcanic and volcaniclastic rocks. The ores are underlain by comformable alteration zones characterized by albite-gedrite-quartz and biotite-muscovite-plagioclase-K-feldspar-quartz +/– garnet assemblages. The present mineralogies are interpreted as medium-grade metamorphic equivalents to the original alteration mineral assemblages. PT-conditions during prograde regional metamorphism are semiquantatively determined to be 510 to 560 °C at approximately 3 kbar. With increasing modal content of gedrite and biotite in the alteration zones, the Mg/Fe ratios and X_Mg's in octahedral positions of these minerals also increase. In the gedrite-bearing strata, whole-rock Mg/Fe ratios remain constant, whereas in the biotite-rich unit the wholerock Mg/Fe trend is parallel to that of the biotites.The trends in the metamorphic mineral composition are interpreted to be a product of original changes in fluid composition during the evolution of a sub-seafloor hydrothermal system. During the initial stage of alteration, Fe-Mn-rich fluids altered the rocks, and during a later stage, the fluids became more Mg-rich, possibly due to entrainment of fresh seawater, and the alteration zones became relatively more Mg-rich. Sulphide precipitation was contemperaneous with Mg metasomatism, suggesting base metal precipitation was a function of the mixing of cool seawater with hydrothermal fluid. It is proposed that early hydrothermal alteration was associated with the deposition of areally extensive Fe-oxide formation, and that Mg metasomatism defines a second stage of hydrothermal activity during which sulphide mineralization overprinted the earlier formed Fe-oxide deposit.     

Mobilization and speciation of arsenic from hydrothermally altered rock containing calcite and pyrite under anoxic conditions

The effects of water residence time and anoxic conditions on the mobilization and speciation of As in a calcite- and pyrite-bearing altered rock excavated during a road-tunnel project has been evaluated using batch and column laboratory experiments. Higher infiltration rates (i.e., shorter water residence times) enhanced the leaching of As due to the higher pH values of the effluents and more rapid transport of dissolved As through the columns. The concentration of As in the effluent also increased under anoxic conditions regardless of the water residence time. This enhanced leaching of As under anoxic conditions could be attributed to a significant pH increase and decreased Fe oxyhydroxide/oxide precipitation compared to similar experiments done under ambient conditions. Processes that controlled the evolution of pH and the temporal release mechanisms of As under anoxic conditions were identical to those previously observed under ambient conditions: the dissolution of soluble phases, pyrite oxidation, co-precipitation and/or adsorption/desorption reactions. Speciation of As in the column experiments could partly be attributed to the pH-dependent adsorption of As species onto Fe oxyhydroxide/oxide precipitates. Moreover, apparent equilibrium of the total As and As[III] concentrations was delayed under anoxic conditions in both batch and column experiments.     

Reaction pathways and reaction progress for the smectite-to-chlorite transformation: evidence from hydrothermally altered metabasites

The transformation from smectite to chlorite has been interpreted as involving either a disequilibrium chlorite/smectite mixed‐layering sequence, or an equilibrated discontinuous sequence involving smectite–corrensite–chlorite. Here, analysis of the smectite to chlorite transition in different geothermal systems leads us to propose that the transformation proceeds via three contrasting reaction pathways involving (i) a continuous mixed‐layer chlorite/smectite series; (ii) a discontinuous smectite–corrensite–chlorite series and (iii) a direct smectite to chlorite transition. Such contrasting pathways are not in accord with an equilibrium mineral reaction series, suggesting that these pathways record kinetically controlled reaction progress. In the geothermal systems reviewed the style of reaction pathway and degree of reaction progress is closely correlated with intensity of recrystallization, and not to differences in thermal gradients or clay grain size. This suggests a kinetic effect linked to variation in fluid/rock ratios and/or a contrast between advective or diffusive fluid transport. The mode of fluid transport provides a means by which the rates of dissolution/nucleation/growth can control the reaction style and the reaction progress of the smectite to chlorite transition. Slow rates of growth are linked to the first reaction pathway involving mixed‐layering, while increasing rates of growth, relative to nucleation, promote the generation of more ordered structures and ultimately lead to the direct smectite to chlorite transition, representative of the third pathway.     

Mobilization and speciation of arsenic from hydrothermally altered rock in laboratory column experiments under ambient conditions

This paper describes the mobilization and speciation of As found in hydrothermally altered rock under oxic column conditions. The altered rock sample was obtained from a tunnel project located in the Nakakoshi area of Hokkaido, Japan, whose geology is represented by slate, shale and sandstone. This area has undergone silicification, pyritization and argillic alteration resulting in As-enrichment of the rock. Results of the column experiments show that the infiltration rate, bulk density and rock bed thickness affected the duration of water residence, which in turn influenced the pH of the rock–water system. Coexisting ions most notably Ca²⁺ at amounts greater than ca. 50 mg/L retarded the mobilization of As. Mobilization of As from the rock with time occurred in two stages: stage 1 (weeks 1–20) with higher As leaching and stage 2 (weeks 20–76) characterized by nearly constant As release. In addition, pore water As concentrations revealed that the columns developed into two regions: the top half where most of the leaching occurred and the bottom part dominated by adsorption. Thus, the mechanism controlling the mobilization of As from the rock is a combination of one or more of the following processes: dissolution of soluble As-bearing fractions, pyrite oxidation and adsorption reactions. Arsenite (As[III]) was the dominant species in the effluent at the start of the experiment in columns with shorter water residence time and lower pH conditions (<8). On the other hand, arsenate (As[V]) was the major inorganic species released from the rock at higher pH (8–9.5) and when the system was close to equilibrium. Speciation of As with depth also indicated that As[III] disappeared around the bottom half of the columns, probably as a result of adsorption and/or oxidation. Arsenic speciation is partially controlled by the pH dependent adsorption of As species. The important adsorbent phases in the rock included Fe–Al oxides/oxyhydroxides, clay minerals and organic matter, which permitted the columns to attenuate additional As loadings including As[III]. Implications of these results on the design of a novel disposal method for these altered rocks include the enhancement of As adsorption through the addition of natural or artificial adsorbents and the utilization of a covering soil with low permeability to minimize rainwater infiltration into the rock.     

Phyllosilicates in hydrothermally altered basalts from DSDP Hole 504B,Leg 83 — a TEM and AEM study

Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or ohvine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from< 100 Å to a few hundred Å. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystal-linity of saponite. by contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of Å thick. The Si/(Si+Al) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe+Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+Al) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite±mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.Contribution No. 488 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan     

Occurrence of pumpellyite in hydrothermally altered basalts from the Vema fracture zone (mid-Atlantic ridge)

Metabasalts with abundant pumpellyite have been dredged in the Vema fracture zone, Atlantic ocean, and contain prehnite+pumpellyite±epidote+chlorite+white mica. The prehnite — pumpellyite association in these rocks differs from the prehnite-epidote association for most of the prehnite — pumpellyite facies metabasalts from the ocean crust described previously. The occurrence of pumpellyite is discussed in terms of temperature conditions, and oxygen fugacity and the pumpellyiterich metabasalts are believed to be recrystallized by hydrothermal circulation of seawater at about 250° C under a very low pressure (<1 kb).The bulk composition of the rocks demonstrates a strong chemical modification during hydrothermal metamorphism, similar to what is observed under greenschist facies conditions, except for potassium which can be uptaken from seawater by the rocks.     

Crystal chemistry of a clinopyroxene series in ultrafemic xenoliths from North-Eastern Brazil

A series of clinopyroxenes (Cpx) in peridotitic spinel nodules from Rio Grande do Norte, North-Eastern Brazil, was investigated by X-ray diffraction and electron microprobe analyses and compared with an analogous series from spinel peridotite nodules from Mt. Leura, Victoria, Australia. The examined Cpx series is mainly characterized by increasing Mg/(Mg+Fe²⁺) ratio (i.e. increase of refractory character of the host nodules), by the substitutions of Al^vi vs Mg in the M1 site and Na vs (Ca+Mg) in the M2 site, whereas Al^iv in the T site remains substantially constant. The strong Na depletion in the Brazilian Cpx causes a different charge balance from that in the Australian Cpx, in which Na remains quite constant. This feature, associated with the differences in polyhedral and cell volumes which are significantly higher in the Brazilian Cpx than in those of the Australian Cpx, suggests that the Brazilian Cpx crystal chemistry may be related to a lower pressure regime, compared with that of the Australian Cpx series.     

Age and geochemistry of Karoo dolerite dykes from northeast Botswana

The Botswana Dyke Swarm is a prominent 800 km long and 100 km wide feature on aeromagnetic maps of southern Africa, but little has been published on its exact age or geochemical composition. New age, trace element and isotope data for this dyke swarm show that is magmatism is indistinguishable from Karoo continental flood basalts. Ar/Ar dating gives an age of 178.9 ± 1.4 Ma. Both high Ti-Zr and low Ti-Zr dolerites occur, but the high Ti-Zr rocks appear to be the dominant type of magmatism. Low Ti-Zr mafic rocks have isotopic and trace element characteristics similar to a combination of a normal mid-ocean ridge basalt source with sedimentary and fluid-enriched components, which are thought to reside in the sub-continental lithospheric mantle. A lithospheric component seen in the high Ti-Zr mafic rocks is similar to that in nephelinites from Zimbabwe.     

Fluid inclusion studies of hydrothermally altered archaean granites around the Witwatersrand Basin

Summary Hydrothermally altered granitic rocks occur along the northern and northwestern edge of what is classically termed the Witwatersrand Basin. Pyrite, chalcopyrite, sphalerite, molybdenite, galena, wurtzite and other sulphides were deposited during this hydrothermal alteration, as were uranium and REE-rich nodules of carbonaceous matter and free gold. Heating and freezing data from secondary fluid inclusions in igneous quartz as well as primary fluid inclusions in vein quartz and carbonate indicate that two main groups of aqueous fluid inclusions exist. The first group has a range of final melting temperatures from 0 °C to –9 °C, corresponding to salinities between 0 and 13 equivalent wt.% NaCl. Homogenization occurred at temperatures between 130 °C and 230 °C. The second group of inclusions generally have final melting temperatures between –14 °C and –26 °C, with salinities ranging between 12 and 30 equivalent wt.% NaCl. Homogenization temperatures range from 120 °C to about 170 °C. The low initial melting temperatures of -60°C to –35°C and SEM-EDX analyses of encrustations formed after evaporation of fluid in opened inclusions indicate as additional components Ca, Cl and S. Rare clathrate melting in both types of fluids indicate the presence of CO₂, CH₄ or some other clathrate compound. The low salinity fluids are interpreted to be of a meteoric, seawater or metamorphic origin, whereas the highly saline fluids are thought to be connate brines or highly evolved formation waters.Zusammenfassung Hydrothermal veränderte granitische Gesteine kommen am nördlichen und nordwest-lichen Rand von dem vor, was man klassisch als Witwatersrand-Becken bezeichnet. Während dieser hydrothermalen Umwandlung wurden Pyrit, Kupferkies, Zinkblende, Molybdänglanz, Bleiglanz, Wurtzit und andere Sulfide abgesetzt, ebenso Uran- und SEE-reiche Knollen aus kohliger Substanz und Freigold. Erhitzungs- und Ausfrierdaten von sekundären Fluideinschlüssen in Gesteinsquarz, ebenso wie von primären Fluideinschlüssen in Gangquarz und Karbonat weisen darauf hin, daß zwei Hauptgruppen von wäßrigen Fluideinschlüssen existieren. Der Bereich der finalen Schmelztemperaturen der ersten Gruppe liegt zwischen 0 °C und –9 °C, was einer Salinität zwischen 0 und 13 äquiv. Gew.-% NaCl entspricht. Homogenisierung erfolgte bei Temperaturen zwischen 130 °C und 230 °C. Die zweite Gruppe von Einschlüssen hat im allgemeinen finale Schmelztemperaturen zwischen –14 °C und –26 °C, mit Salinitäten, die sich zwischen 12 und 30 äquiv. Gew.-% NaCl bewegen. Die Homogenisierungstemperaturen variieren von 120 °C bis ungefähr 170 °C. Die niedrigen initialen Schmelztemperaturen von –60 °C bis –35 °C und SEM-EDX-Analysen von Inkrustationen, die sich nach der Verdunstung der Flüssigkeit in geöffneten Einschlüssen bilden, weisen auf Ca, Cl und S als weitere Bestandteile. Gelegentliches Clathratschmelzen in beiden Typen von Fluiden zeigt die Anwesenheit von CO₂, CH, und einigen anderen Clathratbildnern. Die niedrigsalinaren Fluide werden als von meteorischem, Seewasser oder metamorphem Ursprung gedeutet, während die hochsalinaren Fluide als con.nate brines oder sehr gereifte Formationswässer angesehen werden.

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Studien an Fluideinschlüssen hydrothermal veranderter archaischer Granite um das Witwatersrand-Becken

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With 8 Figures     

Hydrothermally altered dolerite dykes in East Greenland: implications for Ca-metasomatism of basaltic protoliths

Dolerite dykes that cut Tertiary baslats near the Skaergaard intrusion, East Greenland, are extensively altered to metasomatic assemblages indicating large scale mobilisation of calcium and alkalis. The alteration is characterised by replacement of the dolerite by prehnite together with lesser amounts of epidote, amphibole, salite, titanite, calcite and chlorite, resulting in a complete loss of primary igneous texture. A related type of alteration in the same dykes consists of albite, epidote, amphibole and chlorite with only partial loss of primary texture. Textural relations indicate that the albitic alteration occurs first and is progressively overprinted by the later CaAl-silicate dominated alteration with a consequent large addition of calcium and removal of sodium. Quartz is absent at all stages of alteration. The metasomatic reactions are confined to the centres of a group 10 to 20 metres wide, compound dykes, that were intruded late relative to the majority of Tertiary dykes in this part of East Greenland. These dykes contain abundant leucocratic segregations, the pegmatitic and drusy nature of which suggest volatile pressure increased at late stages in dyke cooling, possibly leading to an early episode of autometasomatism. The later CaAl-silicate alteration is localised around these segregations, but affects the surrounding dolerite. Hydrogen isotope data indicates that the fluids responsible for precipitation of the CaAl-silicates were meteoric, and derived from the surrounding basalt-hosted hydrothermal system. The physical attributes of the dykes, including their width, compound nature, and high initial porosity, are all factors that favoured the influx of surrounding pore fluids into the dyke centres, where they became isolated and reacted to form the CaAl-silicate assemblages. Fluid flow into the dyke centres may have been characterised by fluid movement orthogonal to isotherms. Changes in the relative concentrations of aqueous species in fluids coexisting with albite and CaAl-silicates as a function of temperature along such flowlines are considered an important factor in driving the observed Ca–Na exchange.     

Crystal chemistry of Fe-Mg-carpholites

A chemical and X-ray investigation of the Fe-Mg-carpholites of Crete/Greece reveals a large variation in their Fe²⁺/(Fe²⁺+Mg) ratios (0.68 to 0.05). The lattice constants a and b depend linearly on this parameter, and the magnitude of the effect is comparable to that encountered in pyroxenes. The similar behaviour of the lattice dimensions of both these minerals is explained by comparing their structures and pointing out the salient crystal chemical similarities.     

Crystal chemistry and stability of petalite

Mineralogy and Petrology - New data confirm that most petalites are slightly Li-deficient, Al-excessive and (OH)-bearing with respect to the idealized formula LiAlSi4O10. Primary substitution or...     

Crystal chemistry and optics of bazzite from Furkabasistunnel (Switzerland)

Summary The crystal structure of blue bazzite (space group P6/mcc, Z = 2, = 9.501(1),c = 9.178(1) Å) with the composition Be₃(Sc_1.25Fe³⁺ _0.43Mg_0.32Mn_0.03Al_0.02)_=2.05 Si₆O₁₈[Na_0.32(n H₂O)] was refined from X-ray single-crystal data with 422 unique reflections to R = 2.2%. The structure is of the beryl type with octahedra strongly compressed parallel to the c-axis. The octahedral Me-O distance in bazzite is 2.080 A compared to 1.904 Å in beryl. The flattening of octahedra leads to a larger a cell dimension in bazzite compared to beryl (9.209 A). Two-valent cations (mainly Mg) in octahedral coordination are charge balanced by Na at (0, 0, 0) in the structural channels. Polarized single-crystal IR-spectra recorded between 400 and 8000 cm^–1 indicate that H₂O in the structural channels is oriented with the H-H vector perpendicular to thec-axis. The IR-spectra show more absorption bands than known for type II H₂O in beryl thus the existence of more than one H₂O species or even OH-groups is very likely.Refractive indices of the same bazzite were measured using a spindle-stage and employing the wavelength-temperature variation method yielding n_o = 1.6279(3), n_e = 1.6066(5) for 589 nm at 25°C. The birefringence = 0.0213 is significantly larger than the one of near end-member beryl ( = 0.0047). This high birefringence of bazzite is related to the electronic polarizability of octahedral Sc and Fe which increase n_o at a stronger rate than n_e. Transition metals in bazzite are also responsible for a higher refractive index dispersion than found for beryl.

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Kristallchemie und optische eigenschaften des bazzits aus dem Furkabasistunnel

Zusammenfassung Die Kristallstruktur eines blauen Bazzits (Raumgruppe P6/mcc, Z = 2,a = 9.501(1),c = 9.178(1) Å) mit der Zusammensetzung Be₃(Sc_1.25Fe³⁺ _0.43Mg_0.32Mn_0.03Al_0.02)_2=2.05 Si₆)O₁₈[Na_0.32(n H₂O)] wurde von Röntgeneinkristalldaten mit 422 Reflexen zu R = 2.2% verfeinert. Die Struktur ist die des Berylltyps mit der Besonderheit, daß die Oktaeder entlang der c-Achse stark zusammengedrückt sind. Die oktaedrischen Me-O Abstände im Bazzit betragen 2.080 Å, im Gegensatz zu 1.904Å im Beryll. Die Stauchung der Oktaeder im Bazzit führt zu einer längeren a-Achse als im Beryll (9.209 Å). Zweiwertige Kationen (vor allem Mg) in oktaedrischer Koordination werden durch Na auf (0, 0, 0), in den strukturellen Kanalen, neutralisiert. Polarisierte Einkristall-IR-Spektren zwischen 400 and 8000 cm^–1 zeigen, daß H₂O Moleküle in den strukturellen Kanalen mit dem H-H Vektor senkrecht zur c-Achse orientiert sind. Die IR-Spektren weisen außerdem mehr Absorptionsbanden auf, als für den H₂O Typ II bekannt sind, sodaß entweder mehr als eine H₂O Spezies oder sogar OH-Gruppen sehr wahrscheinlich sind.Brechungsindizes am gleichen Bazzit wurden auf einem Spindeltisch mit der Wellenlängen-Temperatur Variationsmethode gemessen und ergaben n_o = 1.6279(3), n_e = 1.6066(5) bei 589 nm und 25 °C. Die Doppelbrechung A = 0.0210 ist deutlich höher als für einen nahezu Endgliedberyll (A = 0.0047). Diese hohe Doppelbrechung des Bazzits wird durch die elektronische Polarisierbarkeit von oktaedrischem Sc und Fe verursacht, die n_o starker ansteigen läßt als n_e. Übergangselemente im Bazzit sind auch für die höhere Dispersion der Brechungsindizes als im Beryll verantwortlich.

     

Rare earth element abundances and distribution patterns in hydrothermally altered basalts: Experimental results

Oceanic tholeiites that have been experimentally reacted with seawater at 500–600° C, 800–1000 bars, have rare earth element (REE) abundances and distribution patterns that are essentially identical to those of the unaltered basalts. Although the data indicate the possibility of some REE leaching and redistribution, the net effects are small and the REE patterns of basalts affected by submarine hydrothermal processes may still be valid petrogenetic indicators.     

Rb-Sr ages of Precambrian dolerite and alkaline dikes,southeast Mysore state,India

Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.