Abstract Low-temperature hydrous alteration of FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, produced silician ferrihydrite, cronstedtite, greenalite, hisingerite, and talc. Ferrihydrite occurs as nanocrystals in ~50 nm diameter granules that form monomineralic masses up to 300 μm across. It is inferred to have formed by the replacement of an igneous sulfide such as pyrrhotite. Electron diffraction shows the ferrihydrite to be a 9-line variety. Si-rich cronstedtite formed thin rims around the ferrihydrite, and talc grew patchily around the cronstedtite. Greenalite formed in ~10 μm cracks through all the above minerals and olivine, and hisingerite microveinlets partially replaced olivine. Igneous minerals remaining include olivine Fa46, magnetite, ilmenite, hornblende, biotite, and trace clinopyroxene. Correlations among the constituents of ferrihydrite determined by electron microprobe, including anhydrous totals, indicate progress during the growth of two charge-balanced exchanges involving silica enrichment: an inverse cronstedtite substitution (MgFe2+,Si) (Fe3+Mn3+)–2 and an inverse hydrogarnet substitution SiH–4. The cronstedtite exchange requires charge and size balance across nearest-neighbor T and O crystal sites, suggesting crystal-interior rather than crystal-surface control. Ferrihydrite’s composition reflects time- and space-related variations in the chemical potentials of components in the hydrous fluid at the site of alteration. An upper limit for SiO2 of 14–15 wt%, or ≈1.0 Si per 5-cation formula unit, would seem to correspond to the limit of availability in ferrihydrite of tetrahedral sites open to the entry of Si. Our EPMA data, projected to zero SiO2, indicate an anhydrous total of ≈83 wt% for end-member ferrihydrite, a number that matches the formula: Fe10O15.9H2O. The geochemical properties of Laramie ferrihydrite are shared by some samples of altered chondritic and Martian meteorites. Ferrihydrite on Earth commonly occurs as a surface deposit; unlike the Laramie occurrence, these lack the microspatial coherence of replacements/ pseudomorphs to show systematic, structure-related element variations. The superior crystal quality of the Laramie ferrihydrite likely contributed to its unique compositional variability.

We present microanalyses of secondary phyllosilicates in altered ferroan metaperidotite, containing approximately equal amounts of end-members serpentine ((Mg,Fe2+)3Si2O5(OH)4) and hisingerite (□Fe3+2Si2O5(OH)4·nH2O). These analyses suggest that all intermediate compositions can exist stably, a proposal that was heretofore impossible because phyllosilicate with the compositions reported here have not been previously observed. In samples from the Duluth Complex (Minnesota, USA) containing igneous olivine Fa36–44, a continuous range in phyllosilicate compositions is associated with hydrothermal Mg extraction from the system and consequent relative enrichments in Fe2+, Fe3+ (hisingerite), Si, and Mn. Altered ferroan–olivine-bearing samples from the Laramie Complex (Wyoming, USA) show a compositional variability of secondary FeMg–phyllosilicate (e.g., Mg–hisingerite) that is discontinuous and likely the result of differing igneous olivine compositions and local equilibration during alteration. Together, these examples demonstrate that the products of serpentinization of ferroan peridotite include phyllosilicate with iron contents proportionally larger than the reactant olivine, in contrast to the common observation of Mg-enriched serpentine in “traditional” alpine and seafloor serpentinites. To augment and contextualize our analyses, we additionally compiled greenalite and hisingerite analyses from the literature. These data show that greenalite in metamorphosed banded iron formation contains progressively more octahedral-site vacancies (larger apfu of Si) in higher XFe samples, a consequence of both increased hisingerite substitution and structure modulation (sheet inversions). Some high-Si greenalite remains ferroan and seems to be a structural analogue of the highly modulated sheet silicate caryopilite. Using a thermodynamic model of hydrothermal alteration in the Fe–silicate system, we show that the formation of secondary hydrothermal olivine and serpentine–hisingerite solid solutions after primary olivine may be attributed to appropriate values of thermodynamic parameters such as elevated a S i O 2 ( a q ) and decreased a H 2 ( a q ) at low temperatures (~200 °C). Importantly, recent observations of Martian rocks have indicated that they are evolved magmatically like the ferroan peridotites analyzed here, which, in turn, suggests that the processes and phyllosilicate assemblages recorded here are more directly relevant to those occurring on Mars than are traditional terrestrial serpentinites.

Serpentine formed from Fe-rich igneous forsterite (Fa38–43) in two samples of peridotite from the Layered Series of the Duluth igneous complex, Minnesota, is enriched to at least 30% of the Fe endmember. In addition to lizardite, chrysotile, and clinochlore, electron microprobe and transmission electron microscope study reveals the presence of metamorphic fayalite (Fa53–80) and Fe-rich phyllosilicates such as magnesian hisingerite, low-Al ferrosaponite, and chamosite. These phyllosilicates are physically mixed on a nano-scale (in part interlayered) with the serpentine. Chlorite and serpentine compositions tend to mirror the bimodal frequency of Fa% in olivine, suggesting a measure of local FeMg–1 exchange equilibrium. A trioctahedral, ferrian ferrosaponite is believed to have replaced the hisingerite, and a more Si-rich ferrosaponite has filled pores in chrysotile and locally replaced it. Both varieties of ferrosaponite are attended unevenly by the presence of up to 1 wt % Na2O and K2O. Mixtures of lizardite with hisingerite show a gradual increase in Ca toward the latter up to 0 08 atoms per formula unit (1 5 wt % CaO). The presence of 10 and 13 wt % MgO in hisingerite indicates at least 25 and 34 mol % solid solution of Mg-serpentine. Both saponite and hisingerite probably contribute to the yellow color of alteration in thin section. The abundance of Fe-rich phyllosilicates (XFe1⁄4 0 5– 0 9) and magnetite in the phyllosilicate microveins argues for a loss of Mg from the rock accompanying the hydrous alteration. This mass change accounts for the growth of magnesian fayalite as a unique product of a low-temperature hydration reaction that ordinarily only consumes olivine. The removal of Mg is inferred to have taken place initially at low fO2 and temperature very roughly 300 C, under the action of an infiltrating acidic, aqueous fluid. This fluid was probably influenced by nearby concentrations of Cu–Ni sulfides, or alternatively it was derived from underlying graphitic, sulfidic metapelites. Mg loss accompanying serpentinization is not exclusively a feature of hydrospheric alteration; it can instead be induced by a hydrothermal fluid of hitherto unrecognized composition. Growth of hisingerite and ferrosaponite (and its partial oxidation) is inferred to have taken place at a late stage under cooler and more oxidizing conditions. Similar MgFe-phyllosilicates and ferroan olivine are found in terrestrial ore deposits, on the ancient (Noachian) cratered surface of Mars, and in Martian meteorites (nakhlites). Other layered ultramafic–mafic intrusions, zoned intrusions, and ophiolites with Cu–Ni sulfide concentrations may conceivably be found locally to show the same style of serpentinization with major Mg depletion.

Abstract Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of T and fO2 in the magma reservoir. In relatively reduced (ΔΝΝΟ <1) siliceous magmas (e.g., Bishop Tuff, Taupo units), Ti behaves compatibly (DTi ≈ 2−3.5), leading to a decrease in TiO2 activity in the melt with cooling and fractionation. In contrast, FeTioxides are poorer in TiO2 in more oxidized magmas (ΔΝΝΟ > 1, e.g., Fish Canyon Tuff, Pinatubo), and the d(aTiO2)/dT slope can be negative. Biotite, FeTi-oxides, liquid, and possibly plagioclase largely maintained equilibrium in the Bishop Tuff magma (unlike the pyroxenes, and cores of quartz, sanidine, and zircon) prior to and during a mixing event triggered by a deeper recharge, which, based on elemental diffusion profiles in minerals, took place at least several decades before eruption. Equilibrating phases and pumice compositions show evolving chemical variations that correlate well with mutually consistent temperatures based on the FeTi-oxides, sanidine-plagioclase, and Δ18Ο quartz-magnetite pairs. Early Bishop Tuff (EBT) temperatures are lower (700 to ~780 °C) than temperatures (780 to >820 °C) registered in Late Bishop Tuff (LBT), the latter defined here not strictly stratigraphically, but by the presence of orthopyroxene and reverse-zoned rims on quartz and sanidine. The claimed similarity in compositions, Zr-saturation temperatures and thermodynamically calculated temperatures (730−740 °C) between EBT and less evolved LBT reflect the use of glass inclusions in quartz cores in LBT that were inherited from the low-temperature rhyolitic part of the reservoir characteristic of the EBT. LBT temperatures as high as 820 °C, the preservation of orthopyroxene, and the presence of reverse-zoned minerals (quartz, sanidine, zircons) are consistent with magma recharge at the base of the zoned reservoir, heating the cooler rhyolitic melt, partly remelting cumulate mush, and introducing enough CO2 (0.4−1.4 wt%, mostly contained in the exsolved fluid phase) to significantly lower H2O-activity in the system.

We describe a sample of Proterozoic banded iron-formation from the inner contact aureole of the Duluth complex near Babbitt, Minnesota, that contains a compositionally unique Fe-rich monoclinic ferromagnesian amphibole. Its content of T Al (1.3 atoms per 23 anhydrous oxygens) and A Na (0.55) places it in a compositional field (with the label clinoferrogedrite) outside those recognized in the IMA nomenclature for amphiboles. With declining Al, it grades into Al-rich grunerite, and finally to Al-poor grunerite at the crystal margins. While spot-to-spot compositional trends define basic gedrite substitutions, they differ systematically from the well-established anthophyllite-gedrite trends found in amphibolites. They resemble instead two instances of sodic gedrite described in the literature that are enriched in A Na apfu and depleted in octahedral Al. These unusual trends may be the result of very high temperatures of formation, or a high activity ratio of Fe + Mg to Si. Associated minerals in our sample include orthopyroxene Fs 74–76 , olivine Fa 85–91 , almandine, quartz, plagioclase, and accessories graphite, pyrrhotite, ilmenite, biotite, apatite, zircon, monazite, and lollingite. Conditions are estimated to have been ≥800 °C and ≈1.3 kbar, with f O 2 close to FMQ-2 log 10 units. Clinogedrite should be considered extremely rare, but not non-existent. In the monoclinic structure the symmetry of the double Si–O chains limits the uptake of gedritic components to amounts lower than found in the orthorhombic structure, where Al is distributed across three of the four distinct tetrahedral sites. We also present electron microprobe data for a highly aluminous cummingtonite grading compositionally into clinogedrite ( A Na = 0.3, M 2 Al + Fe 3+ + 2Ti = 1.0, T Al = 1.1) that was grown from fused and S-augmented Pinatubo dacite in the laboratory at 780 °C, 2.2 and 3.9 kbar. Growth of this MgFe-clinoamphibole was enabled by the capture of Ca into anhydrite away from potential hornblende.

In our 2014 paper ([Joy & Evans 2014][1], p. 349–351), we gave reasons for preferring to use an earlier IMA nomenclature scheme (IMA 1997) for the Mg-Fe-Mn amphiboles ([Leake et al . 1997][2], [2003][3]) rather than the latest version, IMA 2012 ([Hawthorne et al . 2012][4]). [Hawthorne et al

Rock-forming serpentine minerals form flat, cylindrical, and corrugated crystal microstructures, which reflect energetically efficient layering of alternate tetrahedral and octahedral sheets. Serpentinization of peridotite involves internal buffering of the pore fluid, reduction of oxygen fugacity, and partial oxidation of Fe2+ to Fe3+. Sluggish MgFe diffusion in olivine causes precipitation of magnetite and release of H-2. The tectonic environment of the serpentinization process dictates the abundance of fluid-mobile elements in serpentinites. Similar enrichment patterns of fluid-mobile elements in mantle-wedge serpentinites and arc magmas suggest a linkage between the dehydration of serpentinite and arc magmatism.

Eruption of the Bishop Tuff magma preserved equilibrium of exchange components and element concentrations among magnetite, ilmenite, biotite, apatite, zircon, and liquid. Orthopyroxene and clinopyroxene were not in exchange equilibrium with the other MgFe-bearing phases, but they appear to have been in equilibrium among themselves. Internally consistent temperatures recorded by the FeTi-oxide, Ti-in-quartz, and Delta O-18 quartz-magnetite thermometers, coupled with evidence for magmatic corrosion of quartz and sanidine, indicate that an initially low-T (approximate to 700 degrees C), near-H2O-saturated, high-SiO2 rhyolite magma was heated up to >= 800 degrees C and its crystal cargo partially melted by recharge of hotter melt from below. Oxygen fugacity and compositions of biotite, ilmenite, magnetite, and silicate liquid initially adjusted by internal rearrangement of components and conservation of oxygen. Partial melting of feldspars liberated Sr and Ba back into the melt. Mixing during recharge eventually re-introduced compatible elements (e.g., Mg, Ba, Sr) as well as foreign crystals of euhedral ortho- and clinopyroxene, which evidently never totally re-equilibrated with the rhyolite liquid. Introduction of CO2 and accompanying reduction in the aH(2)O during recharge raised crystallization temperatures of quartz and sanidine in the rhyolite sufficient to allow marginal regrowth of these phases with enhanced contents Ti, Ba, and Sr.

Abstract Microprobe analyses of antigorite show that (Al+Cr) and inferred Fe3+ correlate inversely with Si apfu in a Tschermaks substitution. This observation suggests that the uptake of Fe3+ is not simply related to fO₂. For Si = 1.95 apfu estimated Fe3+ = 0.032 apfu (or 0.95 wt% Fe2O3). Such estimates of Fe3+ require high analytical accuracy and precision, and assume a fixed polysomatic formula (e.g., m = 17) and freedom from interlayer sheet-silicate impurities. In many cases the estimates appear to be high. An alternative measure of Fe3+ is provided by the partitioning of total Fe and Mg between antigorite and olivine in well-equilibrated natural antigorite-olivine-magnetite parageneses. Extrapolation of Nernst and Roozeboom partition plots to Fe-free olivine permits an estimate of the Fe3+ content of the average antigorite in this paragenesis, namely 0.42 or 0.64 wt% Fe2O3. The partition estimates are in good agreement with the results of Mössbauer spectroscopy performed here on 14 antigorites from metaperidotites, together with four from the literature. These spectra reveal a range of 0.16 to 1.94 in wt% Fe2O3 in metaperidotite antigorite, with an average of 0.83. In two olivine-bearing rocks, antigorite has Fe3+/ΣFe ratios of 0.13 and 0.15, which corresponds to wt% Fe2O3 = 0.47 and 0.54, respectively. Larger amounts of Fe2O3 occur in some, but not all, vein antigorites. The prograde formation of antigorite in serpentinite from lizardite is accompanied by loss of some cronstedtite component and the precipitation of additional magnetite. The Roozeboom Mg/Fe partition plot is concave down rather than up; in other words the partition coefficient KD is a function of the XMg of olivine. This behavior has been found in other olivinemineral pairs. It can be interpreted to reflect strongly non-ideal solution behavior of MgFe-olivine at low temperatures, viz. WG ≈ 8.5 kJ assuming a symmetrical solution. MgFe-brucite appears to be similarly non-ideal.

Outcrops of spinifex-textured metaperidotite hi the Cerro del Almirez ultramafic massif, S. Spain, are located a few meters upgrade from a unique eclogite-facies (1.6-1.9 GPa, approximate to 680 degrees C), antigorite-out (orthopyroxene-in) isograd. Olivine in the spinifex rock is pleochroic brown hi thin section, and microprobe analysis (carefully avoiding accidental inclusions) shows contents of Cr2O3 and TiO2 in the range 0 to 0.4 and 0 to 0.1 weight percent respectively. These amounts of Cr2O3 and TiO2 are matched by olivine in peridotite xenoliths from the mantle, komatiites, and pseudotachylytes. According to laboratory experiments, at 2 GPa and a likely redox state near QFM, these levels of Cr2O3 signify equilibration at temperatures in excess of 1600 degrees C. Curvature of the elongate olivines, and outcrop-scale alignment of olivine splays suggest a connection with komatiites. Two early studies at Cerro del Almirez emphasized this connection, but all papers from the late-1990s to the present time have advocated a metamorphic origin, analogous to jackstraw-olivine rocks. Some 5 to 6 weight percent or 13 to 15 volume percent of H2O was released at the isograd. We hypothesize that this dehydration water induced embrittlement of chlorite-metaharz-burgite, and frictional slip in shear zones raised the temperature by several hundred degrees C above the ambient, more than enough to melt wet or dry ultramafic rock and form spinifex texture on rapid cooling. The amounts of crystallized melt so formed are orders-of-magnitude larger than found in peridotite pseudotachylyte, but are perhaps not totally inconsistent with dehydration-induced seismic events repeated over a protracted period of time during Miocene subduction of an oceanic plate. The transient ultramafic melts yielded on rapid cooling up to 12 cm long olivines and interstitial aggregates of parallel to radiating orthopyroxene prisms, followed by crystallization from glass of assorted eclogite-facies minerals (chlorite, tremolite, talc).

In two samples of antigorite serpentinite from Sasaguri, Kyushu, Japan, we have found trace amounts of a NiFe-rich silicate whose atomic proportions in terms of the ratio of M to T cations match those of the fibrous MgFe-silicate hydrate balangeroite. Balangeroite in serpentinite at the type locality in Italy is not reported to contain Ni, whereas the balangeroite analogue from Japan has very little MgO. One sample from Japan is nickel-rich (up to 39 wt% NiO), and the other Fe-rich (up to 61 wt% total FeO). The Ni-rich sample appears to contain similar to 2 wt% Cl, and the Fe-rich sample contains up to 2.3 wt% P2O5. A wide miscibility gap in the ternary system Mg-Fe-Ni separates the Italian and Japanese minerals. Anhydrous microprobe totals are low, and we attribute this to a submicroscopic porosity, which is imaged by electron-beam scanning. Owing to its small grain-size (similar to 25 mu m) and very low modal abundance, our identification is as yet not confirmed by physical methods, such as XRD or micro-Raman spectroscopy.

Research Article| January 01, 2010 Abbreviations for names of rock-forming minerals Donna L. Whitney; Donna L. Whitney * 1Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A. *E-mail: dwhitney@umn.edu Search for other works by this author on: GSW Google Scholar Bernard W. Evans Bernard W. Evans 2Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98185, U.S.A. Search for other works by this author on: GSW Google Scholar Author and Article Information Donna L. Whitney * 1Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A. Bernard W. Evans 2Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98185, U.S.A. *E-mail: dwhitney@umn.edu Publisher: Mineralogical Society of America Received: 11 Aug 2009 Accepted: 13 Aug 2009 First Online: 02 Mar 2017 Online ISSN: 1945-3027 Print ISSN: 0003-004X © 2010 American Mineralogist American Mineralogist (2010) 95 (1): 185–187. https://doi.org/10.2138/am.2010.3371 Article history Received: 11 Aug 2009 Accepted: 13 Aug 2009 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Donna L. Whitney, Bernard W. Evans; Abbreviations for names of rock-forming minerals. American Mineralogist 2010;; 95 (1): 185–187. doi: https://doi.org/10.2138/am.2010.3371 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Nearly 30 years have elapsed since Kretz (1983) provided the mineralogical community with a systematized list of abbreviations for rock-forming minerals and mineral components. Its logic and simplicity have led to broad acceptance among authors and editors who were eager to adopt a widely recognized set of mineral symbols to save space in text, tables, and figures. Few of the nearly 5000 known mineral species occur in nature with a frequency sufficient to earn repeated mention in the geoscience literature and thus qualify for the designation “rock-forming mineral,” but a reasonable selection of the most common and useful rock-forming minerals... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50-300 degrees C), where lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich lizardite, magnetite, and dihydrogen gas (+/- brucite) are products of the reaction. At higher temperatures (400-600 degrees C), rates of MgFe diffusion in olivine are orders of magnitude faster, with the result that the growth of Mg-rich antigorite can be accommodated by a compositional adjustment of olivine, eliminating the need to precipitate magnetite and evolve hydrogen. This latter end-member mechanism probably best reflects the situation in the forearc mantle wedge.

Research Article| April 01, 2009 Acceptance of the Mineralogical Society of America Roebling Medal for 2008 Bernard W. Evans Bernard W. Evans University of Washington, Department of Earth and Space Science, Seattle, Washington 98195-1310, U.S.A. Search for other works by this author on: GSW Google Scholar Author and Article Information Bernard W. Evans University of Washington, Department of Earth and Space Science, Seattle, Washington 98195-1310, U.S.A. Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online ISSN: 1945-3027 Print ISSN: 0003-004X © 2009 American Mineralogist American Mineralogist (2009) 94 (4): 639–640. https://doi.org/10.2138/am.2009.535 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Bernard W. Evans; Acceptance of the Mineralogical Society of America Roebling Medal for 2008. American Mineralogist 2009;; 94 (4): 639–640. doi: https://doi.org/10.2138/am.2009.535 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Mr. President, Members and Guests: When I received President Barbara Dutrow’s telephone call in Rome, my first reaction was to wonder what on earth were the Selection Committee thinking? I can name dozens of my contemporaries whose scientific accomplishments easily match mine. Certainly, Barb’s call quite took my mind off the business at hand, which was, of all things, amphibole nomenclature. I can only conclude that my supporters must have written extraordinarily eloquent letters on my behalf, and of course I thank them very much for doing so. I humbly and gratefully accept this award. The list of previous recipients... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

We document an example of serpentinization of olivine and orthopyroxene that produced Virtually no magnetite, but instead relatively Fe-rich yellow-colored lizardite (X(Fe) = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite's identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of Fe(3+) exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables T, f(H2O), f(H2), and a(SiO2) attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization.

It is my honor and pleasure to introduce Bernard Evans as this year’s recipient of the Roebling Medal. In his quest to investigate how mineral chemistry and structure can be used to understand lithospheric evolution, Bernard has made fundamental contributions for 50 years, and is a former recipient, in 1970, of the MSA award. A brief synopsis of the highlights of Bernard’s research over the years must include mention of his classic work on muscovite breakdown and aluminosilicate phase equilibria, accomplished during his years as a postdoc and professor at Berkeley in the 1960s. At that time, Bernard was a pioneer in developing the electron microprobe for use in petrologic studies, transforming the fields of mineralogy and petrology and making possible the important advances in applications of phase equilibria that soon followed, as well as many firsts in microanalysis of minerals. Although Bernard subsequently became well known as a …

A model for the thermodynamic properties of rhonibohedral oxide solid solutions in the system Fe2O3-FeTiO3-MgTiO3-MnTiO3 (containing minor amounts of Al2O3) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is calibrated from the cation-ordering data of Harrison and others (2000; Harrison and Redfern, 2001) and experimental data on Fe+2Ti double left right arrow (Fe+3)(2) exchange between rhombohedral oxide and spinel from Lattard and others (2005) and Evans and others (2006). Successful calibration require introduction of an energetic contribution attributed to short-range cation-order, which reduces the configurational entropy of the solid solution. The resultant thermodynamic model for the rhombohedral oxides is internally consistent with the model for spinel solid solutions of Sack and Ghiorso (1991a, 1991b) and with the endmember thermodynamic properties database of Berman (1988); a new model equation for the isobaric heat capacity of ulvospinel (cubic Fe2TiO4) is proposed and values of the enthalpy of formation, -1490.417 kj/mol, and third law entropy, 184.199 J/K-mol, at 298.15 K and 10(5) Pa are recommended. The new model forms the basis of a revised FeTi-oxide geothermometer/oxygen barometer, which is applied to a newly compiled dataset of natural two oxide pairs from silicic volcanic rocks. Results are compared to previous formulations with the general conclusion that the new model gives a better estimate of oxidation state for magmas that equilibrated tinder conditions more oxidizing than the nickel-nickel oxide buffer. Estimates of oxygen fugacity are fairly insensitive to analytical uncertainties in oxide compositions. By contrast, temperature estimates are especially sensitive to analytical error and to the abundances of "minor" constituents. Application of the geothermometer to oxide pairs that grew under conditions where the rhonibohedral phase was cation disordered (that is high temperature or at oxygen fugacities greater by about one log,,, unit than the nickel-nickel oxide buffer) results in an uncertainty due solely to analytical error of at least 50 degrees C and sometimes as high as 100 degrees C. Temperature estimates from the new geothermometer can be made using either the Fe+2Ti double left right arrow (Fe+3)(2) exchange or Fe+2 double left right arrow Mg exchange between the two oxides. Comparison of the two temperature estimates provides a means of evaluating the internal consistency of coexisting oxide compositions and assessing the extent of disequilibrium. Temperatures calculated from the new model are found to be consistent with experimental phase relations for the stability of cummingtonite in silicic volcanics. Other petrologic constraints on derived temperatures are examined including limits on the width of the miscibility gap and the development of self-reversed remanent magnetization in the rhombohedral series. Software that implements the new thermodynamic model and the two-oxide geothermometer/oxygen barometer is available from http://www.ofm-research.org/.

It is argued that the high-Mg content (mg-number 95 3) of the serpentine minerals in serpentinized peridotite is a consequence of the environmental (FeMg1)-Mg-2 exchange potential imposed on the system by the abundance of olivine and orthopyroxene. Mass balance in the serpentinization reaction then requires the precipitation of an iron-rich mineral that in most cases is magnetite. This causes hydrogen to be evolved in an oxygen-conserved reaction. The low-variance mineral assemblage Ol Srp Brc Mag sets the chemical potentials of H2O, SiO2 and O-2 internally at an early stage in the process, but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown Fe-3 content of the serpentine minerals, particularly lizardite. Whole-rock analyses of highly to completely serpentinized peridotites reveal Fe-3/Fe ratios 04, with an average value (069) similar to that of magnetite (067). This feature may be attributed to the presence of high-Fe-3 lizardite, as has been found in Mssbauer spectroscopy studies. Electron microprobe and scanning electron microcope analyses in the literature exhibit element trends (e.g. decreasing Si vs Fe a.p.f.u.) for olivine-pseudomorph lizardite and, with some exceptions, for bastite lizardite, that show a substitution of the cronstedtite component (Fe-3 charge-balanced on T and M sites). Cronstedtite substitution will be favoured at low temperature and/or low hydrogen fugacity, and in these circumstances less magnetite will be evolved during serpentinization, in some cases none at all. Some bastite lizardites from sea-floor settings show evidence of M-site vacancy substitution of Fe-3 for Fe-2. In the course of progressive serpentinization, micrometer to millimeter-scale variations in SiO2 potential may well be present, but their influence on Fe in lizardite seems to be limited to a few cases of lizardite associated with orthopyroxene. Chrysotile is on average more Mg-rich and less variable in Fe/Mg ratio than lizardite, facts that may be attributed to the greater Fe-3 content of lizardite. Chrysotile veins provide the best record available to us of the environmental (FeMg1)-Mg-2 exchange potential in the pore fluid attending serpentinization. This potential serves as a robust control on serpentine and brucite compositions, although it may fail after olivine and orthopyroxene have been armoured or eliminated, and in more open-system environments (high water/rock ratio) such as on the sea floor or at serpentinite host-rock contacts. The default assumption in microprobe analyses that measured iron is all Fe-2 can lead to inappropriate petrological conclusions in the case of serpentinites.