Barbara L. Dutrow

Nomenclature of the tourmaline-supergroup minerals

Abstract A nomenclature for tourmaline-supergroup minerals is based on chemical systematics using the generalized tourmaline structural formula: XY3Z6(T6O18)(BO3)3V3W, where the most common ions (or vacancy) at each site are X = Na1+, Ca2+, K1+, and vacancy; Y = Fe2+, Mg2+, Mn2+, Al3+, Li1+, Fe3+, and Cr3+; Z = Al3+, Fe3+, Mg2+, and Cr3+; T = Si4+, Al3+, and B3+; B = B3+; V = OH1- and O2-; and W = OH1-, F1-, and O2-. Most compositional variability occurs at the X, Y, Z, W, and V sites. Tourmaline species are defined in accordance with the dominant-valency rule such that in a relevant site the dominant ion of the dominant valence state is used for the basis of nomenclature. Tourmaline can be divided into several groups and subgroups. The primary groups are based on occupancy of the X site, which yields alkali, calcic, or X-vacant groups. Because each of these groups involves cations (or vacancy) with a different charge, coupled substitutions are required to relate the compositions of the groups. Within each group, there are several subgroups related by heterovalent coupled substitutions. If there is more than one tourmaline species within a subgroup, they are related by homovalent substitutions. Additionally, the following considerations are made. (1) In tourmaline-supergroup minerals dominated by either OH1- or F1- at the W site, the OH1--dominant species is considered the reference root composition for that root name: e.g., dravite. (2) For a tourmaline composition that has most of the chemical characteristics of a root composition, but is dominated by other cations or anions at one or more sites, the mineral species is designated by the root name plus prefix modifiers, e.g., fluor-dravite. (3) If there are multiple prefixes, they should be arranged in the order occurring in the structural formula, e.g., “potassium-fluor-dravite.”

Garnet-biotite geothermometry revised: New Margules parameters and a natural specimen data set from Maine

The gamet-biotite geothermometer lias been recalibrated using recently obtained Mar- gules parameters for iron-magnesium-calcium garnet. Mn interactions in garnet, and Al interactions in biotite. as well as the Fe oxidation state of both minerals. Fe-Mg and ΔWAl Margules parameters for biotite have been retrieved by combining experimental results on [6]Al-free and [6]Ahbearing biotite using statistical methods. Margules parameters, per mole of biotite are WBt MgFe= 40 719 - 30T J/mole, ΔWBtAl = WBtFeAl - WBtMgAl = 210190 - 245.40 T J/mole, ΔWBtTi = WBtFeTi - WBtMgTi = 310990 - 370.397 T/mole. Based on this model, the exchange reaction ΔH is 41952 J/mol and ΔS is 10.35 J/(K mol). Estimated uncertainty for this geothermometer is 25 °C. This geothermometer was tested on two data sets. The first consisted of 98 specimens containing garnet and biotite from west-central Maine, which formed under reducing fO₂, with graphite, a limited range of P (∼3 to 4.5 kbar). and a moderate range in T (∼550-650 °C). and which were all analyzed on a single microprobe using the same standards. Results indicate that the Maine staurolite zone averages 574 °C compared with 530 °C previously calculated and that the muscovite-breakdown T is consistent with experimental data. The second set consisted of cordierite-garnet granulites without hyperstliene from Ontario. Results here suggest an average T of 662 °C. compared with significantly lower or higher Ts calculated from other geothermometers. This model reproduces the Perchuk and Lavrent‘eva (1983) experimental Ts with a standard deviation of 12 °C and discriminates the assemblages in the Maine data set better than other models.

Tourmaline in a low grade clastic metasedimentary rock: an example of the petrogenetic potential of tourmaline

Detrital tourmaline grains and their associated tourmaline overgrowths provide a means to unravel the provenance and petrogenetic history of low grade clastic metasedimentary rocks. Evidence derives from tourmaline grains found in a lithic wacke metamorphosed to chlorite zone conditions. The detrital tourmaline cores are diagnostic indicators of the source rocks of the sediment whereas the overgrowths record both diagenetic and metamorphic reactions in the rock. Tourmaline grains consist of a detrital core surrounded by asymmetric overgrowths comprised of inner and outer rims. Abrupt chemical discontinuities between each of these zones implies that volume diffusion within tourmaline was minor under the conditions of formation. Compositions of the detrital cores vary widely, yet can be correlated with source rock types that are consistent with lithic fragments recognizable in the metawacke. At either the analogous or antilogous pole, inner rim compositions proximal to the detrital cores converge, despite the substrate tourmaline composition, indicating an approach to chemical equilibrium. However, significant dufferences in Al and X-site vacancies at the expense of Mg, Na and Ti between the analogous and antilogous poles of the inner rims demonstrate the presence of significant amounts of compositional polarity. Outer rim compositions at either pole also converge but compositional polarity between the analogous and antilogous poles persists. The presence of the inner and outer rims separated by a compositional discontinuity suggests punctuated evolution of the overgrowth. This implies that boron was sporadically available during diagenesis and metamorphism. Based on boron contents of minerals, this may correspond to a mechanism such as boron release due to polytypic change of illite or consumption of illite and/or muscovite. As such, tourmaline growth stages may serve as a monitor of chemical reactions in low grade metamorphic rocks.

Lithium in staurolite and its petrologic significance

Natural metapelitic staurolites contain appreciable amounts of lithium. Lithium contents were determined by ion microprobe with concentrations of representative samples independently analyzed by atomic absorption spectrophotometry for calibration. Seventy-one percent of the analyzed staurolites contain >0.1 wt.% Li2O, although the distribution is skewed to values less than 0.3 wt.%.High Li contents observed in staurolite are attributed to one or more of several factors: initiation of staurolite breakdown, lack of additional host phases for lithium (e.g. biotite), pre-metamorphic Li-rich bulk rock composition, and/or interaction of the rock with Li-rich fluids. Li content is generally not correlated with the modal amount of staurolite in the rock, rather Li values tend to reflect variable host rock Li. Lithium most likely resides in the R2+ tetrahedral site. Its incorporation into the structure is probably related to a coupled substitution with Al: ivLi viA1/3ivR−12+vi□−1/3 When staurolite analyses yield low R2+ and high Al values, the possibility of high Li should be considered after accounting for variable H.Lithium partitions into common pelitic metamorphic minerals in the order staurolite>cordierite>biotite>muscovite> garnet, tourmaline, and chloritoid. Partitioning is non-ideal in staurolite and a function of Fe content. Li in staurolite expands its stability field to a higher T relative to garnet and sillimanite, and to a lower T relative to chloritoid and Al-silicate. Analysis of staurolites for Li may provide further insight into this enigmatic mineral.

Tourmaline-rich pseudomorphs in sillimanite zone metapelites: Demarcation of an infiltration front

The mineralogical community lost a valued colleague and friend with the death of Eugene E. Foord. Gene, a Life Fellow of the Mineralogical Society of America, died at his home on January 8, 1998 at the age of 51 after a three-year battle with lymphoma. Gene was a career scientist at the U.S. Geological Survey where he worked from 1976 until his death in 1998. Gene was an outstanding mineralogist and he will be remembered for his significant contributions to the mineralogy and paragenesis of pegmatites from San Diego County, California. He will also be remembered for his boundless enthusiasm for mineralogy, his dedication to thorough and accurate mineral identifications and descriptions, and his willingness to work with both professional scientists and amateur collectors. Foordite, a tin-niobium oxide was named after Gene ( ̌ Cerný et al. 1988) in honor of his many contributions to the study of niobium-tantalum-tin minerals in pegmatites. Gene enjoyed practical jokes, having learned from the master himself, Richard H. Jahns. In fact, Gene was the “mystery” person responsible for “relocating” the bust of Theodore Hoover from the third floor of the Stanford geology building. Gene was an enthusiastic and animated storyteller and he loved to entertain his friends and colleagues with his outrageously funny stories of his escapades including backyard bouts with birds, avoiding landmines along the Pakistan-Afghanistan border, finding stashes of frozen hummingbirds in a Stanford professor’s freezer, nearly “starving” to death in Labrador, graphic descriptions of the Russian cuisine, toying with guards while in house arrest in China, and many more. Gene was born at Children’s Hospital in Oakland, California, November 20, 1946. Gene, a RH-factor baby, had the distinction of being one of the first survivors of a complete exchange transfusion. However, as a result of the RH incompatibility, Gene was born severely hearing impaired. He moved in 1947 with his parents, Elizabeth and Delbert Foord, and his older brother William, to West Hempstead, New York. When his parents realized that Gene was hearing impaired, they took him to the Manhattan Eye and Ear Infirmary. They were told that their only education option was to enroll Gene in a special school for the deaf. However, his parents were determined to provide Gene with a normal education in their own hometown. Consequently, they joined together with other parents of hearing impaired children in Long Island and formed the Long Island Hearing and Speech Society. This was one of the first

Tourmaline in meta-evaporites and highly magnesian rocks: perspectives from Namibian tourmalinites

Tourmaline from meta-evaporitic tourmalinites of the Duruchaus Formation of central Namibia reveal a common compositional trend that occurs in tourmaline from other meta-evaporite localities. The meta-evaporitic tourmalines are generally sodic, magnesian, moderately-to-highly depleted in Al, and enriched in Fe 3+ and W O 2− (calculated). They typically follow this trend along a join between “oxy-dravite” [Na(Mg 2 Al)(Al 6 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 (O)] and povondraite [Na(Fe 3 3+ ) (Fe 4 3+ Mg 2 ) (Si 6 O 18 ) (BO 3 ) 3 (OH) 3 (O)]. Similar trends occur in the meta-evaporites at Alto Chapare (Bolivia), Challenger Dome (Gulf of Mexico), and Liaoning (China). This chemical feature is attributed to the influence of oxidizing, highly saline, boron-bearing fluids that are associated with these lithologies. In the Namibian tourmalines there are some deviations from this trend, which are considered to be a consequence of later overprints related to sulfate–silicate interactions and/or influx of reactive fluid. Tourmalines occurring in the highly magnesian high-pressure rocks (whiteschists and pyrope–coesite rocks) are distinctly more magnesian and fall close to the dravite and “oxy-dravite” compositions. These latter tourmaline compositions likely reflect the metasomatic processes that produced these unusual bulk compositions and/or the influx of a reactive fluid that eliminated any earlier chemical signatures of meta-evaporitic fluids or protoliths.

Compositional zoning and element partitioning in nickeloan tourmaline from a metamorphosed karstbauxite from Samos, Greece

Abstract Blue-green nickeloan tourmaline from a micaceous enclave of a marble from Samos, Greece, contains unusually high concentrations of Ni (up to 3.5 wt% NiO), Co (up to 1.3 wt% CoO), and Zn (up to 0.8 wt% ZnO). The polymetamorphic karstbauxite sample has an uncommon assemblage of nickeloan tourmaline, calcite, zincian staurolite, gahnite, zincohögbomite, diaspore, muscovite, paragonite, and rutile. The complex geologic history is reflected in multi-staged tourmaline growth, with cores that represent detrital fragments surrounded by two-staged metamorphic overgrowths. Zone-1 metamorphic overgrowths, which nucleated next to detrital cores, are highly asymmetric and exhibit compositional polarity such that narrow overgrowths of brown schorl developed at the (-) c-pole are enriched in Mg, Ti, and F, and depleted in Al, Fe, and X-site vacancies (x⃞ ) relative to wider, gray-blue schorl-to-foitite overgrowths developed at the (+) c-pole. Volumetrically dominant Zone-2 overgrowths are strongly zoned nickeloan dravites with a continuous increase in Mg, Co, Ca, and F at the expense of Fe, Zn, Cr, and V from the Zone-1 interface to the outermost rim. Within Zone 2, Ni reaches a maximum of 0.5 apfu before decreasing in the outer 20-40 µm. Zone-2 overgrowths also exhibit compositional polarity such that, at the (-) c-pole, overgrowths are enriched in Mg, F, Na, Ca, and Cr relative to overgrowths at the (+) c-pole that are, in turn, enriched in Al, Fe, Ni, Co, and X⃞ . Element partitioning involving tourmaline rims and coexisting minerals indicates that relative partitioning of Ni is tourmaline >> staurolite > gahnite; Co is tourmaline > staurolite > gahnite; and Zn is gahnite > staurolite >> tourmaline.

Complex behavior of magma- hydrothermal processes: Role of supercritical fluid

Abstract Magmas emplaced into the upper portions of the earth’s crust are accompanied by extensive hydrothermal activity. Hydrothermal activity is represented as a system of coupled processes that dissipate thermal, mechanical, and chemical energy into the magma’s lithocap, primarily by convection of H2O-rich fluids. To investigate dynamical behavior of the system, a serial experiment was undertaken in which T(t) and P(t) values are computed for a pluton location during the time the region was subjected to near-critical hydrothermal convective flow. The consequent evolution of fluid buoyancy, ∇xρf, ion stability, ΔḠ°, and fracture extension, δL/L0 during this time indicates that variations in density gradients increase smoothly until 70,000 yr then burst into frequent, ≈100-yr oscillations. Oscillations first increase in magnitude then decrease. Oscillatory behavior of state conditions derived from numerical experiments illustrate resonant effects in chemical equilibrium and fracture extension processes and show the sensitivity of the stable mineral assemblage to either of the competing chemical and mechanical transport processes. An oscillatory zoned tourmaline that formed at near-critical conditions of H2O from the Geysers Geothermal deposit appears to provide evidence of nonlinear systematics in hydrothermal activity. Mathematical analogs to this system demonstrate that processes in this system record their dynamical behavior in the supercritical region and suggest that alteration events are generated by the complex, “chaotic” behavior of these processes. This type of behavior appears to be further augmented by strong divergence of H2O-fluid properties toward ± infinity at commonly encountered state conditions in the shallow reaches of magma-hydrothermal activity. System behavior elucidated here arises from affording for connectivity of processes by numerical experiments of hydrothermal activity for a region near the contact of a magma and its lithocap. The cumulative data from numerical experiments, equation-of-state (EOS) relationships, geologic and geochemical observations support the proposition that magma- hydrothermal processes should be thought of as complex dynamical systems whose behavior at state conditions near the supercritical region of the fluid is likely chaotic.

Coupled heat and silica transport associated with dike intrusion into sedimentary rock: effects on isotherm location and permeability evolution

An 11-meter-wide alkalic monchiquite dike recovered from the subsurface of Louisiana has produced a metasomatic aureole in the adjacent interbedded carbonate mudstones and siltstones. The asymmetric contact aureole, which extends nearly 6 m above and 4 m below the intrusion, contains the metamorphic minerals, diopside, pectolite, fluor-apophyllite, fluorite, and garnet. A series of coupled heat and mass transport calculations was undertaken to provide thermal constraints for the aureole, in the absence of robust geothermometric assemblages, and insights into accompanying mass transport associated with the sedimentary rock- dike system. Calculations were completed for systems with homogeneous, anisotropic, and layered permeability, . Transport, dissolution, and precipitation of silica were also incorporated into calculations. All systems modeled indicate that the thermal pulse waned in 3 yr with a return to background temperatures in 10 yr. Heat and fluid transport produce maximum temperature isotherms that are distinctly different in spatial extent and lateral variability for each numerical system. The homogeneous case produced isotherms that pinch and swell vertically above the dike and have large lateral variations, in contrast to the anisotropic case that produced a single large plume above the dike. The layered system case produced the most spatially extensive thermal aureole, unlike that recorded in the rocks. Addition of dissolved silica to the flow system significantly impacts the calculated transport of heat and fluid, primarily due to density changes that affect upwelling dynamics. Although precipitation and dissolution of SiO2 can affect flow through the feedback to permeability, changes were found to be minor for these system conditions. Where decreased, flow was refocused into higher zones, thus mitigating the differences over time. This negative feedback tends to defocus flow and provides a mechanism for lateral migration of plumes. Coupled heat and silica transport produces a complex isotherm geometry surrounding the intrusion due to formation of upwelling and downwelling plumes and lateral translation of plumes, leading to variability in the isotherm pattern that does not reflect the inherent heterogeneity of the initial material properties. Initial heterogeneities in are not a prerequisite for the development of a complicated flow and transport pattern. In addition, if isotherms reflect isograds, these calculations demonstrate that isograds may not form uniform structures with isograd boundaries characterized by their distance from the heat source. Copyright

Thermodynamic properties of stoichiometric staurolite H 2 Fe 4 Al 18 O 48 and H 6 Fe 2 Al 18 Si 8 O 48

A thermodynamic solution model for the ferromagnesian amphiboles is developed. The model accounts explicitly for intersite nonconvergent cation ordering of FeH and Mg between octahedral Ml, M2, M3, and M4 sites and intrasite interaction energies arising from size-mismatch of unlike cations. The model is formulated with 15 parameters: two standard-state contributions, three ordering energies involving the exchange of FeH and Mg between the four crystallographically distinct sites, six reciprocal terms that describe the noncoplanarity of the Gibbs energy of mechanical mixing in composition-ordering space, and four regular-solution type parameters involving FeH-Mg interaction on each site. The model may be readily collapsed to approximations that distinguish cation ordering over only three sites (MI3, M2, and M4) or two sites (M123 and M4), or that assume the absence of ordering (i.e., a macroscopic model), in which case the number of parameters decreases to 10, 6, and 3, respectively. The proposed model is calibrated for the ferromagnesian monoclinic amphiboles, under the assumptions of energetic equivalency of the M I and M3 sites and the absence of excess volume or excess vibrational entropy, using the X-ray site occupancy data of Hirschmann et al. (1994) and the phase equilibrium data of Fonarev and Korolkov (1980). Reference-state thermodynamic quantities for magnesio-cummingtonite [Mg7Sis022(OH)2] and grunerite [Fe7Sis022(OH)2] are derived from previously published results. The calibrated model is internally consistent with the database of Berman (1988) and the work of Sack and Ghiorso (1989) on ferromagnesian orthopyroxene. Gibbs energy of mixing, enthalpy of mixing, and activity-composition relation plots are constructed from the calibration. INTRODUCTION with this compositionally simple subsystem are twofold: (1) only FeH-Mg cation ordering on octahedral sites is Amphiboles are critical petrogenetic indicators in igoperative because there are no major coupled substituneous and metamorphic systems. They have been utitions and no tetrahedral site ordering, and (2) natural lized as temperature, pressure, and reaction-rate sensors parageneses exist to which the resulting thermodynamic for a broad range of PT conditions, and amphibole-bearformulation can be applied directly. ing assemblages have found application as a means of The thermodynamic model proposed here is based upon estimating the fH,o and, indirectly, the fluid composition our experience with other ferromagnesian silicates (Sack present under the conditions ofrock formation. As canand Ghiorso, 1989, 1994a, 1994b, 1994c; Hirschmann, didates for thermodynamic models of solid-solution en1991) and oxide minerals (Ghiorso, 1990a; Sack and ergetics, the amphiboles pose a formidable but fascinatGhiorso 1991a, 1991b; Ghiorso and Sack, 1991). Aling challenge. They are multi site reciprocal solutions that though the amphiboles are structurally more complex, exhibit a high degree of coupled substitution and form our approach to construction of the model is in principle solid-solution series with wide compositional variation. identical. We propose a model that accounts explicitly Despite this apparent complexity, the petrologic imporfor: (1) the temperature dependence of cation ordering of tance of the amphiboles encourages attempts toward a FeH and MgH over available crystallographic sites, and comprehensive thermodynamic formulation. In this pa(2) the excess enthalpy associated with size-mismatch per, we take a first step toward this goal with the forsubstitution of FeH and MgH on individual sites. We mulation and calibration of a thermodynamic model for demonstrate that, as is the case for all mineral solid sothe monoclinic ferromagnesian amphibole solid solutions lutions that exhibit cation ordering, calibration of this (cummingtonite-grunerite series). The reasons for starting model requires three diverse datasets: (I) information on 0003-004X/95/0506-0502