A unique enstatite megacryst with coexisting CR-poor and CR-rich garnet, Weltevreden Floors, South Africa
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Abstract:
A large (17 cm) single crystal of enstatite (En88) contains Mg-ilmenite (MgO ∼ 12 wt%) and inclusions of polyphase garnets. The major garnet is orange in color and is predominantly pyrope-almandine (Py71Al21Gr8) whereas inside this garnet is a second one that is pink in color, and Cr-rich (Cr-Py33Py28Al19Gr20). This Cr-rich garnet itself contians inclusions of Cr-diopside, chromite and ilmenite. The enclosing orange almandine garnet at the boundary with the pink Cr-rich garnet contain rounded Mg-ilmenites together with a string of elongated olivines (Fo86) that are in optical continuity. Also present in the same zone is diopside. Within the enstatite host and in close association with the polyphase garnet assemblage are calcite, Ti-phlogopite and serpentine. Although it is conceivable the Cr-rich garnet and related phases represent the remnant of an earlier pre-existing rock that underwent considerable partial melting it must also be considered that the second garnet may be metamorphic in origin. However, irrespective of the origin, based on orthopyroxene geothermometry and A12O3 content possible P and T of equilibration are in the region of 60kb and 1200°C.Keywords:
Enstatite
Diopside
Almandine
Pyrope
Ilmenite
Phlogopite
Chromite
Pyrope
Enstatite
Sillimanite
Coesite
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Citations (44)
Petrographical and mineral chemical data are given for the eclogites which occur in the garnet‐kyanite micaschists of the Penninic Dora‐Maira Massif between Brossasco, Isasca and Martiniana (Italian Western Alps) and for a sodic whiteschist associated with the pyrope‐coesite whiteschists of Martiniana. The Brossasco‐Isasca (BI) eclogites are fine grained, foliated and often mica‐rich rocks with a strong preferred orientation of omphacite crystals and white micas. Porphyroblasts of hornblende are common in some varieties, whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz, jadeite and garnet. These features differentiate the BI eclogites from the eclogites that occur in other continental units of the Western Alps, which all belong to type C. Garnet, sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist. Sodic pyroxene in the eclogites is an omphacite often close to Jd 50 Di 50 , with very little acmite and virtually no Al IV , and impure jadeite in the leucocratic layers and in the sodic whiteschist. Garnet is almandine with 20–30 mol. % for each of the pyrope and grossular components in the eclogites and a pyrope‐rich variety in the sodic whiteschist. White mica is a variably substituted phengite, and paragonite apparently only occurs as a replacement product of kyanite. Amphibole is hornblende in the eclogites, but the most magnesian glaucophane yet described in the sodic whiteschist. Quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets. The P‐T conditions during the VHP event are constrained in the eclogites by reactions which define a field ranging from 27–28 kbar to 35 kbar and from 680 to 750° C. These temperatures are consistent with the results of garnet‐pyroxene and garnet‐phengite geothermometry which suggest that the eclogites may have equilibrated at around 700° C. In the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite‐pyrope garnet‐glaucophane compatibility. As in the eclogites water activity must have been low. Such conditions are close to the P‐T values estimated for the early Alpine recrystallization of the pyrope‐coesite rock and, like petrographical and mineralogical features, set aside the BI eclogites from the other eclogites of the Western Alps, instead indicating a close similarity to some of the eclogite bodies occurring in the Adula nappe of the Central Alps. An important corollary is that glaucophane stability, at least in Na‐ and Mg‐rich compositions and under very high pressures, may extend up to 700° C, in agreement with the HT stability limit suggested by experimental studies.
Pyrope
Coesite
Omphacite
Grossular
Phengite
Massif
Pyroxene
Pseudomorph
Glaucophane
Amphibole
Almandine
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Citations (81)
Phlogopite
Enstatite
Metasomatism
Pyrope
Peridotite
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Citations (1)
The internal energies and entropies of 21 well-known minerals were calculated using the density functional theory (DFT), viz. kyanite, sillimanite, andalusite, albite, microcline, forsterite, fayalite, diopside, jadeite, hedenbergite, pyrope, grossular, talc, pyrophyllite, phlogopite, annite, muscovite, brucite, portlandite, tremolite, and CaTiO3-perovskite. These thermodynamic quantities were then transformed into standard enthalpies of formation from the elements and standard entropies enabling a direct comparison with tabulated values. The deviations from reference enthalpy and entropy values are in the order of several kJ/mol and several J/mol/K, respectively, from which the former is more relevant. In the case of phase transitions, the DFT-computed thermodynamic data of involved phases turned out to be accurate and using them in phase diagram calculations yields reasonable results. This is shown for the Al2SiO5 polymorphs. The DFT-based phase boundaries are comparable to those derived from internally consistent thermodynamic data sets. They even suggest an improvement, because they agree with petrological observations concerning the coexistence of kyanite + quartz + corundum in high-grade metamorphic rocks, which are not reproduced correctly using internally consistent data sets. The DFT-derived thermodynamic data are also accurate enough for computing the P-T positions of reactions that are characterized by relatively large reaction enthalpies (> 100 kJ/mol), i.e., dehydration reactions. For reactions with small reaction enthalpies (a few kJ/mol), the DFT errors are too large. They, however, are still far better than enthalpy and entropy values obtained from estimation methods.
Grossular
Forsterite
Pyrope
Andalusite
Fayalite
Anorthite
Muscovite
Almandine
Diopside
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Citations (32)
Summary Examination of some pyroxenite nodules from the Jagersfontein kimberlite shows that they have suffered variable deformation followed by different degrees of recovery. Most interes~ting of the nodules is the 'diallage rock', which consists of highly sheared and broken lamellar crystals of diopside containing exsolved enstatite. Petrographic evidence indicates a pre-tectonic exsolution of enstatite and a syntectonic exsolution of pyrope-almandine from enstatite. Deformation occurred in the mantle, prior to incorporation of the nodule in the kimberlite. Compositions of pyroxenes from the diallage rock suggest it re-equilibrated at a temperature of 1000° and at a pressure of about 35 kb. Olivine and phlogopite, accompanied by serpentine, occur only in broken kink-bands in the diallage rock and they are considered to be of secondary origin, precipitated from kimberlite magma at temperatures near 700°
Enstatite
Diopside
Pyrope
Xenolith
Phlogopite
Almandine
Sanidine
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Citations (6)
This paper is the fourth in a series relating the lattice vibrational properties to the thermodynamic properties of minerals. The temperature dependence of the harmonic lattice heat capacity is calculated from a model which uses only elastic, crystallographic, and spectroscopic data for the following minerals: calcite, zircon, forsterite, grossular, pyrope, almandine, spessartine, andradite, kyanite, andalusite, sillimanite, clinoenstatite, orthoenstatite, jadeite, diopside, tremolite, talc, and muscovite. The heat capacities of these minerals reflect structural and compositional differences. The ‘excess’ entropy of pyrope—compared with that of grossular—is shown to arise from low‐frequency optic modes of vibration. The entropy differences between kyanite, andalusite, and sillimanite are well reproduced by the model, although the absolute values calculated are systematically about 3% high. Model values of the heat capacity and entropy are compared with experimental values at 298.15, 700, and 1000°K for the 32 minerals included in papers 1–4 of this series. The average deviation of the entropies at 298°K from well‐determined calorimetric values is ±1.5%. A method is given for obtaining greater accuracy in the model thermodynamic functions by fitting one parameter to experimental data when partial calorimetric data (such as the heat capacity at a single temperature in the range 50–100°K) are available; such a method should permit accurate extrapolation of calorimetric data beyond the range of experiment.
Grossular
Pyrope
Andalusite
Almandine
Andradite
Muscovite
Enstatite
Forsterite
Sillimanite
Periclase
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Citations (204)
A large (17 cm) single crystal of enstatite (En88) contains Mg-ilmenite (MgO ∼ 12 wt%) and inclusions of polyphase garnets. The major garnet is orange in color and is predominantly pyrope-almandine (Py71Al21Gr8) whereas inside this garnet is a second one that is pink in color, and Cr-rich (Cr-Py33Py28Al19Gr20). This Cr-rich garnet itself contians inclusions of Cr-diopside, chromite and ilmenite. The enclosing orange almandine garnet at the boundary with the pink Cr-rich garnet contain rounded Mg-ilmenites together with a string of elongated olivines (Fo86) that are in optical continuity. Also present in the same zone is diopside. Within the enstatite host and in close association with the polyphase garnet assemblage are calcite, Ti-phlogopite and serpentine. Although it is conceivable the Cr-rich garnet and related phases represent the remnant of an earlier pre-existing rock that underwent considerable partial melting it must also be considered that the second garnet may be metamorphic in origin. However, irrespective of the origin, based on orthopyroxene geothermometry and A12O3 content possible P and T of equilibration are in the region of 60kb and 1200°C.
Enstatite
Diopside
Almandine
Pyrope
Ilmenite
Phlogopite
Chromite
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Citations (12)
Grossular
Pyrope
Almandine
Pyroxene
Omphacite
Glaucophane
Closure temperature
Ilmenite
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Citations (14)
Omphacite
Phlogopite
Phengite
Coesite
Pyrope
Geothermobarometry
Lawsonite
Grossular
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Citations (64)