Iron In Biotite Research Articles

Revised Ti-in-biotite geothermometer for ilmenite- or rutile-bearing crustal metapelites

In the present study, the Ti-in-biotite geothermometer was revised using more than 300 natural rutile- or ilmenite-bearing metapelites collected worldwide. The formulation was empirically calibrated as ln[T(°C)]= 6.313+0.224ln(XTi)–0.288ln(XFe)–0.449ln(XMg)+0.15P(GPa), with Xj=j/(Fe+Mg+AlVI+Ti) in biotite, assuming ferric iron content of 11.6mol% of the total iron in biotite. This thermometer is consistent with the well-calibrated garnet–biotite thermometer within error of ±50 °C for most of the calibrant samples and can successfully distinguish systematic temperature changes of different metamorphic zones in both prograde and inverted metamorphic terranes as well as thermal contact aureoles. Thus, the thermometer truthfully reflects real geologic conditions and can be applied to TiO2-saturated metapelites metamorphosed at the crustal level within the calibration ranges (450–840 °C, 0.1–1.9GPa, XTi = 0.02–0.14 in biotite).

The role of decarbonization and structure in the Callie gold deposit, Tanami Region of northern Australia

The Callie deposit is the largest (6.0 Moz Au) of several gold deposits in the Dead Bullock Soak goldfield of the Northern Territory’s Tanami Region, 550 km northwest of Alice Springs. The Callie ore lies within corridors, up to 180 m wide, of sheeted en echelon quartz veins where they intersect the 500-m-wide hinge of an ESE-plunging F1 anticlinorium. The host rocks are the Blake beds, of the Paleoproterozoic Dead Bullock Formation, which consist of a > 350-m-thick sequence of lower greenschist facies graphitic turbidites and mudstones overlying in excess of 100 m of thickly bedded siltstones and fine sandstones. The rocks are Fe-rich and dominated by assemblages of chlorite and biotite, both of which are of hydrothermal and metamorphic origin. A fundamental characteristic of the hydrothermal alteration is the removal of graphite, a process which is associated with bleaching and the development of bedding-parallel bands of coarse biotite augen. Gold is found only in quartz veins and only where they cut decarbonized chloritic rock with abundant biotite augen and no sulfide minerals. Auriferous quartz veins differ from barren quartz veins by the presence of ilmenite, apatite, xenotime, and gold and the absence of sulfide minerals. The assemblage of gold–ilmenite–apatite–xenotime indicates a linked genesis and mobility of Ti, P, and Y in the mineralizing fluids. Geochemical analysis of samples throughout the deposit shows that gold only occurs in sedimentary rocks with high FeO/(FeO+Fe2O3) and low C/(C+CO2) ratios (> 0.8 and < 0.2, respectively). This association can be explained by reactions that convert C from reduced graphitic host rocks into CO2 and reduce ferric iron in the host rocks to ferrous iron in biotite and chlorite. These reactions would increase the CO2 content of the fluid, facilitating the transport of Ti, P, and Y from the host rocks into the veins. Both CO2 and CH4 produced by reaction of H2O with graphite, effervesced under the lower confining pressures in the veins. This would have partitioned H2S into the vapor phase, destabilizing Au–bisulfide complexes; the loss of CO2 and H2S from the aqueous phase caused precipitation of gold, ilmenite, apatite, and xenotime. It is proposed that this process was the main control on gold precipitation. Oxidization of iron in the very reduced wall rocks, resulting in reduction of the fluid, provided a second mechanism of gold precipitation in previously decarbonized rocks, contributing to the high grades in some samples. Although sulfide minerals, especially arsenopyrite, did form during the hydrothermal event, host rock sulfidation reactions did not play a role in gold precipitation because gold is absent near rocks or veins containing sulfide minerals. Sulfide minerals likely formed by different mechanisms from those associated with gold deposition. Both the fold architecture and subsequent spatially coincident sinistral semibrittle shearing ensured that the ore fluids were strongly focused into the hinges of the anticlines. Within the anticlines, a reactive cap of fine-grained, graphitic, reduced Fe-rich turbidites above more permeable siltstones and fine sandstones impeded fluid flow ensuring efficient removal of graphite, and the associated effervescence of CO2 from the fluid caused the precipitation of gold. Exploration for similar deposits should focus on the intersection of east–west shear zones with folds and Fe-rich graphitic host rocks.

Quantitative determination of the oxidation state of iron in biotite using X-ray photoelectron spectroscopy: I. Calibration

X-ray photoelectron spectroscopy (XPS) analyses of Fe(III)/ΣFe were calibrated with nine single crystals of biotite of known Fe(III)/ΣFe content. Peak shape parameters for the component Fe 2+ and Fe 3+ Fe 3p peaks were obtained by a constrained least squares fitting method that minimized the difference between Fe(IIl)/EFe determined by XPS and wet chemistry /electron microprobe (WCEM) analyses. Fe 2+ and Fe 3+ peak separation was estimated from the separation of minima in the second derivative of Fe 3p spectra. The single set of derived peak parameters yielded a good linear correlation ( r = 0.87) between XPS and WCEM values over the sample range of 2–20% Fe(RI)/ΣFe. Target transformation factor analysis of a time series of spectra for a sample displaying progressive oxidation during XPS analysis resulted in Fe 2+ and Fe 3+ component peak shapes largely consistent with the constrained least squares fitting method. Peak asymmetry for the Fe 3+ component was the only parameter that differed for the two methods. Beam damage, which appeared to be restricted to three single crystals with low ΣFe, low Fe/Mg, and high Fe(II)/ΣFe, caused increases in Fe(III)/ΣFe that were proportional to the duration of sample exposure.

Chromium sorption by phlogopite and biotite in acidic solutions at 25°C: Insights from X-ray photoelectron spectroscopy and electron microscopy

Large single crystals of biotite and near endmember phlogopite were reacted with acidic Cr(VI)-and Cr(III)-bearing solutions, at 24–27°C and 1 atm. After reaction of the micas with Cr(VI) solutions, XPS analyses of biotite and phlogopite edges indicated little or no chromium on phlogopite but elevated concentrations of chromium on biotite. The binding energies for Cr2 p are consistent with a Cr(III) species. XPS and SEM of biotites reacted with Cr(VI) solutions indicate that chromium is strongly sorbed by the edges of mica books relative to the basal plane. In fact, much of the chromium associated with the basal plane is sorbed by steps and micron-sized biotite flakes, or, in other words, by layer edges. TEM and SEM of biotite basal planes and edges show no Cr-rich precipitates. After reaction of the micas with a Cr(III) solution at pH = 3, XPS shows similar concentrations of a Cr(III) species on both biotite and phlogopite, and on the edges and basal planes. These results stand in strong contrast to the results from Cr(VI) solutions. XPS indicates that near-surface iron in biotite is more oxidized after reaction with Cr(VI)-bearing relative to Cr(III)-bearing solutions. A comparison of biotites and phlogopites from the Cr(VI) and Cr(III) experiments provides convincing evidence for coupled sorption and reduction of Cr(VI) aq at the biotite edge-fluid interface. Experiments with additions of potassium and sodium perchlorate confirm the importance of potassium in the heterogeneous coupled cation-electron transfer reaction. Initial observations indicate that biotite concentrated Cr(III) more strongly in Cr(VI) solutions than in the Cr(III) solution, at pH = 3 and ionic strength ≈ 10 −3 M, despite the fact that there were orders-of-magnitude more Cr(III) aq in the Cr(III) solution than in the Cr(VI) solutions.

Well Preserved Late Precambrian Paleosols from Northwest Scotland

ABSTRACT Two paleosol profiles were developed side by side on gneiss (Sheigra clay paleosol) and amphibolite (Staca clay paleosol) along the same unconformable contact of Lewisian metamorphic rocks (2.7-1.7 Ga) overlain by Torridonian sandstones (0.8 Ga) in northwestern Scotland. Although altered during burial by compaction and illitization, these profiles are well preserved compared with most other Precambrian paleosols that are metamorphosed to at least greenschist facies. In addition, deformation of pegmatite veins in a manner similar to soil creep can be taken as an indication that upper horizons of these paleosols have been preserved. Soil development in both profiles was characterized by the oxidation of iron in biotite and magnetite, and by hydrolysis of plagioclase, orthoclase, and hornblende. Clay skins, soil-like microfabrics, and pedogenic smectite have been preserved in the profiles. Alteration of silicate grains other than quartz, microcline, and biotite has been intense, but chemical analyses failed to show dramatic removal of alkalis and alkaline earths from the profiles. Microscopic concretions similar to pedogenic carbonate were found in subsurface horizons of the Staca clay. Rare-earth elements are more abundant within the paleosols than in their parent materials. These paleosols developed on a well drained landscape that was buried by Torridonian alluvial fans. Atmospheric oxidation was greater than revealed by studies of Archean paleosols, but probably still short of modern levels. Paleoclimate was subhumid and probably seasonal. Organic carbon and its light isotopic composition in the paleosol is evidence for photosynthetic microbial crusts. Paleosols of such great antiquity do not fit comfortably within modern soil taxonomies, and are for the moment assigned to the informal category of Green Clays.

Laumontite mineralization in rocks exposed north of San Andreas Fault at Cajon Pass, southern California

Laumontite of hydrothermal origin occurs in veins and as a replacement of the anorthite component of plagioclase in granitic and gneissic basement and overlying arkosic sandstone and conglomerate of the San Francisquito(?), Vaqueros and Cajon formations exposed in Cajon Pass north of the San Andreas fault. Accompanying alteration includes exsolution of albite from plagioclase, conversion of orthoclase to microcline and perthite, and oxidation of iron in biotite and magnetite to form a disseminated hematite stain. These changes are pervasive within 1 km of the fault but diminish to no alteration at 4 km. Granitic rocks close to the fault are crushed to highly fractured and contain a mosaic of laumontite veins. Veins occur along joints and minor faults in sedimentary rocks. Most veins are nearly vertical and trend northwest at a more northerly angle than the San Andreas. This mineralization is attributed to deep circulation of ground water in fractured and frictionally heated rocks along the San Andreas fault.

Oxidation of Iron in Biotite by Different Oxidizing Solutions at Room Temperature

AbstractBiotite samples (10–20 µm lepidomelane) were mixed with solutions that contained various oxidants (O2, NaOCl, H2O2, or Br2) and salts (NaCl, KCl, NaCl‐NaBPh4, or none) to compare the effectiveness with which these solutions oxidize structural Fe2+ at 25°C when customary procedures and mica samples at different stages of expansion are used. In general, the treatments that minimized interlayer K exchange did not oxidize detectable amounts of Fe2+, whereas those causing mica expansion oxidized equivalent fractions of the Fe2+ if they were given enough time. The rates of Fe2+ oxidation by the O2 and NaOCl‐O2 solutions were very slow in even the expanded mica and were similar enough to suggest the OCl‐ ions did not contribute to the process under these conditions. The Br2 and H2O2 treatments, on the other hand, oxidized 97 and 85% of the structural Fe2+ in a fully expanded mica, respectively, in a 24‐h period. The relative effects of Br2 and H2O2 solutions probably reflect differences in the oxidants themselves but are confounded by the low pH of the Br2 solution, NaBPh4 interactions and H2O2 decomposition. Indeed, all the results make it clear that more distinction needs to be made between the role of the oxidants and the side efects of the solutions and procedures that are used in oxidation experiments.

Biotite Alteration in Deeply Weathered Granite. I. Morphological, Mineralogical, and Chemical Properties

AbstractBiotite in deeply weathered granitic rocks in southwestern Australia has altered to exfoliated grains composed of biotite, mixed-layer clay minerals, kaolinite, vermiculite, gibbsite, goethite, and hematite. Discrete vermiculite and vermiculite-dominant mixed-layer clay minerals are not major weathering products. Oxidation of octahedral iron in biotite is associated with ejection of octahedral cations, loss of interlayer K, and a contraction of the b-dimension of the biotite sheet. Si, Mg, Ca, Mn, K, and Na are lost from biotite during weathering, and Ti, Al, Ni, and Cr are retained. Fe and water have been added to the grains during weathering. Much Fe occurs as aggregates of microcystalline, aluminum-rich goethite particles on flake surfaces and within etchpits, with smaller amounts occurring as hexagonal arrangements of lath-shaped crystals of goethite on flake surfaces.

The Oxidation of Octahedral Iron in Biotite

AbstractOxidation of octahedral ferrous iron in biotite by saturated bromine water results in a loss of both octahedral and interlayer cations. The hydroxyl adjacent to vacant octahedral cation sites adopt an inclined orientation resulting in a more stable environment for interlayer cations. The only structural change accompanying these processes is a decrease in b-axis dimension which is linearly related to octahedral ferric iron content. These findings are in agreement with observations made on naturally weathered biotites.

Geochemistry of wall-rock alteration and the role of sulfurization in the formation of the Ore Knob sulfide deposit

Chemical analyses of the wall rock gneiss at Ore Knob reveal that iron decreases toward the ore body. Biotite, the principal iron-bearing mineral in the wall rock gneiss, decreases in iron content and abundance toward the ore. Along a given drill core there is a correlation between the percentage of iron in biotite and in the total rock, and the thickness of the ore body; a similar correlation exists between the N (sub gamma ) index of biotite and the thickness of ore.Biotite averages only 8 ppm of copper, and the amount of copper in biotite from drill core samples does not show a significant increase or decrease toward the ore. Chalcopyrite and sulfur, however, increase toward the ore. The distributions of copper and sulfur in the wall rock gneiss support the idea of introduction of these elements from a distant source, whereas the distribution of iron does not support this idea. Calculations indicate that enough iron was removed from the wall rock gneiss to account for all the iron in the ore body. It is suggested that during metamorphism sulfurization was an important ore-forming process at Ore Knob, and that copper and sulfur introduced in hydrothermal solutions combined with iron from the biotite of the wall rock to produce the massive sulfide copper ore body at Ore Knob.