Modeling the thermodynamic parameters of six endmember garnets at ambient and high pressures from vibrational data

Raman and infrared spectroscopic data at ambient and high pressures were used to compute the lattice contribution to the heat capacities and entropies of six endmember garnets: pyrope, almandine, spessartine, grossular, andradite and uvarovite. Electronic, configurational and magnetic contributions are obtained from comparing available calorimetric data to the computed lattice contributions. For garnets with entropy in excess of the computed lattice contribution, the overwhelming majority is found in the subambient temperature regime. At room temperature, the non-lattice entropy is approximately 11.5 J/mol-K for pyrope, 49 J/mol-K for almandine, and 19 J/mol-K for andradite. The non-lattice entropy for pyrope and some for almandine cannot be accounted for by magnetic or electronic contributions and is likely to be configurational in nature. Estimates of low temperature non-lattice entropies for both spessartine and uvarovite are made in absence of calorimetric measurements and are based on low temperature calorimetry of other minerals containing the Mn²⁺ and Cr³⁺ cations as well as on solid solution garnets containing these cations. The estimate for uvarovite non-lattice entropy is approximately 18 J/mol-K, while for spessartine, approximately 45 J/mol-K. Neither of these cations is expected to provide electronic contributions to the entropy. For both iron-bearing garnets, a small electronic or magnetic entropy contribution continues above ambient temperatures. High pressure data on pyrope, grossular and andradite permit calculation of the thermodynamic parameters at high pressures, which are important for computation of processes in the Earth’s mantle. Thermal expansion coefficients of these materials were found to be 1.6, 1.5, 1.6×10⁻⁵ K⁻¹ at 298 K, respectively, using a Maxwell relation. These closely match the literature values at ambient conditions.     

Garnet Types from the Cazadero Area, California

This is the third in a series of papers on glaucophane schistsfrom the Franciscan Formation near Cazadero, California. Previouspapers describe three distinct types of glaucophane-bearingFranciscan metamorphic rocks near Cazadero. The purpose of this study is to investigate the garnets presentin metamorphic types III (bedrock schists) and IV (tectonicblocks) as defined by Coleman & Lee (1963). Twenty-fourgarnet analyses are presented. Sixteen of these are from (aragonite-bearing)type III glaucophane schists, and eight are from type IV glaucophaneschists. Type IV rocks include California eclogites. Type III rocks include metabasalt, metachert, metashale, meta-ironstone,and metacarbonate that were formed under high pressure and relativelylow temperature. These rocks contain garnets that display awide range of composition, but the dominant molecules representedare consistently almandine, spessartine, and grossular. Type IV rocks are mainly metabasalts that were probably formedunder higher temperatures and pressures than type III rocks.There is a distinct difference between garnets from type IIIrocks and those from type IV (including eclogites); the lattercontainless spessartine and more pyrope, and the dominant moleculesare almandine and grossular. The four analyses of garnets fromCalifornia eclogites have an average pyrope content of aboutten molecular per cent, and they extend the range of compositionreported for eclogite garnets. Quantitative spectrographic determinations of minor elementsare listed for each of the garnets described. The values determinedfor some of the minor elements have a wide range and a capriciousdistribution over a few feet of outcrop area. As a group, both the garnets from type III rocks and those fromtype IV are pyralspites with large contents (as much as 35 molecularper cent) of ugrandite. This unusual admixture of the pyralspiteand ugrandite garnet series may have resulted in part from theconditions (high pressures and relatively low temperatures)under which the enclosing rocks were recrystallized.     

Granate mit Zusammensetzungen zwischen Almandin und Grossular aus den westlichen Hohen Tauern, Österreich

Garnets in epidote-bearing gneisses and mica schists from the western Hohe Tauern, Austria, have been analyzed by optical, x-ray powder diffraction, wet chemical and electron microprobe methods.The garnets frequently show zonal structure and their core compositions are in the range 45–52% grossular, 31–44% almandine, 3–13% spessartine, 0–7% andradite and 0–2% pyrop. The refractive indices n=1.780–1.786 and the lattice parameter a=11.68–11.73 Å likewise are intermediate between almandine and grossular. It is suggested that there is complete solid solution between almandine and grossular, at least under the conditions of greenschist to low-temperature amphibolite facies of regional metamorphism of the Tauern area.

---

---

Prof. Dr. F. Karl starb am 15. 8.1972.

---

Dank. Die Mikrosonde wurde von der Stiftung Volkswagenwerk bereitgestellt, und die Probenahme wurde von der Deutschen Forschungsgemeinschaft unterstützt. Wir danken Herrn Dr. P. K. Hörmann (Kiel) für die naßchemische Granatanalyse und Herrn Dr. K. Abraham (Bochum) für die Bereitstellung seines Korrekturprogrammes für Mikrosondenanalysen.     

Single-crystal hydrostatic compression of synthetic pyrope, almandine, spessartine, grossular and andradite garnets at high pressures

The compression of synthetic pyrope Mg₃Al₂ (SiO₄)₃, almandine Fe₃Al₂(SiO₄)₃, spessartine Mn₃Al₂ (SiO₄)₃ grossular Ca₃Al₂(SiO₄)₃ and andradite Ca₃Fe₂ (SiO₄)₃ was studied by loading the crystals together in a diamond anvil cell. The unit-cell parameters were determined as a function of pressure by X-ray diffraction up to 15 GPa using neon as a pressure transmitting medium. The unit-cell parameters of pyrope and almandine were measured up to 33 and 21 GPa, respectively, using helium as a pressure medium. The bulk moduli, K _{T 0}, and their first pressure derivatives, K _{T 0} ^′, were simultaneously determined for all five garnets by fitting the volume data to a third order Birch-Murnaghan equation of state. Both parameters can be further constrained through a comparison of volume compressions between pairs of garnets, giving for K _{T 0} and K _{T 0} ^′ 171(2) GPa and 4.4(2) for pyrope, 185(3) GPa and 4.2(3) for almandine, 189(1) GPa and 4.2 for spessartine, 175(1) GPa and 4.4 for grossular and 157(1) GPa and 5.1 for andradite, where the K _{T 0} ^′ are fixed in the case of spessartine, grossular and andradite. Direct comparisons of the unit-cell volumes determined at high pressures between pairs of garnets reveal anomalous compression behavior for Mg²⁺ in the 8-fold coordinated triangular dodecahedron in pyrope. This agrees with previous studies concerning the compression behaviors of Mg²⁺ in 6-fold coordinated polyhedra at high pressures. The results show that simple bulk modulus–volume systematics are not obeyed by garnets. Received: 29 July 1998 / Revised, accepted: 7 April 1999     

Raman spectra of silicate garnets

The single-crystal polarized Raman spectra of four natural silicate garnets with compositions close to end-members almandine, grossular, andradite, and uvarovite, and two synthetic end-members spessartine and pyrope, were measured, along with the powder spectra of synthetic pyrope-grossular and almandine-spessartine solid solutions. Mode assignments were made based on a comparison of the different end-member garnet spectra and, in the case of pyrope, based on measurements made on additional crystals synthesized with ²⁶Mg. A general order of mode frequencies, i.e. R(SiO₄)>T(metal cation)>T(SiO₄), is observed, which should also hold for most orthosilicates. The main factors controlling the changes in mode frequencies as a function of composition are intracrystalline pressure (i.e. oxygen-oxygen repulsion) for the internal SiO₄-vibrational modes and kinematic coupling of vibrations for the external modes. Low frequency vibrations of the X-site cations reflect their weak bonding and dynamic disorder in the large dodecahedral site, especially in the case of pyrope. Two mode behavior is observed for X-site cation vibrations along the pyrope-grossular binary, but not along the almandine-spessartine join. Received: 3 December 1996 / Revised, accepted: 13 April 1997     

接触交代夕卡岩型矿床中石榴子石和辉石成分特点及其与矿化的关系

本文对中国十四个接触交代钙夕卡岩矿床和钙-镁夕卡岩矿床中的三百多个样品的石榴子石和辉石成分进行了电子探针分析。不同矿床类型的石榴子石和辉石成分代表着钙夕卡岩矿床的十个矿种(Fe、Fe-Cu、Pb-Zn、W、Sn、Sn-Mo-Bi-W、、W-Bi-Cu-Mo、Cu-Zn、Cu-Sn、W-Zn-Cu)和钙-镁夕卡岩矿床的三个矿种(Fe-Cu、Mo、Pb-Zn)。石榴子石和辉石成分变化范围大,大多数石榴子石是含锰铝榴石+铁铝榴石+镁铝榴石小于15％(摩尔百分数)的钙铝榴石-钙铁榴石固溶体;大多数辉石是含小于5％的锰钙辉石的透辉石-钙铁辉石固溶体。有些Pb-Zn钙-镁夕卡岩矿床中的辉石显示出Mn含量有所增加。只有Sn和W钙夕卡岩矿床及Pb-Zn钙-镁夕卡岩矿床含(Sps+Alm+Pyr)总量大于15％的石榴子石。石榴子石和辉石成分与夕卡岩矿床金属矿化类型之间有某些联系。     

On recasting analyses of garnet into end-member molecules

Most published analyses of garnet deviate from structural ideality. Consequently, compositions expressed as molecular percentages of end-member molecules may differ if different re-calculation sequences have been used. A suitable standard calculation procedure is presented, and is demonstrated to be satisfactory by its application to 69 published analyses of garnets both common and rare.It is seldom necessary to use molecules other than pyrope, almandine, spessartine, grossular, andradite, uvarovite and hydrogrossular, and most analyses can be recast into four or less molecules which exceed 3% of the garnet. This means that most analyses can be visually displayed in a composition tetrahedron.It is suggested, that the percentage number of cations which can be allocated to garnet molecules is a figure useful for assessment of analytical quality. More than 95% of the cations can be so allocated in the majority of the analyses considered.Full details of the proposed scheme are appended together with a worked example which demonstrates the abbreviated procedure which applies to most common garnets. A compilation is given of the common physical properties which have been measured for synthetic end-member garnets of the types used in the calculation scheme.S.A. UMP Publication No. 5.     

Origin of garnet phenocrysts in calc-alkaline rocks

A large number of garnet phenocrysts from Palaeozoic rhyodacites and granodiorite porphyrites from Central and Northeastern Victoria have been analyzed using the electron microprobe. These garnets, from an area of several thousand square miles, are very uniform in composition (dominantly almandine, with subordinate pyrope and minor grossular and spessartine). They show minor zoning with a very thin outer rim slightly richer in almandine and spessartine than the remainder of the phenocryst. They are surrounded by a complex intergrowth of cordierite and hypersthene forming a reaction rim. Resorbed quartz phenocrysts are typically associated with the garnet phenocrysts. The uniform composition, the conspicuous size and the subhedral-euhedral form of the garnet phenocrysts indicate that they crystallized directly from the acid calc-alkaline magma at an early stage of its crystallization. High pressure experimental work on a natural garnet-bearing rhyodacite glass demonstrates that almandine-rich garnet and quartz are near-liquidus phases at 18 and 27 kb , but garnet does not appear until well below the liquidus at 9 kb. A comparison of the composition of the experimentally crystallized garnets with the natural garnets suggests that these acid calc-alkaline magmas began to crystallize at pressures between 9 and 18 kb, i.e. at depths corresponding to the lower crust or upper mantle.     

Mineralogy of New Caledonian metamorphic rocks

A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouégoa destrict. The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments; calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional rocks. Garnets occur in metasediments throughout the lawsonite zone as rare tiny crystals (<0.03 mm diam.). Garnets first appear in metabasalts in lawsonite-epidote transitional rocks. Garnets are widespread and abundant in epidote-zone metasediments and metabasalts. 45 garnets from rocks representative of all lithologies and metamorphic grades have been analysed with an electron-probe microanalyser. The garnets were consistently zoned. Garnets in lawsonite and low-grade epidote zones show a “bell-type” zoning with cores enriched in Mn relative to Fe and rims enriched in Fe, Mg and frequently Ca. Garnets from high-grade epidote-zone metapelites and metabasalts show, in addition, a shallow oscillatory zoning with complimentary variations in Mn and Fe equivalent to 5 mole- % spessartine and almandine. The Fe-for-Mn substitutional zoning, believed to be caused by a diffusion/saturation effect similar to that of the Rayleigh fractional model (Hollister, 1966), appears to have had superimposed on it the effects of parent-rock chemistry and metamorphic grade which control in a complex manner the composition of the cores and the rims of garnets. Garnets from different rock types and metamorphic grade are compositionally distinct. Garnets from lawsonite-zone rocks, irrespective of parent-rock chemistry appear to be spessartine. Garnets from epidote-zone metaigneous rocks and most metasediments are almandine. Garnets from epidote-zone metasediments with bulk-rock compositions which are manganiferous, or have high oxidation ratios, or both, may be spessartine-rich. Garnets from metabasalts are consistently more pyropic in both core and rim compositions than garnets from pelitic metasediments; the pyrope content of cores and rims of garnets from equivalent rock types and mineral assemblages increases with increasing metamorphic grade. Cores of garnets from epidote-zone pelites are richer in grossular than garnets from lower-grade pelites. The reaction which brings almandine garnet into Ouégoa district blueschist assemblages simultaneously with the replacement of lawsonite by epidote involves components of chlorites and sodic amphiboles and can be represented by the following simplified equation: ferroglaucophane+Fe-rich chlorite+lawsonite → glaucophane+Mg-rich chlorite+epidote+almandine.     

<Emphasis Type="Italic">P-T</Emphasis> conditions of the Jandagh metapelitic schists,Northeastern Isfahan Province,Iran

The metapelitic schists of Jandagh or simply Jandagh metapelites can be divided into four groups based on mineral assemblages: (1) quartz-muscovite schists, (2) quartz-muscovite-biotite schists, (3) garnet-muscovite-chlorite schists, and (4) garnet-muscovite-staurolite schists. The Jandagh garnet-muscovite-chlorite schists show the first appearance of garnets. These garnets contain 58–76% almandine, 1–18% spessartine, and 8–20% grossular. Microprobe analysing across the garnets demonstrates an increase in Mg# from core to rim. This is a feature of the prograde metamorphism of metapelites. Well-preserved garnet growth zoning is a sign that metapelites were rapidly cooled and later metamorphic phases had no effect here. The appearance of staurolite in garnet-muscovite-chlorite schists signifies a beginning of the amphibolite facies. The absence of zoning in staurolite suggests that its formation and growth during prograde metamorphism occurred at a widely spaced isograde. Thermobarometric investigations show that the Jandagh metapelites were formed within a temperature range of 400–670°C and pressures of 2.0–6.5 kbar. These results are in agreement with the mineral paragenetic evidence and show the development of greenschist and amphibolite facies in the area studied.     

Chemical composition and evolution of the garnets in the Astamal Fe-LREE distal skarn deposit,Qara-Dagh–Sabalan metallogenic belt,Lesser Caucasus,NW Iran

The chemistry of garnet can provide clues to the formation of skarn deposits. The chemical analyses of garnets from the Astamal Fe-LREE distal skarn deposit were completed using an electron probe micro-analyzer. The three types of garnet were identified in the Astamal skarn are: (I) euhedral coarse-grained isotropic garnets (10–30 mm across), which are strongly altered to epidote, calcite and quartz in their rim and core, with intense pervasive retrograde alteration and little variation in the overall composition (Adr_94.3–84.4 Grs_8.5–2.7 Alm_1.9–0.2) (garnet I); (II) anhedral to subhedral brecciated isotropic garnets (5–10 mm across) with minor alteration, a narrow compositional range along the growth lines (Adr_82–65.4 Grs_21.9–11.7 Alm_11.1–2.4) and relatively high Cu (up to 1997 ppm) and Ni (up to 1283 ppm) (garnet II); and (III) subhedral coarser grained garnets (> 30 mm across) with moderate alteration, weak diffusion and irregular zoning of discrete grossular-almandine-rich domains (Adr_84.2–48.8 Grs_32.4–7.6 Alm_19.9–3.5) (garnet III). In the third type, the almandine content increases with increasing grossular/andradite ratio and increasing substitutions of Al for Fe^3 +.Almost all three garnet types have been replaced by fine-grained, dark-brown allanite that is typically disseminated and has the same relief as andradite. The Cu content increases while Ni content decreases slightly towards the rim of garnet II and garnet III. Copper in garnet II is positively correlated with increasing almandine content and decreasing andradite content, indicating that the almandine structure, containing relatively more Fe^2 +, is more suitable than andradite and grossular to host divalent cations such as Cu^2 +. Nickel in garnet II is positively correlated with increasing andradite content, total Fe, and decreasing almandine content. This is because Ni^2 + substitutes for Fe^3 + in the Y (octahedral) position. There are unusual discrete grossular-almandine rich domains within andraditic garnet III, indicating the low diffusivity of Ca compared to Fe at high temperatures.     

Garnet-bearing granite from the Třebíč pluton, Bohemian Massif (Czech Republic)

Summary Garnet occurs as a significant mineral constituent of felsic garnet-biotite granite in the southern edge of the Třebíč pluton. Two textural groups of garnet were recognized on the basis of their shape and relationship to biotite. Group I garnets are 1.5–2.5 mm, euhedral grains which have no reaction relationship with biotite. They are zoned having high X_Mn at the rims and are considered as magmatic. Group II garnets form grain aggregates up to 2.5 cm in size, with anhedral shape of individual grains. The individual garnet II grains are usually rimmed by biotite and have no compositional zoning. The core of group I garnets and group II garnets contains 67–80 mol% of almandine, 5–19 mol% of pyrope, 7–17 mol% of spessartine and 2–4 mol% of grossular. Biotite occurs in two generations; both are magnesian siderophyllites with Fe/(Fe + Mg) = 0.50–0.69. The matrix biotite in granites (biotite I) has high Ti content (0.09–0.31 apfu) and Fe/(Fe + Mg) ratio between 0.50 and 0.59. Biotite II forms reaction rims around garnet, is poor in Ti (0.00–0.06 apfu) and has a Fe/(Fe + Mg) ratio between 0.61 and 0.69. The textural relationship between biotite and garnet indicates that garnet reacted with granitic melt to form Ti-poor biotite and a new granitic melt, depleted in Ti and Mg and enriched in Fe and Al. In contrast to the host durbachites (hornblende-biotite melagranites), which originated by mixing of crustal melts and upper mantle melts, the origin of garnet-bearing granites is related to partial melting of the aluminium-rich metamorphic series of the Moldanubian Zone.     

The genetic significance of almandine-pyrope phenocrysts in the calc-alkaline Borrowdale Volcanic Group,Northern England

A suite of garnet-bearing andesites and dacites from the Ordovician of N. W. England is described and major- and trace-element analyses of the garnet phenocrysts are presented. The garnets are of almandine-pyrope composition, with minor amounts of spessartine and grossular, and often show marked reversed zoning; the crystal becoming progressively enriched in pyrope towards the margin. Garnets from the dacites are consistently richer in almandine and spessartine than are those from the andesites.From a consideration of the chemistry of the garnet phenocrysts and host rocks, especially La and Y abundances, it is shown that garnet could not have been removed from the magma in quantities sufficient to affect the liquid composition. Consequently the magma must have evolved by some process other than crystal fractionation. It is proposed that the magma was generated by the partial melting of oceanic crust along an ancient Benioff zone, stored at depth (possibly at the crust/mantle interface) long enough for garnet to nucleate, and then transferred rapidly to the surface. Isobaric crystallisation of the garnet phenocrysts at depth could explain the reversed zoning observed.     

Simulation of thermodynamic mixing properties of garnet solid solutions at high temperatures and pressures

We present results of high temperature, high pressure atomistic simulations aimed at determining the thermodynamic mixing properties of key binary garnet solid solutions. Computations cover the pressure range 0–15 GPa and the temperature range 0–2000 K. Through a combination of Monte-Carlo and lattice-dynamics calculations, we derive thermodynamic mixing properties for garnets with compositions along the pyrope–almandine and pyrope–grossular joins, and compare these with existing experimental data. Across the pressure–temperature range considered, simulations show virtually ideal mixing behaviour in garnet on the pyrope–almandine join, while large excess volumes and enthalpies of mixing are predicted for garnet along the pyrope–grossular join. Excess heat capacities and entropies are also examined. These simulations shed additional light on the link between the behaviour at the atomic level and macroscopic thermodynamic properties: we illustrate the importance of certain atomistic Ca–Mg contacts in the pyrope–grossular solid solutions. For simulation techniques of this type to become sufficiently accurate for direct use in geological applications such as geothermobarometry, there is an urgent need for improved experimental determinations of several key quantities, such as the enthalpies of mixing along both joins.     

西藏甲玛铜多金属矿石榴子石矿物学特征及成因意义

甲玛铜多金属矿主要的工业矿体赋存于矽卡岩中,石榴石矽卡岩是主要的矽卡岩类型,因此,研究石榴子石的矿物学特征及其成因具有重要的意义。本文综合前人研究成果,重点对采于甲玛矿区不同钻孔的、不同空间位置的石榴子石进行了矿石学及电子探针分析研究,并系统对比其矿物学特征。对18个石榴子石测点和项目组其它电子探针分析成果表明,甲玛铜多金属矿的石榴子石均为钙质系列,由贫Ti的钙铁榴石和富Ti、Mg、Mn的钙铝榴石组成。受流体氧逸度的制约,矿区中心以钙铁榴石为主,边缘以钙铝榴石为主,从深部至浅部钙铁榴石含量减少,钙铝榴石含量增加。此外,石榴子石"锯齿状"环带成分暗示了其流体过程的多期多阶段性。     

REE in skarn systems: A LA-ICP-MS study of garnets from the Crown Jewel gold deposit

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe³⁺) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids.Garnets from the Crown Jewel deposit range from Adr₃₀Grs₇₀ to almost pure andradite (Adr_>99). Fe-rich garnets (Adr_>90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE³⁺ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr_>90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE³⁺ following a coupled, YAG-type substitution mechanism , whereas Eu²⁺ substitutes for X²⁺ cations. Thermodynamic data (e.g., H_mixing) in grossular-andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions.Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.     

Correlations between the infrared spectrum and the composition of garnets in the pyrope-almandine-spessartine series

The i.r. spectrum of 13 analyzed garnets of the pyralspite group has been investigated in the 1400-200 cm^–1 region, and correlations have been found between the spectrum and the chemical composition. The results include: typical features in the spectrum of the end-members pyrope, almandine and spessartine; relationships between the spectrum and the pyrope percentage in pyrope-almandine solid solutions; and the influence of the CaO (grossularite) amount on the shape of the low-frequency absorption bands. These data allow a semiquantitative determination of the pyrope percentage in pyrope-almandine solid solutions.     

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 ν_OH 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, ?_int[1mol_H2O^?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 ppm_H2O, the values being low and very high for the garnet solid solutions (PyrAlm)_SS and close-to-end-member Gross_SS, 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 ν_OH spectra, which showed similar shape only for members of the (PyrAlm)_SS, yielded integral absorption coefficients, α_int (cm^?2), which allowed the calculation of integral molar absorptivities, ?_int, using the “water” values of NRA. The ?_int 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, ?_int was in the 10³ range with an average of ?_int=3630±1580[1mol_H2O^?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.     

广西拉么Zn—Cu矿床中夕卡岩的主要矿物特点与Ca—Fe—Si—C—O体系T—lgfO2图解

用电子探针数据研究了该矿床中主要夕卡岩矿物特点及其变化规律。采用理想结晶溶液固溶体位置混合模式,计算了石榴子石和辉石矿物对中钙铁榴石和钙铁辉石的摩尔分数。在计算纯固相和流体相参加的反应抵达平衡时的边界方程中,加上理想结晶固溶体中端元组分摩尔分数的修正项后,我们便可以计算出有纯固相、流体相和理想结晶溶液固溶体参加反应的在给定T、P条件下的lgfo_2值。计算结果表明:无变度点4周围的Hed+And+Wol组合对本矿区具有重要的地质地球化学意义。     

Diffusion of divalent cations in garnet: multi-couple experiments

We demonstrate the possibility of studying several diffusion couples in a single run, i.e. under almost similar P–T–t– conditions, allowing direct comparison of the diffusion rates in different diffusion couples. Thus the duration of experimental study and the risk of failure of expensive experimental equipment can be decreased considerably. The diffusion experiments were carried out in piston-cylinder apparatus. Gem-quality garnets of almandine, spessartine and grossular compositions together with inclusion-rich eclogitic garnets were embedded in a powder of natural pyrope and annealed together under dry conditions at P = 1.9–3.2 GPa and T = 1,070–1,400°C. Diffusion profiles were measured by electron microprobe and fitted numerically on the basis of multicomponent diffusion theory. The datasets derived from different diffusion couples yields parameters of the Arrhenius equation for Ca, Mg and Fe in natural eclogitic garnets and Mg, Mn and Fe in gem-quality garnets. We have also studied the effect of grain-boundary diffusion in the sintered pyrope matrix on interdiffusion on the basis of 2D modeling. Under conditions analogous to those of our experimental runs, we show that observed irregularities in some measured diffusion profiles (not applied for the diffusion modeling) can be directly related to the superposition of local grain-boundary diffusion on dominant volume diffusion.