Volume 104, Issue 7

As iron sulfide mineral phases are important sedimentary sinks for naturally occurring or contaminant metals, it is important to know the fate of metals during the diagenetic transformation of primary sulfide minerals into more stable phases, such as pyrite (FeS 2 ). Furthermore, the trace metal content of pyrite has been proposed as a marine paleoredox proxy. Given the diverse low-temperature diagenetic formation pathways for pyrite, this use of pyrite requires validation. We, therefore, studied nickel (Ni) and cobalt (Co) incorporation into freshly precipitated mackinawite (FeS m ), and after experimental diagenesis to pyrite (FeS 2 ) using S 0 as an oxidant at 65 °C. Metal incorporation was quantified on bulk digests using ICP-OES or ICP-AES. Bulk mineralogy was characterized with micro-X‑ray diffraction (micro-XRD), documenting the transformation of mackinawite to pyrite. Epoxy grain mounts were made anoxically of mackinawite and pyrite grains. We used synchrotron-based micro-X‑ray fluorescence (µXRF) to map the distribution of Co and Ni, as well as to collect multiple energy maps throughout the sulfur (S) K -edge. Iron (Fe) and S K -edge micro-X‑ray absorption near edge spectroscopy (µXANES) was used to identify the oxidation state and mineralogy within the experimentally synthesized and diagenetically transformed minerals, and map end-member solid phases within the grain mounts using the multiple energy maps. Metal-free FeS m transformed to pyrite, with residual FeS m detectable. Co- and Ni-containing FeS m also transformed to pyrite, but with multiple techniques detecting FeS m as well as S 0 , implying less complete transformation to pyrite as compared to metal-free FeS m . These results indicate that Co and Ni may inhibit transformation for FeS m to pyrite, or slow it down. Cobalt concentrations in the solid diminished by 30% during pyrite transformation, indicating that pyrite Co may be a conservative tracer of seawater or porewater Co concentrations. Nickel concentrations increased several-fold after pyrite formation, suggesting that pyrite may have scavenged Ni from the dissolution of primary FeS m grains. Nickel in pyrites thus may not be a reliable proxy for seawater or porewater metal concentrations.

The effect of aluminum (Al) on the elasticity of majorite-pyrope garnets was investigated by means of ultrasonic interferometry measurements on well-fabricated polycrystalline specimens. Both velocities and elastic moduli increase almost linearly with increasing Al content within analytical uncertainty. No significant variation of the velocities and elastic moduli is observed across the tetragonal-to-cubic phase transition at majorite with the pyrope content up to 26 mol% along the majorite-pyrope system. The elasticity variation of majorite-pyrope garnets is largely dominated by the Al content, while the phase transition as a result of cation ordering/disordering of Mg and Si via substitution of Al on octahedral sites cannot significantly affect elastic properties. Seismic velocity variations of a garnet-bearing mantle transition zone are therefore dominated by garnet composition (e.g., Al, Fe, Ca, and Na) rather than the tetragonal-to-cubic phase transition because of cation ordering/disordering.

Here we present a study on the quenchability of hydrous mafic melts. We show via hydrothermal experiments that the ability to quench a mafic hydrous melt to a homogeneous glass at cooling rates relevant to natural samples has a limit of no more than 9 ± 1 wt% of dissolved H 2 O in the melt. We performed supra-liquidus experiments on a mafic starting composition at 1–1.5 GPa spanning H 2 O-undersaturated to H 2 O-saturated conditions (from ~1 to ~21 wt%). After dissolving H 2 O and equilibrating, the hydrous mafic melt experiments were quenched. Quenching rates of 20 to 90 K/s at the glass transition temperature were achieved, and some experiments were allowed to decompress from thermal contraction while others were held at an isobaric condition during quench. We found that quenching of a hydrous melt to a homogeneous glass at quench rates comparable to natural conditions is possible at water contents up to 6 wt%. Melts containing 6–9 wt% of H 2 O are partially quenched to a glass, and always contain significant fractions of quench crystals and glass alteration/devitrification products. Experiments with water contents greater than 9 wt% have no optically clear glass after quench and result in fine-grained mixtures of alteration/devitrification products (minerals and amorphous materials). Our limit of 9 ± 1 wt% agrees well with the maximum of dissolved H 2 O contents found in natural glassy melt inclusions (8.5 wt% H 2 O). Other techniques for estimating pre-eruptive dissolved H 2 O content using petrologic and geochemical modeling have been used to argue that some arc magmas are as hydrous as 16 wt% H 2 O. Thus, our results raise the question of whether the observed record of glassy melt inclusions has an upper limit that is partially controlled by the quenching process. This potentially leads to underestimating the maximum amount of H 2 O recycled at arcs when results from glassy melt inclusions are predominantly used to estimate water fluxes from the mantle.

The Dahutang tungsten deposit, located in the Yangtze Block, South China, is one of the largest tungsten deposits in the world. Tungsten mineralization is closely related to Mesozoic granitic plutons. A drill core through a pluton in the Dalingshang ore block in the Central segment of the Dahutang tungsten deposit shows that the pluton is characterized by multi-stage intrusive phases including biotite granite, muscovite granite, and Li-mica granite. The granites are strongly peraluminous and rich in P and F. Decreasing bulk-rock (La/Yb) N ratios and total rare earth element (ΣREE) concentrations from the biotite granite to muscovite granite and Li-mica granite suggest an evolution involving the fractional crystallization of plagioclase. Bulk-rock Li, Rb, Cs, P, Sn, Nb, and Ta contents increase with decreasing Zr/Hf and Nb/Ta ratios, denoting that the muscovite granite and Li-mica granite have experienced a higher degree of magmatic fractionation than the biotite granite. In addition, the muscovite and Li-mica granites show M-type lanthanide tetrad effect, which indicates hydrothermal alteration during the post-magmatic stage. The micas are classified as lithian biotite and muscovite in the biotite granite, muscovite in the muscovite granite, and Li-muscovite and lepidolite in the Li-mica granite. The Li, F, Rb, and Cs contents of micas increase, while FeO T , MgO, and TiO 2 contents decrease with increasing degree of magmatic fractionation. Micas in the muscovite granite and Li-mica granite exhibit compositional zonation in which Si, Rb, F, Fe, and Li increase, and Al decreases gradually from core to mantle, consistent with magmatic differentiation. However, the outermost rim contains much lower contents of Si, Rb, F, Fe, and Li, and higher Al than the mantle domains due to metasomatism in the presence of fluids. The variability in W contents of the micas matches the variability in Li, F, Rb, and Cs contents, indicating that both the magmatic and hydrothermal evolutions were closely associated with W mineralization in the Dahutang deposit. The chemical zoning of muscovite and Li-micas not only traces the processes of W enrichment by magmatic differentiation and volatiles but also traces the leaching of W by the fluids. Therefore, micas are indicators not only for the magmatic–hydrothermal evolution of granite, but also for tungsten mineralization.

A topological model (TM) is presented for the complex crystal structures characteristic of some minerals. We introduce a tractable method for applying the TM to characterize defects in these complex materials. Specifically, we illustrate how structural groups, each with a motif containing multiple atoms, provide lattices and structures that are useful in describing dislocations and disconnections in interfaces. Simplified methods for determining the shuffles that accompany disconnection motion are also described. We illustrate the model for twinning in albite owing to its potential application for constraining the rheological properties of the crust at conditions near the brittle-plastic transition, where plagioclase is a major constituent of common rock types. While deformation twins in plagioclase are often observed in crustal rocks, the interpretation of the stress states at which they form has not advanced. The concept of structural groups makes an analysis of the twinning process easier in complex minerals and explicitly predicts the interface structure of the deformation twins.

Phoxite, (NH 4 ) 2 Mg 2 (C 2 O 4 )(PO 3 OH) 2 (H 2 O) 4 , is a new mineral species from the Rowley mine, Maricopa County, Arizona, U.S.A., and it has potential uses in agricultural applications for soil conditioning, fertilizing, and as a natural pesticide. It was found in an unusual bat-guano-related, post-mining assemblage of phases that include a variety of vanadates, phosphates, oxalates, and chlorides, somecontaining NH + . Other secondary minerals found in association with phoxite are antipinite, aphithitalite, 4 bassanite, struvite, thenardite, and weddellite. Crystals of phoxite are colorless composite blades upto about 0.4 mm. The streak is white, and the luster is vitreous to oily. The Mohs hardness is 2½, the tenacity is brittle, fracture is irregular, there is fair {100} cleavage, and the measured density is 1.98(2) g/cm 3 . Phoxite is optically biaxial (–) with α = 1.499(1), β = 1.541(1), γ = 1.542(1) (white light); 2 V = 16(1)°; dispersion r < v , slight; orientation Y = b , X ^ a ≈ 9° in obtuse β. Electron microprobe analyses yielded the empirical formula [(NH 4 ) 1.77 K 0.23 ] Σ2 Mg 2.00 (C 2 O 4 )(PO 3 OH) 2 (H 2 O) 4 , with the C and H content inferred from the crystal structure. Raman spectroscopy confirmed the presence of NH 4 and C 2 O 4 . Phoxite is monoclinic, P 21/ c , with a = 7.2962(3), b = 13.5993(4), c = 7.8334(6) Å, β = 108.271(8)°, V = 738.07(7) Å 3 , and Z = 2. In the crystal structure of phoxite ( R 1 = 0.0275 for 1147 I o > 2σ I reflections), bidentate linkages between C 2 O 4 groups and Mg-centered octahedra yield chains, which link to one another via PO 3 OH tetrahedra to create undulating [Mg 2 (C 2 O 4 )(PO 3 OH) 2 (H 2 O) 4 ] 2– sheets. Strong hydrogen bonds link the sheets into a “soft framework,” with channels containing 44 NH + . The NH + forms both ordered hydrogen bonds and electrostatic bonds with O atoms in the framework. Phoxite is the first mineral known to contain both phosphate and oxalate groups as essential components.

Garnet is a common metamorphic and igneous mineral with extensive solid solution that can be stable to mantle depths ≥400 km. High- T and/or high- P garnet may contain oriented lamellae of other minerals, most commonly simple oxides (e.g., rutile, ilmenite), apatite, and, in ultrahigh- P cases, silicates including pyroxene and amphibole. Lamellae have classically been considered to be precipitation features preserving a record of former garnet chemistry richer in the lamellae nutrients (e.g., Ti 4+ ). Such microtextural origins in precipitation systems (e.g., alloys) have long been studied via the crystallographic orientation relationships (COR) that form between a host and a separating phase, and by the shape-preferred orientation (SPO) of the lamellae. Recently, however, alternative hypotheses to precipitation have been suggested that require emplacement of lamellae in garnet by fluids, or co-growth, overgrowth, or inheritance mechanisms. These hypotheses posit that lamellae cannot be used to study former garnet chemistry. Moreover, they predict that lamellae phases, SPO, and COR should differ widely between localities, as lamellae formation will be controlled by various local rock-specific factors such as fluid presence, fluid chemistry, or mineral growth sequence. On the other hand, if lamellae characteristics are largely consistent between localities, it likely reflects control by precipitation energetics, rather than external factors. There have been few comparative COR studies in geologic systems, but the integrative assessment of COR, SPO, and lamellae assemblages should fingerprint lamellae growth process. To test the precipitation and alternative hypotheses, we collected large electron backscatter diffraction (EBSD) data sets for rutile, ilmenite, and apatite lamellae in garnet from the Brimfield Schist, Connecticut (≥1000 °C metamorphism; Central Maine Terrane, U.S.A.). We analyzed these data alongside published EBSD data for rutile, ilmenite, and corundum from metapegmatites metamorphosed in the eclogite facies from the Austrian Alps (Griffiths et al. 2016). The apatite data set is the first of its kind, and reveals that apatite preferentially aligns its close-packed direction parallel to that of garnet ( c -axis apatite //<111> garnet ). We also recognize a rutile-garnet COR related to those in meteorites with Widmanstätten patterns that are unequivocal products of exsolution. This is the first identification of direct similarities between silicate-oxide and metal-metal COR of which we are aware. Significantly, this rutile-garnet COR is found in diverse geologic settings including Connecticut and Idaho (U.S.A.), Austria, Germany, Greece, and China over a broad range of bulk-rock compositions. Results for all lamellae minerals show that COR are largely consistent between localities and, furthermore, are shared between apatite, ilmenite, and corundum. Moreover, between 70% and 95% of lamellae have COR and there is a dominant COR for each lamellae phase. Calculations show that d - spacing ratios of host-lamellae pairs can successfully predict the most commonly observed specific COR (those COR with two or more axial alignments with the host). These results, especially similarity of COR from markedly different geologic settings and a low diversity of lamellae minerals, are fully consistent with lamellae formation by precipitation (likely via exsolution). In contrast, the alternative hypotheses remain unsupported by COR results as well as by mineralogical and petrological evidence. Lamellae with similar traits as those in this work should thus be considered precipitates formed during unmixing of garnet compositions originally stable at elevated or extreme pressures and temperatures.

Several geothermobarometric tools have focused on clinopyroxene due to its prevalence in igneous rocks, however clinopyroxene produced in high-silica igneous systems is high in iron and low in aluminum, causing existing geothermometers that depend on aluminum exchange to fail or yield overestimated temperatures. Here we present a new clinopyroxene-liquid geothermometer recommended for use in natural igneous systems with bulk SiO 2 ≥ 70 wt%, which contain clinopyroxene with Mg# ≤ 65 and Al 2 O 3 ≤ 7 wt%. (1)T∘ C=300− 1.89− 0.601XcaTsCpx− 0.186XDiHd2003Cpx+4.71Xsio2liq+77.6XTio2liq+10.9XFeOliq+33.6XMgOliq+15.5XCaoliq+15.6XKO0.5liq$$T\left( ^{\circ }C \right)=300\left[ \begin{array}{*{35}{l}} -1.89-0.601\left( X_{\text{caTs}}^{\text{Cpx}} \right)-0.186\left( X_{\text{DiH}{{\text{d}}_{2003}}}^{\text{Cpx}} \right)+4.71\left( X_{\text{si}{{\text{o}}_{2}}}^{\text{liq}} \right)+77.6\left( X_{\text{Ti}{{\text{o}}_{2}}}^{\text{liq}} \right) \\ +10.9\left( X_{\text{FeO}}^{\text{liq}} \right)+33.6\left( X_{\text{MgO}}^{\text{liq}} \right)+15.5\left( X_{\text{Cao}}^{\text{liq}} \right)+15.6\left( X_{\text{K}{{\text{O}}_{0.5}}}^{\text{liq}} \right) \\ \end{array} \right]$$ The new geothermometer lowers calculated temperatures by ~85 °C on average relative to Putirka (2008, Eq. 33) and reduces the uncertainty by a factor of two (standard error of estimate ±20 °C). When applied to natural systems, we find this new clinopyroxene-liquid geothermometer reconciles many inconsistencies between experimental phase equilibria and preexisting geothermometry results for silicic volcanism, including those from the Bishop Tuff and Yellowstone caldera-forming and post-caldera rhyolites. We also demonstrate that clinopyroxene is not restricted to near-liquidus temperatures in rhyolitic systems; clinopyroxene can be stable over a broad temperature range, often down to the solidus. An Excel spreadsheet and Python notebook for calculating temperature with this new geothermometer may be downloaded from GitHub at http://bit.ly/cpxrhyotherm

High-pressure single-crystal X‑ray diffraction patterns on five synthetic Mg-Al tourmalines with near end-member compositions [dravite NaMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 OH, K-dravite KMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 OH, magnesio-foitite ☐(Mg 2 Al)Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 OH, oxy-uvite CaMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 O, and olenite NaAl 3 Al 6 Si 6 O 18 (BO 3 ) 3 O 3 OH, where o represents an X -site vacancy] were collected to 60 GPa at 300 K using a diamond-anvil cell and synchrotron radiation. No phase transitions were observed for any of the investigated compositions. The refined unit-cell parameters were used to constrain third-order Birch-Murnaghan pressure-volume equation of states with the following isothermal bulk moduli ( K 0 in GPa) and corresponding pressure derivatives ( K 0 ʹ = ∂ K 0 /∂ P ) T : dravite K 0 = 97(6), K 0 ʹ = 5.0(5); K-dravite K 0 = 109(4), K 0 ʹ = 4.3(2); oxy-uvite K 0 = 110(2), K 0 ʹ = 4.1(1); magnesio-foitite K 0 = 116(2), K 0 ʹ = 3.5(1); olenite K 0 = 116(6), K 0 ʹ = 4.7(4). Each tourmaline exhibits highly anisotropic behavior under compression, with the c axis 2.8–3.6 times more compressible than the a axis at ambient conditions. This anisotropy decreases strongly with increasing pressure and the c axis is only ~14% more compressible than the a axis near 60 GPa. The octahedral Y - and Z -sites’ composition exerts a primary control on tourmaline’s compressibility, whereby Al content is correlated with a decrease in the c -axis compressibility and a corresponding increase in K 0 and K ʹ. Contrary to expectations, the identity of the X -site-occupying ion (Na, 0 K, or Ca) does not have a demonstrable effect on tourmaline’s compression curve. The presence of a fully vacant X site in magnesio-foitite results in a decrease of K 0 ʹ relative to the alkali and Ca tourmalines. The decrease in K 0 ʹ for magnesio-foitite is accounted for by an increase in compressibility along the a axis at high pressure, reflecting increased compression of tourmaline’s ring structure in the presence of a vacant X site. This study highlights the utility of synthetic crystals in untangling the effect of composition on tourmaline’s compression behavior.

The 13 single-crystal adiabatic elastic moduli ( C ij ) of a C 2 /c jadeite sample close to the ideal composition (NaAlSi 2 O 6 ) and a natural P 2 /n diopside-rich omphacite sample have been measured at ambient condition by Brillouin spectroscopy. The obtained C ij values for the jadeite sample are: C 11 = 265.4(9) GPa, C 22 = 247(1) GPa, C 33 = 274(1) GPa, C 44 = 85.8(7) GPa, C 55 = 69.3(5) GPa, C 66 = 93.0(7) GPa, C 12 = 84(1) GPa, C 13 = 66(1) GPa, C 23 = 87(2) GPa, C 15 = 5.4(7) GPa, C 25 = 17(1) GPa, C 35 = 28.7(6) GPa, C 46 = 14.6(6) GPa. Voigt-Reuss-Hill averaging of the C ij values yields aggregate bulk modulus KS = 138(3) GPa and shear modulus G = 84(2) GPa for jadeite. Systematic analysis combing previous single-crystal elasticity measurements within the diopside-jadeite solid solution indicates that the linear trends are valid for most C ij values. The v p and v s of omphacite decrease with diopside content, though the velocity changes are small as diopside component exceeds 70%. We also found that both the isotropic v p and v s, as well as the seismic anisotropy of eclogite, changed strongly with the bulk-chemical composition. The relationship between the anisotropic velocities of eclogite and the chemical composition can be a useful tool to trace the origin of the eclogitic materials in the Earth’s mantle.

The elasticity of single-crystal hydrous pyrope with ~900 ppmw H 2 O has been derived from sound velocity and density measurements using in situ Brillouin light spectroscopy (BLS) and synchrotron X‑ray diffraction (XRD) in the diamond-anvil cell (DAC) up to 18.6 GPa at room temperature and up to 700 K at ambient pressure. These experimental results are used to evaluate the effect of hydration on the single-crystal elasticity of pyrope at high pressure and high temperature ( P - T ) conditions to better understand its velocity profiles and anisotropies in the upper mantle. Analysis of the results shows that all of the elastic moduli increase almost linearly with increasing pressure at room temperature, and decrease linearly with increasing temperature at ambient pressure. At ambient conditions, the aggregate adiabatic bulk and shear moduli ( K S 0, G 0 ) are 168.6(4) and 92.0(3) GPa, respectively. Compared to anhydrous pyrope, the presence of ~900 ppmw H 2 O in pyrope does not significantly affect its K S0 and G 0 within their uncertainties. Using the third-order Eulerian finite-strain equation to model the elasticity data, the pressure derivatives of the bulk [(∂ K S /∂ P ) T ] and shear moduli [(∂ G/ ∂ P ) T ] at 300 K are derived as 4.6(1) and 1.3(1), respectively. Compared to previous BLS results of anhydrous pyrope, an addition of ~900 ppmw H 2 O in pyrope slightly increases the (∂ K S /∂ P ) T , but has a negligible effect on the (∂ G/ ∂ P ) T within their uncertainties. The temperature derivatives of the bulk and shear moduli at ambient pressure are (∂ K S /∂ T ) P = –0.015(1) GPa/K and (∂ G/ ∂ T ) P = –0.008(1) GPa/K, which are similar to those of anhydrous pyrope in previous BLS studies within their uncertainties. Meanwhile, our results also indicate that hydrous pyrope remains almost elastically isotropic at relevant high P - T conditions, and may have no significant contribution to seismic anisotropy in the upper mantle. In addition, we evaluated the seismic velocities ( v P and v S ) and the v P / v S ratio of hydrous pyrope along the upper mantle geotherm and a cold subducted slabs geotherm. It displays that hydrogen also has no significant effect on the seismic velocities and the v P / v S ratio of pyrope at the upper mantle conditions.

A series of synthetic Mid-Ocean Ridge Basalt (MORB) glasses with Fe 3+ /Fe TOT from 0 to 1, determined previously by Mössbauer spectroscopy, were used to test methods for quantifying Fe 3+ /Fe TOT by Raman spectroscopy. Six numerical data reduction methods were investigated, based on conventional approaches as well as supervised and unsupervised machine learning algorithms. For the set of MORB glass standards, with fixed composition, the precision of all methods was ≤±0.04 (1 St.dev.) However, Raman spectra recorded for 42 natural MORB glasses from a wide range of locations revealed a strong correlation between the spectra and composition, despite the latter varying only over a relatively limited range, such that the methods calibrated using the glass standards are not directly applicable to the natural samples. This compositional effect can be corrected by using a compositional term that links spectral variations to the Fe 3+ /Fe TOT value of the glass. The resulting average Fe 3+ /Fe TOT determined by Raman spectroscopy was 0.090 ± 0.067 (n = 42). This value agrees with the latest Fe K -edge XANES and wet-chemistry estimates of 0.10 ± 0.02. The larger uncertainty of the Raman determination reflects the sensitivity of Raman spectroscopy to small changes in the glass structure. While this sensitivity is detrimental for high precision Fe 3+ /Fe TOT determinations, it allows the major element composition of natural MORB glasses to be determined within 1 mol% through the use of an artificial neural network. This suggests that Raman spectrometers may be used to determine the composition of samples in situ at difficult to access locations that are incompatible with X-ray spectrometry (e.g., mid-ocean ridges).

Lepageite, a new arsenite-antimonite mineral, was discovered in a granitic pegmatite hosted byserpentinites of the Szklary massif, Lower Silesia, southwest Poland. Lepageite is a primary mineral formed during injection of an evolved LCT-type melt related to anatectic processes within the metasedimentary-metavolcanic complex of the nearby Góry Sowie Block, ~380 Ma, into serpentinite of the Szklary massif and its contamination by fluid-mobile serpentinite-hosted elements, amongothers As and Sb, transported in the form of H 2 AsO– and HSbO 2 species at pH ≈ 9–11 and a low redox 3 potential of –0.7 to –0.3 V.

Synchrotron powder X‑ray diffraction (XRD) experiments and transmission electron microscopy (TEM) observations of heat-treated sillimanite at various pressures were conducted to clarify the detailed phase relation between sillimanite and mullite. Under TEM, heat-treated sillimanite frequently showed anti-phase boundary (APB)-like textures with a displacement vector of 1⁄2[001] sil . Additional scanning TEM energy-dispersive X‑ray spectroscopy analysis of regions with APB-like texture showed that they were clearly enriched in Al and accompanied by very fine, Si-rich glass inclusions, which indicates that the APB-like textures are composed of fine mullite. Moreover, synchrotron XRD patterns of these samples clearly showed double peaks of newly formed mullite and remnant sillimanite, indicating that the compositional transformation from sillimanite to mullite and glass is discontinuous. We separately determined the cell parameters of the sillimanite and mullite from the XRD pattern and found that the b axial length of the sillimanite increased with the treatment temperature, reflecting disordering of tetrahedral Al and Si in the sillimanite. In contrast, the positions of the deconvoluted mullite peaks indicated that the a axial length of mullite decreased as experimental pressure increased, owing to enrichment of the Si component. By projecting the cell parameters onto the a–b axial plane, the detailed changes in the crystallographic state of the sillimanite and mullite could be easily and comprehensively identified. On the basis of our results, we propose a new P-T diagram for the Al 2 SiO 5 system that shows the transformation boundary between sillimanite and mullite + SiO 2 -rich melt and the contour of the Al/Si order parameter of sillimanite.