Chloritization and its bearing on uranium mineralization in Madyalabodu area,Cuddapah district,Andhra Pradesh

Uranium mineralization in Madyalabodu area, Cuddapah district, Andhra Pradesh, is spatially related to chloritized and brecciated quartzite of the Gulcheru Formation in the immediate vicinity of E-W to ESE-WNW trending basic dyke. Chloritization transgresses the lithological boundaries. Whole-rock geochemical data indicate enrichment of MgO and Al₂O₃ coupled with depletion of SiO₂, Na₂O, K₂O, CaO and TiO₂ in the chlorite-rich zone. Fe₂O₃ and FeO do not vary significantly in the altered and the unaltered zones. Electron Probe Microanalysis (EPMA) data reveal that the chlorites in contact with uranium minerals are enriched in MgO and depleted in FeO than in the others. Considering the petrological evidence, geochemical signature and structural constraints, it appears that chlorite acted more as an adsorbent rather than as a reductant in facilitating uranium mineralization. Uraninite crystallized later from the uranium originally adsorbed on chlorites. Chloritization might also have facilitated mineralization through the generation of nascent hydrogen, H₂S and lowering pH of uranium-bearing solution.     

Sea floor basalt alteration: Some chemical and Sr isotopic effects

Major element, Sr isotope and trace element data for 16 elements are reported for various weathered zones in three submarine basalt pillows. During the initial stages of alteration, it appears that SiO₂, Al₂O₃, CaO, S, and Ga are lost from the basalt, whereas Fe₂O₃, total Fe, MnO, K₂O, H₂O, Cl, B, Rb, and Cs increase. Sr⁸⁷/Sr⁸⁶ ratios also increase during weathering. MgO, Na₂O, P₂O₅, Ba, Ni and Cu show significant (10–50%) but less consistent changes. TiO₂, Zr, Sr and V show only minor changes (<7%). Zn, Cr, Co, Y and Nb show no changes outside experimental error.     

High-pressure phengite decomposition in the Weissenstein eclogite,Münchberger Gneiss Massif,Germany

The petrography, mineral chemistry and petrogenesis of a sample from the Weissenstein eclogite, Bavaria, Germany, has been investigated. The total mineral assemblage comprises garnet, clinopyroxene_I+II, quartz, amphibole_I+II, rutile, phengite, epidote/allanite, plagioclase, biotite, apatite, pumpellyite, titanite (sphene), zircon, alkali feldspar and calcite. Textural observations combined with geothermobarometry (Fe/Mg distribution between clinopyroxene/garnet and phengite/garnet; jadeite-content of omphacite, Si-content of phengite, and An-content of plagioclase) provide indications of two different stages in the metamorphic evolution of the rock. The main phengitequartz-eclogite mineral equilibration occurred at minimum P=13–17kbar, minimum T=620±50° C; the retrograde symplectite stage (clinopyroxene_II, amphibole_II, biotite, plagioclase) occurred at P _total between 12 and 8.5 kbar. Reactions of the symplectite stage are:

1. phengite (core) + Na₂O_aq + CaO_aq=phengite (rim) + biotite + plagioclase + K₂O_aq + H₂O
2. phengite (core) + clinopyroxene_I + Na₂O_aq=phengite (rim + biotite + plagioclase + amphibole_II + SiO₂ + K₂O_aq + CaO_aq + H₂O
3. clinopyroxene_I + SiO₂ + K₂O_aq + H₂O=clinopyroxene_II + plagioclase+amphibole_II + Na₂O_aq + CaO_aq

The phengite decomposition produces H₂O, whereas the clinopyroxene decomposition consumes H₂O. The estimated P-T-conditions for the Weissenstein eclogite are in the same order of magnitude as those for other eclogite bodies from the Alps and Caledonides believed to be related to subduction processes.     

Solution behavior of C-O-H volatiles in FeO-Na2O-Al2O3-SiO2 melts in equilibrium with liquid iron alloy and graphite at 4 GPa and 1550°C

In order to elucidate the solution behavior of carbon and hydrogen in iron-bearing magmatic melts in equilibrium with a metallic iron phase and graphite at oxygen fugacity (fO₂) values 2–5 orders of magnitude below the iron-wustite buffer equilibrium, fO₂ (IW), experiments were carried out at 4 GPa and 1550°C with melts of FeO-Na₂O-SiO₂-Al₂O₃ compositions. Melt reduction in response to an fO₂ decrease was accompanied by a decrease in FeO content. The values of fO₂ in the experiments were determined on the basis of equilibrium between Fe-C-Si alloy and silicate liquid. Infrared and Raman spectroscopy showed that carbon compounds are formed in FeO-Na₂O-SiO₂-Al₂O₃ melts: CH₄ molecules, CH₃ complexes (Si-O-CH₃), and complexes with double C=O bonds. The content of CO₂ molecules and carbonate ions (CO ₃ ^2? ) is very low. In addition to carbon-bearing compounds, dissolved hydrogen occurs in melt as H₂ and H₂O molecules and OH^? groups. The spectral characteristics of FeO-Na₂O-SiO₂-Al₂O₃ glasses indicate the occurrence of redox reactions in the melt, which are accompanied at decreasing fO₂ by a significant decrease in H₂O and OH^?, a slight decrease in H₂, and a significant concomitant increase in CH₄ content. The content of species with the double C=O bond increases considerably at decreasing fO₂ and reaches a maximum at ΔlogfO₂(IW) = ?3. According to the obtained IR spectra, the total water content (OH^? + H₂O) in the glasses is 1.2–5.8 wt % and decreases with decreasing fO₂. The high H₂O contents are due largely to oxygen release related to FeO reduction in the melt. The total carbon content at high H₂O (4.9–5.8 wt %) is approximately 0.4 wt %. The carbon content in liquid iron alloys depends on silicon content and, probably, oxygen solubility and ranges from 0.3 to 3.65 wt %. Low carbon contents were observed at a significant increase in Si content in liquid iron alloy, which may be as high as ～13 wt % at fO₂ values 4–5 orders of magnitude below fO₂(IW).     

Alkali granite-syenite-carbonatite association in Munnar Kerala,India; implications for rifting,alkaline magmatism and liquid immiscibility

Occurrence of carbonatite is reported from the Munnar area, Kerala, where an alkali granite-syenite-carbonatite association is seen emplaced along the intersection zone of the Attur and Kerala fault-lineaments. The carbonatites are of two varieties, a calcite-rich sovite and a very coarse grained, calcite and dolomite bearing alvikite. Higher levels of SiO₂, Al₂O₃ and CaO are characteristic of these as compared to the composition of typical carbonatites. The transition element levels are high whereas the incompatible elements show lower values. The low Sr values, lower amount of apatite and absence of rare metal minerals preclude a primary carbonatite magma. The associated syenite and alkali granite have higher K₂O, K₂O/Na₂O, K/Rb, K/Ba and transition element levels. Petrochemical features suggest the rock association to be a result of separation of an immiscible fraction of less viscous carbonate liquid during cooling and ascent from a more viscous polymerized alkali silicate phase. The pre-requisites for melt equilibration and liquid immiscibility were achieved through volatile degassing related to crustal warping and rifting. The unique alkaline association of Munnar, which shows spatial relationships with deep-seated faults as well as a probable triple-point junction, is suggested to be a signature of late Precambrian alkaline magmatism which manifested in the Indian shield as a precursor to the rifting of the continental margin.     

A model of the OH positions in olivine,derived from infrared-spectroscopic investigations

Polarized infrared (IR) spectroscopy of olivine crystals from Zabargad, Red Sea shows the existence of four pleochroic absorption bands at 3,590, 3,570, 3,520 and 3,230 cm^?1, and of one non pleochroic band at 3,400 cm^?1. The bands are assigned to OH stretching frequencies. Transmission electron microscopy (TEM) shows no oriented intergrowths in this olivine; it is concluded that OH is structural. On the basis of the pleochroic scheme of the absorption spectra it is proposed that [□O(OH)₃] and [□O₂(OH)₂] tetrahedra occur as structural elements, assuming that the vacancies are on Si sites. If M2 site vacancies were assumed [SiO₃(OH)] and [SiO₂(OH)₂] tetrahedra occur as structural elements.     

Petrology of mafic inclusions from Itinome-gata,Japan

Various types of mafic inclusions up to 30 cm in size occur in lapilli tuff of alkali basalt at Itinome-gata crater, northeastern Japan. They are divided into the following four groups: amphibolite, hornblendite—hornblende gabbro, leucogabbro, and pyroxene gabbro. Also occurring with the mafic inclusions are lherzolite and websterite inclusions and megacrysts of Mg-rich olivine and chromian diopside. New analyses are presented for twenty five representative mafic inclusions, eight clinopyroxenes, six orthopyroxenes, and fifteen brown hornblendes. There are conspicuous chemical differences between the mafic inclusions and lherzolite and websterite inclusions: the former have higher TiO₂, Al₂O₃, total FeO, CaO, Na₂O, and K₂O, and lower MgO than the lherzolites and higher TiO₂, Al₂O₃, total FeO, and alkalis, and lower MgO than the websterites. The petrographic and chemical gradations among these three are not easily recognized. It is indicated that the Moho in this region is a boundary between mafic and ultramafic phases. The mineral assemblages of the mafic inclusions and the compositions of their essential minerals show that all of them recrystallized or crystallized under approximately the same temperature — pressure conditions, within the range of 600–1000° C and 6–9 kb. The following is hypothetically considered. The old and thick tholeiites or high-alumina basalts (may be pre-Silurian) making up the basement of the Japanese Islands had been subjected to the high T/P type metamorphism during Cretaceous time, and changed to amphibolites. In the cataclastic stage, complete melting of the lower part of the amphibolites occured locally and formed a gabbro magma. This gabbro magma moved upward slightly and produced hornblendite, hornblende gabbro and leucogabbro magmas by differentiation under wet conditions and a pyroxene gabbro magma under less wet conditions. Namely, the mafic inclusions are thought to be of fragments of the lower crust.     

Xitieshanite—A new ferric sulphate mineral

Xitieshanite is a new ferric sulfate mineral discovered in the oxidation zone of a Pb-Zn deposit at Xitieshan, Qinghai Province, China. The typical crystal of xitieshanite is a rhombic rectangle. It is bright green in colour with a light yellow tint. Luster vitrous Translucent to almost transparent. Streak yellow. Cleavage imperfect. Fracture uneven or conchoidal. H. (Vickers)=62.6kg/mm². Specific gravity=1.99_obs(2.02_calc,) Pleochroism strong, and axial colours: X=colourless to pale yellow, Y=pale yellow, Z=light yellow with greenish tint. It is optically positive, biaxial, 2V=77°,r v. Refractive indices:N _x =1.536,N _y =1.570,N _z =1.628. Extinction parallel and inclined. Elongation positive and negative. X-ray single-crystal study shows it is monoclinic. Space groupP2₁/a. Unit cell parameters:a=14.102,b=6.908,c=10.673 Å, β=111.266°,V=968.9, ų,Z=4. The powder pattern of xitieshanite gave the strongest lines: 6.67(6)(201), 6.09(5)(110), 5.69(5)(011), 4.96(10)(002), 4.81(10)(211), 4.21(5)(112), and 3.90(9)(211). Chemical analysis gave Al₂O₃ 0.01, Fe₂O₃ 26.15, FeO 0.18, MgO 0.03, CaO 0.09, K₂O 0.03, Na₂O 0.07, SO₃ 27.69, H₂O 45.02, total 99.27%, corresponding to the chemical formula: Fe²⁺ (SO₄)(OH) · 7H₂O. The DTA curve shows respectively three strong endothermic peaks at 85°, 170°, and 735°C, and a weak peak at 460°C. The TGA curve shows a loss of weight in three different steps. The infrared spectral curve of xitieshanite demonstrates that it has two principal absorption bands at 3,350 and 1,225–1,003 cm^?1 and two subordinate bands at 1,620 and 603 cm^?1.     

Mineralogy of the ejected plutonic blocks of the soufriere volcano St. Vincent: Olivine,pyroxene, amphibole and magnetite paragenesis

Ejected plutonic blocks from the Soufrière volcano, St. Vincent, consist of the mineral phases plagioclase (An₉₆?An₈₉, average An₉₃), olivine (Fo₇₉?Fo₆₇, most frequent interval Fo₇₄?₇₂), salite containing 5–6 percent Al₂O₃, hastingsitic amphibole and magnetite containing 6% Al₂O₃, 4% MgO and 7% TiO₂. Aluminous salitic pyroxenes are not confined to alkaline suites but also occur as high temperature phases in certain calc-alkaline suites. Similarly low silica amphiboles with comparatively high Fe⁺³+Ti contents are unusual in common sub-alkaline rocks. Magnetite in the St. Vincent blocks is an example of a single homogeneous iron oxide phase precipitated in equilibrium with the other four phases in the blocks. Conditions of crystallisation are just as important as silica activity in determining the compositions of phases separating from basaltic magmas at relatively shallow depths (P _tot< 8 kb). It is argued mainly on crystal chemical grounds that the mineral assemblage in the St. Vincent blocks can crystalline from a sub-alkaline magma under conditions of relatively high temperature, high water vapor pressure, and oxygen fugacity.     

High temperature solvent growth of anorthite on the join CaAl2Si2O8-SiO2

Stoichiometric anorthite, CaAl₂Si₂O₈, Pˉ1, with sharp a, b, c, and d diffractions was grown, using a CaV₂O₆ solvent, by cooling at 2 ° C per hour from 1450 to 750 ° C in air. Euhedral crystals up to 5 × 3× 0.5 mm, with prominent {010} and well-developed {110} and {001}, were obtained by spontaneous nucleation. Nonstoichiometric anorthite with excess SiO₂ (CaAl₂Si₂O₈+Si₂Si₂O₈) was grown on the join CaAl₂Si₂O₈-SiO₂. Chemical analysis of the synthetic anorthite, having the highest SiO₂ content, with maximum vacancies on M-site gave □_0.110 Na_0.006Ca_0.884Al_1.80Si_2.20O₈, and X-ray diffraction showed a trend from stoichiometric Pˉ1 through diffuse Pˉ1 to body-centered Iˉ1 probably reflecting increasing disorder caused by a higher Si/Al ratio in the T-sites and the positional disorder accompanied by M-site vancancies. Annealing of the nonstoichiometric diffuse Iˉ1 anorthite in air at 1050 ° C for 14 days resulted in exsolution of minute SiO₂ inclusions due to probable ordering in the T-sites and filling of M-sites by Ca. Stoichiometric Pˉ1 anorthite was not obtained by annealing at 1050 ° due to appreciable solubility of SiO₂ in CaAl₂Si₂O₈. Metastable hexagonal CaAl₂Si₂O₈ was found to be a twinned monoclinic crystal with cell dimensions: a = 10.24 (2), b = 17.74 (3), c= 14.99 (5) ?, β = 92.05 (5) °, space group C2.     

Calorimetric study of the stability of spinelloids in the system NiAl2O4-Ni2SiO4

Enthalpies of solution in molten 2 PbO · B₂O₃ at 974 K were measured for four spinelloids, phases I (0.75 NiAl₂O₄ · 0.25 Ni₂SiO₄), II (0.60 NiAl₂O₄ · 0.40 Ni₂SiO₄), III and IV (0.50 NiAl₂O₄ · 0.50 Ni₂SiO₄) in the system NiAl₂O₄ · Ni₂SiO₄. The enthalpies (in cal per 4-oxygen mol) of formation from NiAl₂O₄ and Ni₂SiO₄ spinels are: phase I, 945±366; phase II, 1072±360; phase III, 2253±390; phase IV, 3565±544. Using these enthalpy data in combination with phase relations at high pressure at 1373 K, positive entropies of formation of the spinelloids from NiAl₂O₄ and Ni₂SiO₄ spinels were estimated (in cal mol^?1 K^?1): phase I, 1.2; phase II, 1.5; phase III, 2.0–2.3; phase IV, 3.0–3.1. The thermochemical data obtained above suggest that the spinelloids are “entropy-stabilized” phases with partially disordered cation distributions. The configurational entropies of the spinelloids were calculated based on the observed cation distribution in each spinelloid phase. The positive entropies of formation of the spinelloids from the spinel endmembers are due primarily to the configurational entropies although small positive vibrational entropy changes may also exist.     

The role of water in the 1944 Vesuvius eruption

The P-T path of magma associated with the 1944 Vesuvius eruption has been outlined on the basis of probe mineralogy and the relationships between the crystallising phases. Equilibrium P-T values, obtained from the reactions:

1. CaMgSi₂O_6(liq) = CaMgSi₂O_6(cpx)
2. NaAlSi₃O_{8 (liq)} = NaAlSi₃O_{8 (plag)}
3. CaAl₂Si₂O_{8 (plag)}=CaAl₂SiO_6(cpx)+SiO_2(liq) have been established for three intracrustal crystallisation stages: I) 8.0 kbar and 1255 °C; II) 4.0 kbar and 1178 °C; III) 0.5 kbar and 1105 °C.

The H₂O content in the magma has been estimated from an experimental calibration of \(a_{^{CaMgSi_2 O_6 } }^{liq}\) as a function of \(X_{H_2 O}^{liq}\) at P _tot = 2 kbar. The estimated water contents of the magma for the three stages, I) 0.7%; II) 0.9%; III) 1.1%, are consistent with the pattern of activity of the 1944 Vesuvius eruption and with the relationship between the lavas. The shallow depth of H₂O-saturation of the magma, 0.24 kbar at 1100 °C, is consistent with the eruption sequence of lava flows followed by lava fountain activity.     

The oxygen isotope geochemistry of igneous rocks

Oxygen isotope analyses have been obtained for 443 igneous rock and mineral samples from various localities throughout the world. Detailed studies were made on the Medicine Lake, Newberry, Lassen, Clear Lake, S. E. Guatemala, Hawaii and Easter I. volcanic complexes and on the Bushveld, Muskox, Kiglapait, Guadalupe, Duluth, Nain, Egersund, Lac St. Jean, Laramie, Skaergaard, Mull, Skye, Ardnamurchan and Alta, Utah plutonic complexes, as well as upon several of the zoned ultramafic intrusions of S. E. Alaska. Basalts, gabbros, syenites and andesites are very uniform in O¹⁸/O¹⁶, commonly with δ-values of 5.5 to 7.0 per mil. Many rhyolite obsidians, particularly those from oceanic areas and the Pacific Coast of the United States, also lie in this range; this indicates that such obsidians are differentiates of basaltic or andesitic magma at high temperatures (about 1,000° C). They cannot represent melted sialic crust. The only plutonic granites with such low δ-values are some of the hypersolvus variety, suggesting that these also might form by fractional crystallization. Obsidians from the continental interior, east of the quartz-diorite line, have higher δ-values. This is compatible with their having assimilated O¹⁸-rich sialic crust. A correlation generally exists between the O¹⁸/O¹⁶ ratios of SiO₂-rich differentiates and the chemical trends in volcanic complexes. High O¹⁸/O¹⁶ ratios accompany those trends having the lower Fe/Mg ratios, while ferrogabbro trends are associated with depletion in O¹⁸. Variations in oxygen fugacity may be responsible for these effects, as abundant early precipitation of magnetite should lead to both O¹⁸-enrichment and Fe-depletion in later differentiates. Plutonic granites have higher O¹⁸/O¹⁶ ratios than their volcanic equivalents, because (a) their differentiation occurred at much lower temperatures, or (b) they are in large part derived from O¹⁸-rich sialic crust by partial melting or assimilation. Also, the oxygen isotope fractionations among coexisting minerals are distinctly larger in plutonic rocks than in volcanic rocks. This is in keeping with their lower crystallization temperatures and their longer cooling history, which promotes post-crystallization oxygen isotope exchange. Hydrated obsidians and perlites have δO¹⁸-values that are much different from their primary, magmatic values. A correlation exists between D/H and O¹⁸/O¹⁶ ratios in hydrated volcanic glass from the western U.S.A., proving that the isotopic compositions are a result of exchange with meteoric waters. The O¹⁸ contents of the glasses appear to be about 25 per mil higher than their associated waters; hence, these hydrated glasses have not simply absorbed H₂O, but they have exchanged with large quantities of it. The igneous rocks from Mull, Skye, Ardnamurchan and the Skaergaard intrusion are all abnormally depleted in O¹⁸ relative to “normal” igneous rocks. This is a result of their having exchanged at high temperatures with meteoric water that was apparently abundant in the highly jointed plateau lavas into which these igneous rocks were intruded. In part, this exchange occurred with liquid magma and in part with the crystalline rock; in the latter case the feldspar was more easily exchanged and has become much more depleted in O¹⁸ than has coexisting quartz or pyroxene. The later differentiates of the Muskox intrusion are markedly O¹⁸-rich, but this is not a result of fractional crystallization. It is in large part a result of deuteric exchange between feldspars and an oxygen-bearing fluid (H₂O ?) that was either O¹⁸-rich or had a relatively low temperature. This phenomenon was also observed in a number of granophyres from other localities, particularly those containing brick-red alkali feldspar. The exchanged feldspars in all these examples are turbid or cloudy, and may be filled with hematite dust. It is concluded that most such feldspar in nature is the result of deuteric exchange and is probably drastically out of oxygen isotopic equilibrium with its coexisting quartz.     

Ertixiite—a new mineral from the Altay Pegmatite Mine,Xinjiang, China

Ertixiite (Na₂Si₄O₉), a new mineral found in a miarolitic cavity of the Altay Pegmatite Mine, Xinjiang, China, is associated with topaz, apatite, quartz, cleavelandite, etc. The mineral is white, granular, and transparent. HNV=570.08?850.96 kg/mm² (Moh’s 5.8?6.5), D=2.35, N=1.502. Cubic system,a=5.975 Å, V=213.311 Å, Z=1,D _x =2.34g/cm³. The chemical composition of ertixiite (the average of six samples) is: Na₂O 17.97, CaO 2.82, SiO₂ 77.86, Al₂O₃ 1.45, FeO 0.05, total 100.15. The strongest lines in the X-ray powder pattern are 3.443(2, 111), 2.647(2. 210), 2.674(2,210), 1.996(8,221), 1.798(10,311), and 1.492(2,400).     

早子沟金矿成矿过程中的元素迁移规律及金富集机制探讨

早子沟金矿位于秦岭造山带西段夏河—合作断裂带上,成矿作用与断裂构造关系密切,蚀变带受断裂构造控制,本文通过对矿区蚀变带内常量元素的迁移规律研究,通过计算得到矿区明显带出的组分为SiO2、Al2O3、K2O、MnO,而Fe2O3、Na2O则明显带入,探讨了元素迁移的性状及其与蚀变类型的关系,并对矿床金富集机制做了简要分析。     

Clinopyroxene on the join CaMgSi2O6-CaFe3+AlSiO6-CaTiAl2O6 at low oxygen fugacity

Subsolidus phase relations on the join CaMgSi₂O₆-CaFe³⁺ AlSiO₆-CaTiAl₂O₆ were studied by the ordinary quenching method at \(f_{O_2 } = 10^{ - 11} \) atm and 1,100°C. Crystalline phases encountered are clinopyroxene_ss (ss:solid solution) (Cpx_ss), melilite (Mel), perovskite (Pv), spinel_ss (Sp_ss), magnetite_ss (Mt_ss) and anorthite (An). There is no Cpx_ss single phase field, and the following assemblages were found; Cpx_ss+Mel, Cpx_ss+Mel+Sp_ss, Cpx_ss+Mel+Pv, Cpx_ss+Mel+Sp_ss+Pv, Cpx_ss+Pv+Sp_ss+An, Sp_ss+Pv+Mel+An+Cpx_ss, Mel+Mt_ss+An+Sp_ss+Cpx_ss+liquid and Mel+Mt_ss+An+Sp_ss+Cpx_ss+Pv. Mössbauer spectral study revealed that Cpx_ss contains both Fe²⁺ and Fe³⁺ in the octahedral site, and it was confirmed that the CaFe³⁺ AlSiO₆ content in the Cpx_ss at low \(f_{O_2 } \) is considerably less than that in the Cpx_ss crystallized in air, whereas the CaFe²⁺Si₂O₆ component increases. The maximum solubility of CaTlAl₂O₆ component in the Cpx_ss at low \(f_{O_2 } \) is higher than that in air. The decrease of CaFe³⁺ AlSiO₆ in the Cpx_ss at low \(f_{O_2 } \) may cause increase of CaTial₂O₆ in the Cpx_ss.     

Synthesis and physical properties of piemontite Ca2Al3-pMn p 3+ (Si2O7/SiO4/O/OH)

Mn³⁺-bearing piemontites and orthozoisites, Ca₂(Al_3-pMn³⁺ _p)-(Si₂O₇/SiO₄/O/OH), have been synthesized on the join Cz (p = 0.0)-Pm (p = 3.0) of the system CaO-Al₂O₃-(MnO·MnO₂)-SiO₂-H₂O atP = 15 kb,T= 800 °C, and \(f_{O_2 } \) of the Mn₂O₃/MnO₂ buffer. Pure Al-Mn³⁺-piemontites were obtained with 0.5≦p≦1.75, whereas atp=0.25 Mn³⁺-bearing orthozoisite (thulite) formed as single phase product. The limit of piemontite solid solubility is found near p=1.9 at the above conditions. Withp>1.9, the maximum piemontite coexisted with a new high pressure phase CMS-X1, a Ca-bearing braunite (Mn _0.2 ²⁺ Ca_0.8)Mn ₆ ³⁺ O₈(SiO₄), and quartz. Al-Mn³⁺-piemontite lattice constants (LC),b ₀,c ₀,V ₀, increase with increasingp:

Phase relationships in granulitic metapelites from the Ivrea-Verbano zone (Northern Italy)

Paragneisses of the Ivrea-Verbano zone exhibit over a horizontal distance of 5 km mineralogical changes indicative of the transition from amphibolite to granulite facies metamorphism. The most obvious change is the progressive replacement of biotite by garnet via the reaction: a $${\text{Biotite + sillimanite + quartz }} \to {\text{ Garnet + K - feldspar + H}}_{\text{2}} {\text{O}}$$ which results in a systematic increase in the modal ratio g = (garnet)/(garnet + biotite) with increasing grade. The systematic variations in garnet and biotite contents of metapelites are also reflected by the compositions of these phases, both of which become more magnesian with increasing metamorphic grade. The pressure of metamorphism has been estimated from the Ca₃Al₂Si₃O₁₂ contents of garnets coexisting with plagioclase, sillimanite and quartz. These phases are related by the equilibrium: b $$\begin{gathered} 3 CaAl_2 {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} \rightleftharpoons Ca_3 Al_2 {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} + 2 Al_2 {\text{SiO}}_{\text{5}} + {\text{SiO}}_{\text{2}} \hfill \\ plagioclase garnet sillimanite quartz \hfill \\ \end{gathered} $$ which has been applied to these rocks using the available data on the mixing properties of plagioclase and garnet solid solutions. Temperature and f _{H 2O} estimates have been made in a similar way using thermodynamic data on the biotite-garnet reaction (a) and the approximate solidus temperatures of paragneisses. Amphibolite to granulite facies metamorphism in the Ivrea-Verbano zone took place in the P-T ranges 9–11 kb and 700–820 °C. The differences in temperature and pressure of metamorphism between g= 0 and g = 1 (5 kms horizontal distance) were less than 50° C and approximately 1 kb. Retrogression and re-equilibration of garnets and biotites in the metapelites extended to temperatures more than 50° C below and pressures more than 1.5 kb below the peak of metamorphism, the degree of retrogression increasing with decreasing grade of the metamorphic “peak”. The pressure and temperature of the peak of metamorphism are not inconsistent with the hypothesis that the Ivrea-Verbano zone is a slice of upthrusted lower crust from the crust-mantle transition region, although it appears that the thermal gradient was too low for the zone to represent a near-vertical section through the crust. The most reasonable explanation of the granulite facies metamorphism is that it arose through intrusion of mafic rocks into a region already undergoing recrystallisation under amphibolite facies conditions.     

Synthetic Mn3+-kyanite and viridine, (Al2-xMn x 3+ ) SiO5, in the system Al2O3-MnO-MnO2-SiO2

Experiments on the join Al₂SiO₅-“Mn₂SiO₅” of the system Al₂O₃-SiO₂-MnO-MnO₂ in the pressure/temperature range 10–20 kb/900–1050° C with gem quality andalusite, Mn₂O₃, and high purity SiO₂ as starting materials and using /_O2-buffer techniques to preserve the Mn³⁺ oxidation state had following results: At 20 kb/1000°C orange-yellow kyanite mixed crystals are formed. The kyanite solid solubility is limited at about (Al_1.88Mn _0.12 ³⁺ )SiO₅ and, thus, equals approximately that on the join Al₂SiO₅-“Fe₂SiO₅” (Langer and Frentrup, 1973) indicating that there is no Jahn-Teller stabilisation of Mn³⁺ in the kyanite matrix. 5 mole % substitution causes the kyanite lattice constants a _o, b _o, c _o, and V _o to increase by 0.015, 0.009, 0.014 Å, and 1.6 ų, resp., while α, β, γ, remain unchanged. Between 10 and 18 kb/900°C, Mn³⁺-substituted, strongly pleochroitic (emeraldgreen-yellow) andalusite_ss (viridine) was obtained. At 15 kb/900°C, the viridine compositional range is about (Al_1.86Mn _0.14 ³⁺ )SiO₅-(Al_1.56Mn _0,44 ³⁺ )SiO₅. Thus, Al→Mn³⁺ substitutional degrees are appreciably higher in andalusite than in kyanite, proving a strong Jahn-Teller effect of Mn³⁺ in the andalusite structure, which stabilises this structure type at the expense of kyanite and sillimanite and, thus, enlarges its PT-stability range extremely. 17 mole % substitution cause the andalusite constants a _o, b _o, c _o, and V _o to increase by 0.118, 0.029, 0.047 Å and 9.4 ų, resp. At “Mn₂SiO₅”-contents smaller than about 7 mole %, viridine coexists with Mn-poor kyanite. At “Mn₂SiO₅”-concentrations higher than the maximum kyanite or viridine miscibility, braunite (tetragonal, ideal formula Mn²⁺Mn³⁺[O₈/Si0₄]), pyrolusite and SiO₂ were found to coexist with the Mn³⁺-saturated ky _ss or and _ss, respectively. In both cases, braunites were Al-substituted (about 1 Al for 1 Mn³⁺). Pure synthetic braunites had the lattice constants a _o 9.425, c _o, 18.700 Å, V _o 1661.1 ų (ideal compn.) and a _o 9.374, c _o 18.593 ų, V _o 1633.6 ų (1 Al for 1 Mn³⁺). Stable coexistence of the Mn²⁺-bearing phase braunite with the Mn⁴⁺-bearing phase pyrolusite was proved by runs in the limiting system MnO-MnO₂-SiO₂.     

Serpentinization at Burro Mountain,California

The Burro Mountain ultramafic complex, Monterey County, California, consists of dunites and peridotites which are partially or wholly serpentinized. Primary minerals in both rock types are olivine, enstatite, diopside, and picotite which upon alteration yield chrysotile, lizardite, brucite, magnetite, talc, tremolite, and carbonate. Electron microprobe analyses show that enstatite, En_85.8 to En_90.8, alters to “bastite” composed only of lizardite (5.0–12.0 weight percent FeO), whereas olivine, Fo_90.8 to Fo_91.6, forms lizardite+chrysotile+brucite with or without magnetite. The chrysotile ranges from 3.0 to 5.0 weight percent FeO, the brucite from 16.0 to 43.0 weight percent FeO. As Serpentinization proceeds, the alteration products are enriched in FeO relative to MgO. Serpentinization probably originates in a changing \(P_{O_2 }\)-T environment by two different reactions:

1. (a)
   Olivine+enstatite+H₂O+O₂?Mg, Fe⁺² chrysotile+Mg, Fe⁺³, Fe⁺² lizardite with or without magnetite.