Natural Garnet Research Articles

Thermal diffusivity of garnets at high temperature

Thermal diffusivity (D) of garnets with diverse chemical compositions was measured using the laser-flash technique, which is accurate (±2%) and isolates the lattice component from direct radiative transfer. Temperatures ranged from ~290 to ~1,600 K (unless limited by melting). Seven synthetic (e.g., YAG, GGG) and 15 natural garnets with two types of ionic substitution [Ca3(Fe,Al)2Si3O12 and (Mg,Fe,Ca)3Al2Si3O12] and varying amounts of OH- were examined. Cation substitution or hydroxyl incorporation lowers D from end-member values. Thermal diffusivity is constant once the temperature (T) exceeds a critical value (Tsat) of ~1,100 to 1,500 K. From ~290 K to Tsat, the measurements are best represented by 1/D=A+BT+CT2 where A, B, and C are constants. These constants vary little among diverse chemical compositions, suggesting that the oxygen sublattice controls heat transport. Higher order terms are needed only when Tsat is low, such as Ant Hill garnet wherein 1/D=0.049403+0.0032299T−2.3992T2×10−6+6.0168T3×10−10(1/D in s/mm2; T in K). The mean free path (λ, computed from D and sound velocities) is slightly larger than the lattice parameter above Tsat, in accord with phonon–phonon interactions requiring non-localized modes. At most temperatures, λ is nm-sized. Large values of λ are obtained by extrapolation to a few Kelvins, suggesting that boundary scattering can only be important at extremely cold temperatures. The observed behavior with T and chemical composition is consistent with the damped harmonic oscillator model. Phonon transport is best represented by inverse thermal diffusivity wherein 1/D goes as Tn where n is between 1 and 3 up to ~200 K, depends on a quadratic or cubic polynomial at moderate T, but is constant above Tsat. The predicted and observed temperature response of 1/D mimics the well-known form for heat capacity, in that acoustic modes control heat transport near cryogenic temperatures, optic phonons dominate above ambient temperature, and a limit analogous to that of Dulong and Petit is reached at very high temperature, due to full population of discrete phonon states.

Rates of Fe, Mg, Mn, and Ca diffusion in garnet

Extraction of the thermal and chemical histories preserved by modified compositional zoning in garnet is compromised by large uncertainties in experimentally determined diffusion coefficients for principal divalent cations. Some of this uncertainty derives from long down-temperature extrapolations, but much of it arises from an inability to account accurately for large variations in diffusivity as a function of garnet composition. Numerical simulation of stranded diffusion profiles in partially resorbed natural garnets constrains diffusivities at low temperature, and reveals a systematic dependence of diffusion rates on host garnet composition. A comprehensive model incorporating a simple compositional dependence allows accurate prediction of diffusion rates for Fe, Mg, Mn, and Ca for nearly all garnet compositions across the full range of geologically relevant temperatures, pressures, and oxygen fugacities.

Distinct local environments for Ca along the non-ideal pyrope–grossular solid solution: A new model based on crystallographic and EXAFS analysis

A multi-technique approach (based on electron microprobe analysis, structure refinement, and EXAFS analysis at the Ca K-edge) was used to characterise the local geometry of Ca in synthetic and natural garnet compositions referable to the pyrope–grossular solid solution. Multi-shell fits of the EXAFS data indicate that Ca assumes the standard [4 + 4]-fold coordination (the polyhedral shape being a triangular dodecahedron with Ca1–O = 2.30–2.31(1) and Ca2–O = 2.45–2.46(1) Å) when Ca > 1.50 atoms per formula unit (apfu), but assumes a nearly regular [8]-fold coordination with Ca–O = 2.35–2.36 (1) Å when (Mg, Fe 2+, Mn 2+) > 1.50 apfu. Therefore, in the pyrope-dominant structure the Ca1–O distance lengthens and the Ca2–O distance shortens to converge towards the value observed for the Mg2–O bond in pyrope. This finding is consistent with many distinct structural features observed in solid solution terms with (Mg, Fe 2+, Mn 2+) > 1.50 apfu or Ca > 1.50 apfu, as well as with the anomalous properties of the intermediate terms observed both in the short-range and in the long-range perspective. The presence of two distinct Ca coordinations in the pyrope (almandine, spessartine)-like and in the grossular-like structure, and thus of an isosymmetric transition at the intermediate composition, can help to explain both the strong and asymmetric non-ideality of the solid solution between pyrope (almandine) and grossular, as well as the differences in the ability to incorporate some trace elements (such as REE and actinides) which are commonly used as process-specific indicators. This feature must be taken into account when building theoretical models of the garnet solid solutions, which are at the moment the most promising approach for calculating thermodynamic properties or for interpreting and predicting trace-element behaviour in this crucial mineral phase.

X-ray powder refinement of a natural garnet from Diamantina, Minas Gerais, Brazil

The chemical composition, [Mg0.015 (2),Al1.20 (2),Si2.99 (1),Ca3.00 (2),Ti0.027 (3),Mn0.017 (3), Fe0.75 (3)]O12 (magnesium alumin­ium silicon calcium titanium manganese iron dodeca­oxide), of a natural garnet collected at Diamantina, Minas Gerais, Brazil was determined by electron microprobe analysis. The structure refinement from X-ray powder data is insensitive to the small amounts of Ti, Mn and Mg. The determined cation distribution using the Rietveld method is {Ca32+}[Al1.338 (8)3+,Fe0.662 (8)3+](Si34+)O122−, which is in agreement with a solid solution of the grossular-andradite series. The arrangement of the cations around the O atom is best described as a highly distorted tetrahedral.

The CaGeO3–Ca3Fe2Ge3O12 garnet join: an experimental study

Germanate garnets are often used as isostructural analogues of silicate garnets to provide insight into the crystal chemistry and symmetry of the less accessible natural garnet solid solutions. We synthesised two series of germanate garnets at 3 GPa along the joinVIIICa3VI(CaGe)IVGe3O12–VIIICa3VIFe2IVGe3O12 at 900 °C and 1,100 °C. Samples with compositions close to the CaGeO3 end-member consist of tetragonal garnet with a small amount of triclinic CaGe2O5. Samples with nominal compositions between XFe=0.4 and 1.0 consist of a mixture of tetragonal and cubic garnets; whereas, single-phase cubic garnets were obtained for compositions with XFe>1.2 (XFe gives the iron content expressed in atoms per formula unit, and varies between 0 and 2 along the join). Run products which were primarily single-phase garnet were investigated using Mossbauer spectroscopy. Spectra from samples synthesised at 1,100°C consist of one well-resolved doublet that can be assigned to Fe3+ in the octahedral site of the garnet structure. A second doublet, present primarily in samples synthesised at 900°C, can be assigned to Fe2+ at the octahedral sites of the garnet structure. The relative abundance of Fe2+ decreases with increasing iron content. Transmission electron microscopy analyses confirm this tendency and show that the garnets are essentially defect-free. The unit-cell parameters of tetragonal VIIICa3VI(CaGe)IVGe3O3 garnet decrease with increasing synthesis temperature, and the deviation from cubic symmetry becomes smaller. Cubic garnets show a linear decrease of unit-cell parameter with increasing iron content. The results are discussed in the context of iron incorporation into VIIIMg3VI(MgSi)IVSi3O3 majorite.

Precipitation of pyroxenes and Mg2SiO4 from majoritic garnet: simulation of peridotite exhumation from great depth

AbstractOur experimental simulations of the exhumation path of mantle peridotites show that high‐temperature (1400 °C) decompression of lherzolite from 14 to 13 and 12 GPa results in exsolution of interstitial blebs of diopside and Mg2SiO4 (wadsleyite) lamellae from majoritic garnet. At lower pressures (from 8 to 5 GPa, at T = 1400 °C) only enstatite exsolves as blebs at garnet boundaries. Continuous high‐temperature decompression from 14 to 7 GPa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims. No intracrystalline precipitation of pyroxene was observed in garnet, although such lamellae are found in some natural garnet peridotites. The explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens. Our data suggest that Mg2SiO4 and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites, whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths.

Single‐crystal elasticity of grossular‐ and almandine‐rich garnets to 11 GPa by Brillouin scattering

The high‐pressure elasticity of grossular‐rich Grs87And9Pyp2Alm2 and almandine‐rich Alm72Pyp20Sps3Grs3And2 natural garnet single crystals were determined by Brillouin scattering to 11 GPa in a diamond anvil cell. The experiments were carried out using a 16:3:1 methanol‐ethanol water mixture as pressure medium. The aggregate moduli as well as their pressure derivatives were obtained by fitting the data to Eulerian finite strain equations. The inversion yields KS0 = 165.0 ± 0.9 GPa, G0 = 104.2 ± 0.3 GPa, (∂KS/∂P)T0 = 3.8 ± 0.2, and (∂G/∂P)0 = 1.1 ± 0.1 for the grossular‐rich composition and KS0 = 174.9 ± 1.6 GPa, G0 = 95.6 ± 0.5 GPa, (∂KS/∂P)T0 = 4.7 ± 0.3, and (∂G/∂P)0 = 1.4 ± 0.1 for the almandine‐rich garnet. Both individual and aggregate elastic moduli of the two garnets define nearly linear modulus pressure trends. The elastic anisotropy of the garnets increases weakly in magnitude with compression. Isothermal compression curves derived from our results are generally consistent with static compression data under hydrostatic conditions, and the effects of nonhydrostaticity on previous diffraction data can be identified. The pressure derivatives obtained here are generally lower than those reported in high‐pressure polycrystalline ultrasonic elasticity studies. In combination with earlier Brillouin scattering data for pyrope, our results allow us to constrain the effect on elastic moduli of Fe2+‐Mg2+ substitution in pyrope‐almandine, Ca2+‐Mg2+ in pyrope‐grossular, and Fe3+‐Al3+ substitution in andradite‐grossular at high pressures. This new data set thus allows us to place improved constraints on the compositional dependence of seismic velocities in the rocks of the upper mantle.

Recalibration of mutually consistent garnet–biotite and garnet–cordierite geothermometers

Garnet–biotite and garnet–cordierite geothermometers have been consistently calibrated, using the results of Fe 2+–Mg cation exchange experiments and utilizing recently evaluated nonideal mixing properties of garnet. Nonideal mixing parameters of biotite (including Fe, Mg, Al VI, and Ti) and of cordierite (involving Fe and Mg) are evaluated in terms of iterative multiple least-square regressions of the experimental results. Assuming the presence of ferric Fe in biotite in relation to the coexisting Fe-oxide phases (Case A), and assuming the absence of ferric Fe in biotite (Case B), two formulae of garnet–biotite thermometer have been derived. The garnet–cordierite geothermometer was constructed using Margules parameters of garnet adopted in the garnet–biotite geothermometers. The newly calibrated garnet–biotite and garnet–cordierite thermometers clearly show improved conformity in the calculated temperatures. The thermometers give temperatures that are consistent with each other using natural garnet–biotite–cordierite assemblages within ±50 °C. The effects of ferric Fe in biotite on garnet–biotite thermometry have been evaluated comparing the two calibrations of the thermometer. The effects are significant; it is clarified that taking ferric Fe content in biotite into account leads to less dispersion of thermometric results.

Hydrogen in some natural garnets studied by nuclear reaction analysis and vibrational spectroscopy

A suite of 11 gem-quality, optically completely clear garnet crystals with a broad variety of compositions in the space of the end members pyrope–almandine–spessartine–grossular–andradite–goldmanite were analyzed for trace amounts of “water” by nuclear reaction analysis, NRA, based on the reaction 1H(15N, αγ)12C, and by single-crystal absorption spectroscopy in the νOH vibrational range using microscope-FTIR-spectroscopic methods. The aim was to establish a calibration of the highly sensitive IR method with high areal resolution for “water” determination in garnets, by studying garnets of a wide compositional range, and to check for compositional dependencies of the integral molar absorptivities of the “water” component, ɛint[1molH 2 O −1cm−2], in the nominally “water”-free garnets. The results of NRA show a broad variation of water contents in the range (14 ± 3) to (950 ± 80) wt ppmH 2 O, the values being low and very high for the garnet solid solutions (PyrAlm)SS and close-to-end-member GrossSS, respectively. There were no indications of inhomogeneities in the OH distribution, except possibly for one of the garnets (grossular, variety hessonite, from Tanzania). The quantitative evaluation of the complex νOH spectra, which showed similar shape only for members of the (PyrAlm)SS, yielded integral absorption coefficients, αint (cm−2), which allowed the calculation of integral molar absorptivities, ɛint, using the “water” values of NRA. The ɛint values obtained varied in a wide range but with no obvious correlation with the composition of the garnet except for the extremely high values, in the 104 range, of the two specimen with compositions close to end-member grossular. In all other garnets, ɛint was in the 103 range with an average of ɛint=3630±1580[1molH 2 O −1cm−2]. Therefore, this value is proposed for the use in routine “water” determinations of compositionally different garnets by the micro-IR method, except for garnets near to end-member grossular.

Alkalic magmas generated by partial melting of garnet pyroxenite

Many oceanic-island basalts (OIBs) with isotopic signatures of recycled crustal components are silica poor and strongly nepheline (ne) normative and therefore unlike the silicic liquids generated from partial melting of recycled mid-oceanic-ridge basalt (MORB). High-pressure partial-melting experiments on a garnet pyroxenite (MIX1G) at 2.0 and 2.5 GPa produce strongly ne-normative and silica-poor partial melts. The MIX1G solidus is located below 1350 and 1400 8C at 2 and 2.5 GPa, respectively, slightly cooler than the solidus of dry peridotite. Chemographic analysis suggests that natural garnet pyroxenite compositions straddle a thermal divide. Whereas partial melts of compositions on the silica-excess side of the divide (such as recycled MORB) are silica saturated, those from silica-deficient garnet pyroxenites can be alkalic and have similar- ities to low-silica OIB. Although the experimental partial melts are too rich in Al2O3 to be parental to highly undersaturated OIB suites, higher-pressure (4-5 GPa) partial melting of garnet pyrox- enite is expected to yield more appropriate parental liquids for OIB lavas. Silica-deficient garnet pyroxenite, which may originate by mixing of MORB with peridotite, or by recycling of other mafic lithologies, represents a plausible source of OIB that may resolve the apparent contradiction of strongly alkalic composition with iso- topic ratios characteristic of a recycled component.

Growth of Y3Al5O12 Crystals for Jewelry

The mechanisms responsible for the coloration of Y3Al5O12 (YAG) crystals are analyzed. The conditions are optimized for growing, in a reducing atmosphere, uniformly colored YAG-based crystals reproducing the color of both red and green natural garnets. The compositions ensuring a wide range of hues are Y,Zr,Ln3(Al,Sc)2Al3O12 (red), Y,Yb,Ln3(Al,Zr)2Al3O12 (green), and Y,Eu,Tb3(Al,Zr)2Al3O12 (blue). Data on the microhardness and microbrittleness of the crystals and their chromaticity diagrams are presented.

Characterization of trace Nd and Ce site preference and coordination in natural melanites: a combined X-ray diffraction and high-energy XAFS study

Natural melanite garnets from carbonatitic rocks have been studied by a multitechnique approach based on both experimental (chemical, diffractometric and spectroscopic) methods and full-multiple scattering calculations of the X-ray absorption near-edge structure. In particular, the site location and geometry of trace amounts of neodimium (from 176 to 1074 ppm) and cerium (791 ppm) in natural garnets have been studied by fluorescence-detected X-ray absorption fine-structure spectroscopy (XAFS) at high energy. The measurements, done at both Nd K (43569 eV) and Ce K (40443 eV) edges, demonstrate that, in all the samples, the trace elements are located in the dodecahedral X site and not in ill-defined defects. The local geometry around the two rare-earth elements is compatible with their ionic radius and is compared with that of Ca, the major element at the X site, as determined by single-crystal X-ray diffraction data. This work represents the first example of direct investigation of trace-level REE coordination in natural garnets, and confirms the great relevance for the Earth Sciences of the use of fluorescence XAFS at high energy.

Ion irradiation-induced amorphization and nano-crystal formation in garnets

The radiation susceptibility of garnet structure types (A 3B 2(XO 4) 3, I a3d, Z=8) has been examined by 1.0 MeV Kr 2+ irradiation with in situ transmission electron microscopy over the temperature range of 50–1070 K. The target garnets included five natural garnets: almandine, andradite, pyrope, grossular and spessartine, and five synthetic garnets incorporating various contents of actinides. The synthetic garnets were silicates (N-series) and ferrate–aluminates (G-series). The critical amorphization temperature ( T c), above which amorphization does not occur, were determined to be 1050 K for N77, 1130 K for N56, 1100 K for G3, 890 K for G4 and 1030 K for andradite. T c of the synthetic garnets increased with increasing mass density of the target and as the electronic-to-nuclear stopping power ratio decreased. During ion irradiation of the G3 garnet at a temperature of 1023 K near the T c, nano-crystals were produced in which the unit-cell parameter of the original garnet was halved (∼0.64 nm) as a result of a radiation-induced recrystallization process, without evidence for ordering of the cations.

Mie scattering and charge transfer phenomena as causes of the UV edge in the absorption spectra of natural and synthetic almandine garnets

The UV edge in the electronic absorption spectra of minerals, in many cases influencing their colour, is generally interpreted as the low-energy wing of very strong UV bands caused by ligand–metal charge transfer (CT) transitions (e.g. Burns 1993). However, Mie scattering theory shows that the presence of randomly distributed submicroscopic inclusions with narrow size distribution and a refractive index ni in a matrix with different refractive index nm may give rise to a λ-dependent, band-like scattering (e.g. Kortum 1969). Such scattering bands have so far not been considered as contributing to the UV edge. Single-crystal electronic absorption spectra of eight natural almandine-rich garnets (Alm60–Alm88), two synthetic almandine samples (Alm100), all of different colours, and synthetic spessartine were studied by means of a Zeiss microscope-spectrometer in the range 40 000–20 000 cm−1. Special techniques of spectral measurements with crossed analyzer and polarizer, which enable the registration of the scattering effect directly, were used as well. Four of the above garnets were also investigated using transmission electron microscopy. Different types of inclusions, from 10 to several 100 nm in size, were observed in the garnet matrices. They are abundant in cores of synthetic garnets, but very rare in most natural almandines studied. Electronic absorption spectra of the natural almandine garnets show largely varying UV edge position and, hence, intensity at a given wavenumber which correlates with the intensities of spin-forbidden dd bands of Fe3+ ions at 27 000 and 28 000 cm−1, superimposed on the long energy slope of the UV absorption. There are also positive correlations between Ti4+ and Fe3+ content, the latter recalculated on the basis of garnet stoichiometry, and UV edge intensity. Thus, the presence of Ti4+ and Fe3+ ions in octahedra, even in very low concentrations (0.0n at. pfu), leads to CT phenomena, that probably involve Fe2+ ions in edge-shared dodecahedral position and intensifies ligand- to-metal CT. The different colours of natural almandine garnets with similar Fe2+ contents studied here are caused by this effect. Consistent with the absence of inclusions in most natural garnets studied, λ-dependent scattering plays no role in their UV absorption. In contrast, in synthetic almandine and spessartine crystals, a different intensity of UV absorption was observed in inclusion-free rims and inclusion-enriched cores. Some of the latter demonstrate typical scattering patterns when measured at crossed polarizers.

New opportunities in trace elements structural characterization: high-energy X-ray absorption near-edge structure spectroscopy

Garnets in lower crustal mafic and ultramafic rocks usually contain rare-earth elements (REE) in trace concentrations. Direct characterization of REE at trace levels in natural garnets is not available in the literature because of the difficulty of obtaining structural information by means of conventional diffraction methods. Here, the characterization of Nd at trace levels (176-1029 p.p.m.) in a set of natural garnets performed by means of Nd K-edge X-ray absorption near-edge structure spectroscopy is presented, showing the capability of high-energy XANES for REE in trace structural determinations.

Ion irradiation effects in natural garnets: Comparison with zircon

The behavior of garnet ( A 3 B 2( XO 4) 3; I a3d; Z=8 ) under ion-beam irradiation was investigated in order to compare its radiation susceptibility to another orthosilicate: zircon, ZrSiO 4. Five natural end-member compositions were examined by in situ transmission electron microscopy during irradiation with 1.0 MeV Kr 2+ over the temperature range of 50–1070 K. The critical amorphization temperature, above which amorphization does not occur, was 1030 K for andradite, but could not be determined for the other garnet composition because the T c was higher than the highest temperature of the experiment. Based on topologic criterion, the degree of structural freedom in garnet is ∼−2.25 and for zircon ∼−1.5. Based on topology the critical amorphization dose for garnet should be higher than that of zircon; however, the average amorphization dose of garnet (0.20 dpa) is lower than that of zircon (0.37 dpa) at room temperature. This may be the result of the assumed value for the displacement energies, E d, used in the calculation of dpa. Garnet did not decompose, while zircon decomposes to SiO 2+ZrO 2 during the ion irradiation at high temperature. This behavior may be related to the phase relations of garnet which melts congruently and zircon which decomposes to ZrO 2+SiO 2.

Stability of osumilite coexisting with spinel solid solution in metapelitic granulites at high oxygen fugacity

Experiments were carried out in the quartz-saturated part of the system KFMASH at hematitemagnetite buffer conditions in a piston-cylinder apparatus using synthetic biotite, K-feldspar, quartz, and sillimanite 80 -quartz 20 gel. Natural garnet and sillimanite were used as seeds. Three bulk compositions were used: X Mg = 0.81, 0.72, and 0.53, and the experiments were vapor absent. In the bulk composition with the highest X Mg , dehydration-melting of biotite produced osumilite coexisting with spinel solid solution within the P-T window 7-8.5 kbar and 850-1000 °C. However, this mineral pair coexisted with cordierite at P < 7.5 kbar, and with sillimanite above 7.5 kbar. There is no appreciable change in the lower thermal stability of osumilite with lowering of X Mg in the bulk composition, but both the upper pressure and upper thermal stabilities are reduced. With increasing pressure, osumilite + spinel breaks down to an orthopyroxene + sillimanite assemblage. There is no overlap of the stability fields of osumilite and sapphirine. Rather, sapphirine-bearing assemblages are produced at the expense of osumilite with increasing temperature. Sapphirine, however, did not appear in the bulk composition having X Mg = 0.53. Instead, a wide stability field of spinel + cordierite + orthopyroxene + sillimanite is noted at P > 7 kbar and T > 950 °C. Garnet + orthopyroxene + sillimanite become stable at lower pressures with decreasing X Mg in the bulk composition. The effects of additional components in natural assemblages, such as Zn in spinel and F and Ti in biotite, on the KFMASH equilibria are evaluated qualitatively. The experimental results were applied to six high-grade terranes, where osumilite + spinel coexistence at peak metamorphic conditions has been reported. The deduced P-T window for the osumilite + spinel association is entirely consistent with independent PT estimates for the natural occurrences. It is argued that osumilite + spinel assemblage should be characteristic of metamorphism of highly magnesian pelites at high f O₂ along a prograde path of high dT/dP.

Melt growth of spessartine (Mn 3Al 2Si 3O 12)

The habit of spessartine (Mn 3Al 2Si 3O 12) garnet crystals grown in nature is determined by the presence of a large icositetrahedral crystal form {1 1 2} and a minor rhombic dodecahedron form {1 1 0}. This is in contrast with all other natural garnet crystals for which {1 1 0} is always more important than {1 1 2}. Theoretical growth forms derived from attachment energy calculations indicate that {1 1 0} should be the dominant form for all natural garnets. Spessartine crystals were grown experimentally at atmospheric pressure and at temperatures of 1175–1195°C from a molten stoichiometric mixture of tephroite (Mn 2SiO 4), γ -alumina (Al 2O 3) and quartz (SiO 2). Their habit is identical to that of natural grown spessartine crystals. The specific {1 1 2} spessartine habit may be caused by the presence of Mn 3+ during the growth as an impurity.

Hornblende–garnet–plagioclase thermobarometry: a natural assemblage calibration of the thermodynamics of hornblende

Calibrations are presented for an independent set of four equilibria between end-members of garnet, hornblende, plagioclase and quartz. Thermodynamic data from a large internally-consistent thermodynamic dataset are used to determine the ΔG° of the equilibria. Then, with the known mixing properties of garnet and plagioclase, the non-ideal mixing in amphibole is derived from a set of 74 natural garnet–amphibole–plagioclase–quartz assemblages crystallised in the range 4–13 kbar and 500–800 °C. The advantage of using known thermodynamic data to calculate ΔG° is that correlated variations of composition with temperature and pressure are not manifested in fictive derived entropies and volumes, but are accounted for with non-ideal mixing terms. The amphibole is modelled using a set of ten independent end-members whose mixing parameters are in good agreement with the small amount of data available in the literature. The equilibria used to calibrate the amphibole non-ideal mixing reproduce pressures and temperatures with average absolute deviations of 1.1 kbar and 35 °C using an average pressure–temperature approach, and 0.8 kbar with an average pressure approach. The mixing data provide not only a basis for thermobarometry involving additional phases, but also for calculation of phase diagrams in complex amphibole-bearing systems.

Sites are Separable in Garnets with ALCHEMI

It is shown in this paper that – in contrast to the accepted belief in the literature – it is possible to determine if a minority component is located on the dodecahedral, octahedral or tetrahedral sites in a garnet single crystal. Previous literature regarded the dodecahedral sites indistinguishable form the tetrahedral sites. Our prediction about the separability is based upon dynamical Bloch-wave calculations and proved experimentally in a transmission electron microscope (TEM) on two different natural garnets (almandine and grossular). The crystallographic positions of the minority components are determined in an ALCHEMI experiment that makes use of the channeling of electrons in special directions of a single-crystal sample.