مطالعه کانی شناسی و فرآیندهای اسکارنی شدن در اسکارن مس- آهن آوان، شمالخاور خاروانا، شمال باختر ایران

Systematic infrared and nuclear magnetic resonance investigations of common crustal minerals were undertaken to better understand the geologic significance of minor components of structural hydrous species within these nominally anhydrous minerals. The absolute hydrogen concentration in three alkali feldspars and eight plagioclase samples was measured with 1H nuclear magnetic resonance spectroscopy. The mid-infrared integral absorption coefficient was determined to be 15.3 ? 0.7 ppm-1?cm-2, allowing quantitative analysis of OH and H2O in feldspars with infrared spectroscopy. A survey of hydrous species in igneous feldspars found that feldspars contain structural OH (0-512 ppm H2O), H2O (0-1350 ppm H2O), and NH4+ (0-1500 ppm NH4+) groups as well as fluid inclusions and alteration products. Composition and crystal structure influence the type of hydrous species that can be incorporated into feldspars, but the concentration and speciation of structural hydrogen is at least partially determined by the geologic environment. The diffusivity of H in OH-bearing plagioclase was determined at 800-1000?C (D0=5.7?2.5x10-4 m2/sec and Q=224?33 kJ/mol). A millimeter-sized volcanic feldspar phenocryst would be expected to lose a significant proportion of its OH concentration on the timescale of a typical eruption (hours to weeks). The structures and compositions of low albite and ussingite, Na2AlSi3O8(OH), are similar. The strong hydrogen bonding in ussingite is found to be fundamentally different from the hydrogen bonding environment of OH in feldspars. Comparison of the infrared spectra of structural isomorphs reedmergnerite, NaBSi3O8, and low albite suggest that OH is incorporated in both structures through protonation of the most underbonded oxygen site. The concentration of structural OH in diopside was determined for four granulite facies siliceous marble samples from the Adirondacks, New York. Diopside OH concentration increases monotonically with increasing estimated water fugacity for each outcrop. Hydrogen concentration is correlated to Ti concentration in zoned grossular skarn garnets from Birch Creek, CA. Decrease of Ti and H from garnet cores to rims may be related to the solubility of Ti in the skarn-forming fluid. Skarn garnets from an Adirondacks, NY, wollastonite ore deposit exhibit a large range of OH concentrations broadly related to rock type that are due to recrystallization and partial dehydration.

The Antamina Cu-Zn skarn, in the central Andes of Peru, is the largest skarn in the world with resources of ~2,968 Mt averaging 0.89% Cu, 0.77% Zn, 11 g/t Ag and 0.02% Mo as of 2015. The deposit measures ~2.5 km long by ~1.5 km wide with a vertical extension > 2.2 km and remains open at depth. The host rocks are structurally stacked limestones and marls of the Cretaceous Jumasha and Celendin formations. Skarn and mineralisation occur in and around the Antamina Porphyry Complex (APC), which consists of 4 porphyry centres emplaced between 10.95 ± 0.20 Ma and 10.24 ± 0.23 Ma (U-Pb zircon). The core of the main ore zone is composed of 3 contiguous porphyry centres (Oscarina, Taco-Bornita, Usupallares) oriented lengthwise from NE to SW; the fourth centre (Condorcocha) is located ~1 km north of Taco. This study combines field observations of cross-cutting relationships with U-Pb (zircon) and Re-Os (molybdenite) geochronology into a spatio-temporal model that explains how Antamina became a giant ore deposit. At least 11 intrusive phases were identified at the Antamina deposit. The major porphyry phases documented in each intrusive centre at Antamina are classified as P1 (early), P2 (inter-mineralisation), and P3 (late inter-mineralisation). Sub-phases are denoted in alphabetical order as P2a, P2b, etc. General characteristics of the major phases are as follows: P1 is the causative skarn-forming intrusion in each porphyry centre. P1 ages range from 10.95 ± 0.20 Ma (Taco) to 10.24 ± 0.23 Ma (Usupallares). Endoskarn and exoskarn are genetically related to P1 porphyries. Stockwork quartz veins (≤ 40% volume) and hydrothermal biotite (potassic alteration) are locally abundant in the central porphyry complex. P2 intrusions cut P1 intrusions, related skarns, and early quartz veins. Quartz stockwork veins (± pyrite ± chalcopyrite ± molybdenite) extend outward from P2 porphyries and cross-cut P1 skarns. Locally, P2 contains xenoliths (up to several m long) of P1 exoskarn, P1 endoskarn, and refractory quartz vein fragments. P3 cross-cuts P1, P2, and early veins. Neither P2 nor P3 contain endoskarn alteration, although they do contain veins and disseminations of secondary biotite overprinted by sericite-chlorite alteration. Molybdenite mineralisation occurs in two stages across the Taco-Bornita and Usupallares zones. Stage I molybdenite occurs in skarns; Re-Os ages include 10.58 ± 0.07 Ma and 10.44 ± 0.05 to 10.39 ± 0.05 Ma. Stage II molybdenite + quartz veins cut across P2 and P3 porphyries and Re-Os ages include 9.99 ± 0.04 and 9.68 ± 0.05 Ma. In general, U-Pb zircon and Re-Os molybdenite ages decrease from northeast to southwest along the structurally-controlled axis of the deposit. Together, these ages constrain the duration of magmatic-hydrothermal activity at Antamina to ~1.1 Ma. The skarns formed in sequence with porphyry emplacement, then coalesced to form a continuous body of skarn that comprises the giant Antamina deposit. Systematic patterns of alteration and mineralisation are centred on the multi-phase porphyry centre; outward from this centre, alteration consists of hydrothermal biotite (i.e., potassic alteration) transitioning into endoskarn, exoskarn, bleached marble, and fluid escape structures in the most distal reaches of the deposit. Skarn garnet colour changes from pink to red to brown in endoskarn and from red to brown and green (from proximal to distal) in exoskarn. Garnet becomes more andradite-rich and grossularite-poor from proximal to distal skarns, and clinopyroxene becomes more hedenbergite-rich and diopside-poor along the same trend. Mineralisation and metal zoning shows an outward progression from Mo ± Cu in the central porphyry, to Cu (± Ag, Bi)-Zn-Pb from proximal to distal exoskarns. The ore mineralogy is dominated by molybdenite, chalcopyrite, bornite, and sphalerite with lesser galena and minor Bi-Ag-S minerals. Fluid inclusions in a unidirectional solidification texture (UST) sample consist of four types assigned to primary and secondary assemblages. Primary inclusions are highly saline with liquid-vapour-solid phases and contain up to 5 translucent daughters plus a triangular opaque daughter. Secondary inclusions display variable proportions of liquid-vapour-solid phases, and are generally less saline and more vapour-rich than primary inclusions. Primary inclusions homogenise by halite disappearance, whereas secondary inclusions homogenise to the liquid state. Lithostatic pressure estimates from primary fluid inclusions range from 1.2 to 0.95 kbar, which equates to formation depth range from ~4.6 to 3.5 km. The formation of the giant Antamina skarn deposit is attributed to the emplacement of multiple fertile porphyries along a NE-trending fault zone into reactive wall rocks between 10.95 ± 0.20 Ma (oldest U-Pb zircon age) and 9.68 ± 0.05 Ma (youngest Re-Os molybdenite age). Miocene surface uplift (on the order of 3.5 km), coupled with Pleistocene glaciation, exposed the top of the deposit, preserving the ore body at a favourable erosional level.

The aim of this study ist o emphasize on the origin of copper-gold bearing skarn mineralization at the Batu Hijau deposit which is located at the southwestern corner of Sumbawa Island, Indonesia. Although most skarn are derived from limestones, nolimestone is known in the Batu Hijau deposit. Ca-rich andesitic volcaniclastic host rocks favor skarn alteration within the Batu Hijau deposit. The t ype of skarn can be classified as calcic-exoskarn, and locally controlled by faults and fractures. Two major stages consisting of four sub-stages of skarn forming processes can be divided by the mineral assemblages of skarn as prograde and retrograde stages. The prograde skarn consists of clinopyroxene and garnet ± magnetite formed at the trapping temperature of 440°-480 °C with 34-38 wt% NaCl eq. while retrograde skarn alteration is dominated by Fe-rich minerals such as amphibole and epidote formed at the trapping temperature down to 340°-360°C with 4-8 wt% NaCl eq. Opaque minerals include chalcopyrite, pyrite, sphalertite, and minor galena and bismuth- telluride. Gold was precipitated in the retrograde stage associated with bismuthtelluride minerals. The sulfur isotope data of skarn ranges from +0.1 to +1.7‰ (sulfide), and porphyry systems range from 0.04 to1.4‰ and 10‰ to 15‰ (sulfide and sulfate respectively). According to the fluid inclusion and sulfur isotope data, the origin of skarn and porphyry system can be suggested to be that the magmatic origin. Furthermore, the sulfur isotope data of the deposit evidently shows that a porphyry- related skarn mineralization exhibiting transition from one style to the next can be relatively rapid. The result of this research has indicated that the range of porphyry-related deposits, skarn and porphyry systems can form during a single prolonged hydrothermal event.

The Kwoiek Area of British Columbia contains a pendant or screen of metamorphosed sedimentary and volcanic rocks almost entirely surrounded by a portion of the Coast Range Batholith, and intruded by several dozen stocks. The major metamorphic effects were produced by the quartz diorite batholithic rocks, with minor and later effects by the quartz diorite stocks. The sequence of important metamorphic reactions in the metasedimentary and metavolcanic rocks, ranging in grade from chlorite to sillimanite, is: 1. chlorite + carbonate + muscovite → epidote + biotite 2. chlorite + carbonate → actinolite + epidote 3. chlorite + muscovite → garnet + biotite 4. chlorite + epidote → garnet + hornblende 5. chlorite + muscovite → garnet + staurolite + biotite 6. chlorite + muscovite → aluminum silicate + biotite 7. muscovite + staurolite → garnet + aluminum silicate + biotite 8. staurolite → garnet + aluminum silicate Continuous reactions, occurring between reactions 5 and 7, are: A. chlorite + (high Ti) biotite + Al2O3 (from plagioclase?)→ garnet + staurolite + (low Ti) biotite + O2 B. muscovite (phengitic) → garnet + staurolite +muscovite (less phengitic) + O2 (?) Detailed electron microprobe work on garnet, staurolite, biotite, and chlorite shows that: (1) The garnet porphyroblasts are zoned according to a depletion model, called the Rayleigh depletion model, which assumes equilibrium between the edge of a growing garnet and the minerals which are unzoned, notably biotite, chlorite, and muscovite, but which assumes disequilibrium within the garnet. (2) The staurolite porphyroblasts are also zoned, and from their zoning patterns reactions A, B, and 5 are documented. Progressive reduction of iron with increasing grade of metamorphism is also inferred from the staurolite zoning patterns. (3) During a late period of falling temperature garnet continued to grow and the biotite and chlorite reequilibrated. The biotite, chlorite, and garnet edge compositions can vary from point to point in a given thin section, indicating that the volume of equilibrium at the final stage of metamorphism was only a few cubic microns. (4) The horizon within the garnet that grew at maximum temperature can be identified. The Mg/Fe ratio of this horizon, if the garnet composition is a limiting composition in the Al2O3 - K2O - FeO - MgO tetrahedron, increases systematically with increasing metamorphic grade. Biotite and chlorite compositions also show a general increase in Mg/Fe ratio with increasing metamorphic grade, but staurolite appears to show the reverse effect. (5) The Mg/Fe ratio at the maximum temperature horizon of the garnet porphyroblasts is a function of its Mn content as evidenced from the study of five garnet-bearing rocks, collected from one outcrop area, with the same assemblage but with differing proportions of minerals. An important implication of zoned minerals is that the effective composition of a system in a phase lies on the join between the homogeneous minerals (if there are two) and not within three-or- four-phase fields when a zoned mineral, such as garnet or staurolite, is present in the assemblage. Study of the three aluminum silicates found in the Kwoiek Area showed that a constant pressure change in polymorphs from andalusite to kyanite to sillimanite took place with increasing temperature. This transition series is best explained by the metastable formation of andalusite. Photographic materials on pages 15, 121, 160, 162, and 164 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

Eocene magmatism with intermediate-acid tuffs and volcanic rocks, the host to the Nodoushan deposit in Yazd province, intruded by Oligocene early diorite and later granite plutonic rocks. The former involved in iron skarn (containing epidote and euhedral grossularite) to the north and northern part of the deposit, the latter contributed to fault-controlled Zn-Pb deposit. The structural features controlled both the mineralization and consequent alterations which ranged from silicification (central) to argillic (northern). Propylitic alteration and dolomitization considered the minor ones, the iron contamination of which could be provided by earlier iron mineralization. Sulfide minerals dominantly pyrite, sphalerite and galena followed by chalcopyrite and late stage copper minerals such as covellite, digenite, bornite, chalcocite. Oxide minerals developed to the depth of 40m as a result of faults. Sphalerite which is of high-Fe type was characterized by extensive chalcopyrite disease, the iron content of which provided by earlier iron concentration. The concentration of chalcopyrite exsolution along sphalerite margins as well as galena veinlets is due to the thermal shock of later stage hydrothermal fluids that deposited galena and chalcopyrite. Negligible fossil replacements indicate both mineralization and alteration. It was concluded that the Zn-dominant mineralization was deposited under the structural controlling faults which reflects part of its earlier iron mineralization.

Three types of base metal occurrences discovered along the Barisan Range, Sumatera are skarn, sedex and hydrothermal styles. The skarn styles include Lokop, Latong and Tuboh, while Dairi and Tanjung Balit belong to sedex and hydrothermal deposits, respectively. The Lokop deposit is dominated by galena with minor pyrite and is hosted within interbedded meta-sandstone, slate, phyllite, hornfels and quartzite of the Kluet Forma- tion. The Skarn Latong deposit consists of galena with minor sphalerite and chalcopyrite with skarn minerals of magnetite, garnet and calcite. It is hosted within the meta-limestone of the Kuantan Formation. The Skarn Tuboh deposit is dominated by sphalerite with minor galena, pyrite, manganese, hematite and magnetite. It is hosted within interbedded meta-sandstone and meta-limestone of the Rawas Formation. The Dairi deposit belongs to the sedimentary exhalative (sedex) type. It is hosted within the sedimentary sequence of the Kluet Formation. Two ore types known are Julu and Jehe mineralization. The Julu mineralization referring to as sediment exhalative (sedex), was formed syngenetically with carbonaceous shale. Ore mineralogies consist of galena, sphalerite and pyrite. The deposit was formed within the temperature range of 236-375°C with salinity ranges from 9,3-23% wt.NaCl. The Jehe mineralization which belongs to the Mississippi Valley-Type, is hosted within dolostone of the Jehe Member. Ores comprise galena, sphalerite, pyrite, chalcopyrite, tetrahedrite and tenantite. The deposit was formed at temperature range from 193-400°C with salinity up to 38.2% wt.NaCl. The Tanjung Balit deposit belongs to the hydrothermal mineralization. The deposit is hosted within the sedimentary sequence of the Silungkang Formation. Ores consist of chalcopyrite, galena and sphalerite with minor gold and silver. Hydrothermal minerals such as silica, illite, montmorillonite, pyropilite, muskovite, siderite, diaspor, dickite, magnesite, chlorite, carbonate, rhodochrosite, analcime, alunite, smectite, ankrite, calcite, dolomite, sericite and zeolite are common found. The Tanjung Balit mineralization is formed at the temperature range from 185-350°C and belongs to meso-epithermal type.

Carbonate-hosted talc deposits in the contact aureole of an igneous intrusion (Hwanggangri mineralized zone, South Korea): geochemistry, phase relationships, and stable isotope studies

The Ali-Abad porphyry copper deposit is located in the Central Iranian magmatic belt. As a result of subduction magmatism, Oligo-Miocene leucogranitoid rocks have intruded into conglomerates of lower Cretaceous age. Due to contact metamorphism the following mineral assemblage has occurred in the altered conglomerate: Garnet (andradite-grossular) + epidote+ quartz + calcite + pyrite Skarnification have only occurred in reactive carbonate clasts of the conglomerates. Chemical composition of garnets and classic discrimination scheme imply the porphyry copper related skarn-type mineralization. Garnets are characterized by abrupt chemical zoning area and thin bands. REE pattern of skarn garnet and host granite have similar trends, so it is concluded that garnets are hydrothermal in origin. The garnet mineralization episode occurred during skarnification, giving hydrothermal fluid temperatures of 422-472 oC, likely this episode corresponds to the fluid boiling and high fO2 of garnet forming processes in the skarn system. Probably, the temporal decrease of hydrothermal fluid pressure led to rapid growths of andradite zones i.e., extensive hydrofracturing, porosity of conglomerate could be the dominant conditions which led to boiling of hydrothermal fluids and precipitation of andradite garnet.

International audience

The Variscan basement of Northern Apennines (Northern Italy) is a polymetamorphic portion of continental crust. This thesis investigated the metamorphic history of this basement occurring in the Cerreto Pass, in the Pontremoli well, and in the Pisani Mountains. The study comprised fieldwork, petrography and microstructural analysis, determination of the bulk rock and mineral composition, thermodynamic modelling, conventional geothermobarometry, monazite chemical dating and Ar/Ar dating of muscovite. The reconstructed metamorphic evolution of the selected samples allowed to define a long-lasting metamorphic history straddling the Variscan and Alpine orogenesis. Some general petrological issues generally found in low- to medium-grade metapelites were also tackled: (i) With middle-grade micaschist it is possible to reconstruct a complete P-T-D path by combining microstructural analysis and thermodynamic modelling. Prekinematic white mica may preserve Mg-rich cores related to the pre-peak stage. Mn-poor garnet rim records the peak metamorphism. Na-rich mylonitic white mica, the XFe of chlorite and the late paragenesis may constrain the retrograde stage. (ii) Metapelites may contain coronitic microstructures of apatite + Th-silicate, allanite and epidote around unstable monazite grains. Chemistry and microstructure of Th-rich monazite relics surrounded by this coronitic microstructure may suggest that monazite mineral was inherited and underwent partial dissolution and fluid-aided replacement by REE-accessory minerals at 500-600°C and 5-7 kbar. (iii) Fish-shaped white mica is not always a (prekinematic) mica-fish. Observed at high-magnification BSE images it may consist of several white mica formed during a mylonitic stage. Hence, the asymmetric foliation boudin is a suitable microstructure to obtain geochronological information about the shearing stage. (iv) Thermodynamic modelling of a hematite-rich metasedimentary rock fails to reproduce the observed mineral compositions when the bulk Fe2O3 is neglected or determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe2O3 diagrams.

Geology of the Ramona pegmatites, San Diego County, California

The Qozlou Fe skarn deposit is located at the Abhar–Mahneshan belt of the Central Iranian Zone. It is associated with Upper Eocene porphyritic granite that intruded into the Upper Cretaceous impure carbonaceous rocks. The Qozlou granite has high-K calc-alkaline affinity and is classified as subduction-related metaluminous I-type granitoids. Skarn aureole in the Qozlou is composed of endoskarn and exoskarn zones, with the exoskarn zone being the main skarn and mineralized zone. It includes garnet skarn, garnet-pyroxene skarn, pyroxene skarn, epidote skarn, and pyroxene-bearing marble sub-zones. The Qozlou Fe deposit is 300 m long and 5–30 m wide. Magnetite is the main ore mineral associated to pyrite, chalcopyrite, and pyrrhotite. Garnet, clinopyroxene, actinolite, epidote, calcite, and quartz occur as gangue minerals. Covellite, hematite, and goethite were formed during the supergene processes. The ore and gangue minerals have massive, banded, disseminated, brecciated, vein–veinlets, replacement, and relict textures. EPMA data indicate that garnets have andradite–grossularite compositions (Ad39.97–100–Gr0–49.62) and clinopyroxenes have diopsidic composition (En29.43–42.5–Fs14.31–20.99–Wo43.08–50.17). Based on mineralogical and textural criteria, skarnification processes in the Qozlou skarn can be categorized into three discrete stages: (1) isochemical (metamorphic–bimetasomatic), (2) metasomatic prograde, and (3) metasomatic retrograde. Anhydrous calc-silicate minerals (garnet and clinopyroxene) were formed during the prograde metasomatic stage, while ore minerals and hydrous calc-silicate minerals were formed during the retrograde ore-forming sub-stage. Temperature and ƒO2 conditions range between 430 and 550 °C and 10−26 and 10−23, respectively, for the metasomatic prograde stage. The retrograde metasomatizing fluids had likely ƒS2 = 10−6.5 and temperatures < 430 °C at the beginning of the ore-forming sub-stage.

The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and of the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the SE part of the Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piedmontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dikes. Moreover, in this part of the Island, the relationships between the uplift of the plutonic bodies and the final deformations of the tectonic stack are well exposed. To improve the geological knowledge of SE Elba, the authors carried out a 1:10.000 geological survey of the Calamita Promontory (mostly made up of the Porto Azzurro Unit) and performed petrographic and meso-/micro-structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, likely pre-Carboniferous basement (Mt. Calamita Fm.), which is unconformably overlain by the ?Triassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt.Calamita Fm., five main lithofacies were recognized and mapped. In particular, garnet-bearing, albite micaschist (lithofacies a) geometrically underlie a phyllitic-quartzitic unit (lithofacies b); Porphyroids-like rocks (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of the lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello Geothermal Field, whereas the other lithofacies can be probably correletable with the ?Ordovician formations of the Tuscan Metamorphic Units. The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (Dx), recognized in the Mt.Calamita Fm., which is defined by pre- Alpine schistosity and mineralogic relics (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the Greenschists facies, which deformed also the Mesozoic covers; c) a following folding event (D3) which probably occurred during or immediately after the strong thermometamorphic imprint (including the magnetite-rich skarn bodies), due to the Neogene magmatic intrusions; d) Subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Mt.Calamita Fm. and the Mesozoic cover) and between the latter and the overlying tectonic Units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch). A final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and N-S trending high –angle normal fault systems, locally sealed by hydrothermal, sometimes Fe-rich mineralizations. The lithostratigraphic, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that defined for the Larderello geothermal region. Thus, the Mt.Calamita area can be considered as a little older, but similar geological model for all the future interpretations of the deep structure of southern Tuscany crossed by the Crop 18 profile.

Ore-forming mechanisms and spatio-temporal framework for intrusion-related deposits in NE China

Temperature, T, and pressure, P, behaviour of absorption bands caused by spin-forbidden dd -transitions of Fe 3+ in octahedral sites of the andradite and epidote structures were studied at T from 300 to 750 K and P up to 10 GPa. The temperature dependence of the integral intensity of the 6 A 1g → 4 T 1g ( 4 G) and 6 A 1g → 4 T 2g ( 4 G) bands resemble the behaviour of spin-allowed dd -bands of the other 3d N ions: an increase on heating in case of the centrosymmetric position of Fe 3+ in the Y-site of andradite and nearly constant values in the strongly distorted M3-site of the epidote structure. Increasing T lowers the intensity of the crystal field-independent band 6 A 1g → 4 A 1g 4 E g ( 4 G) in both minerals. The influence of P on the intensity of the dd -bands is insignificant. In andradite garnet, the local thermal expansion coefficient α loc of the Fe 3+ -centred octahedra Y, as derived from the high-T spectra, is much lower than that of Cr 3+ -centred octahedra Y in chromium-bearing garnets. On the other hand, judging from T-induced shifts of 6 A 1g → 4 T 1g ( 4 G) and 6 A 1g → 4 T 2g ( 4 G) bands, α loc of FeO 6 is much higher in epidote than in andradite, being comparable to values in AlO 6 - or CrO 6 -octahedra. This indicates that α loc values of Fe 3+ -centred octahedra may vary strongly between different structural matrices, possibly as a consequence of differences in covalent bonding of Fe 3+ compared to Cr 3+ . In andradites, T and P produce energy shifts of the crystal field-independent band 6 A 1g → 4 T 1g 4 E g ( 4 G) in opposite directions, thus showing a decrease and an increase, respectively, in the covalence of the Fe 3+ -O bonds. In epidote, the effect of both T and P on the energy of the crystal field independent transition is much smaller than in andradite. In epidote, on the other hand, P causes a strong high-energy shift of the 6 A 1g → 4 T 2g ( 4 G) transition, indicating a strong increase of the crystal-field strength Dq, and thus significant compressibility of the Fe 3+ -centred M3-octahedra with k ≅ 75 GPa. Up to ca. 10 GPa, there appear no additional bands in the NIR range of the high-P spectra of both minerals. This is indicative for the absence of P-induced reduction of Fe 3+ .