Phase equilibria among pumpellyite,lawsonite, epidote and associated minerals in low grade metamorphic rocks

Phase relations of pumpellyite, epidote, lawsonite, CaCO₃, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe³⁺ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe²⁺ and Mg are treated as a single component because variation in Fe²⁺/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:

1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H₂O
2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H₂O
3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H₂O
4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H₂O
5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H₂O + CO₂
6. pumpellyite + chlorite + quartz = epidote + actinolite + H₂O

     

Alteration of the upper oceanic crust,DSDP site 417: mineralogy and chemistry

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts.

1. Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage.
2. Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H₂O, increase of Fe³⁺/Fe^T, and possibly some losses of Ca and Mg occurred during this stage.
3. Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H₂O, and occasionally P were added to the rocks, Fe³⁺/Fe^T increased, and Ca, Mg, Si and occasionally Al and Na were lost.
4. Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe³⁺/Fe^T and H₂O occurred.
5. Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H₂O, and CO₂ in the rocks.
6. A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.

Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.     

Optical spectroscopy study of natural Fe, Ti-bearing calcic amphiboles

Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-contents ranging from 0.111 to 0.729 atom per formula unit (a.p.f.u.) and from 0.479 to 2.045 a.p.f.u., respectively, were studied by means of optical absorption spectroscopy and microprobe analysis. Thirteen samples were also studied by Mössbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-metal and metal-metal charge transfer transitions involving both iron and titanium ions (O^2?→ Fe³⁺, Fe²⁺, O^2?→ Ti⁴⁺ and Fe²⁺ + Ti⁴⁺→ Fe³⁺ + Ti³⁺). A broad intense Y-polarized band ～22?000?cm^?1 (ν_1/2?≈?3700?cm^?1) in spectra of two low iron amphiboles with a relatively low Fe³⁺/Fe_total ratio, both from eclogite-like rocks in kimberlite xenoliths, was attributed to electronic Fe²⁺(M3) + Ti⁴⁺(M2)→Fe³⁺(M3)+Ti³⁺(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involving Ti⁴⁺ and Fe²⁺ in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.     

Die Glaukophangesteine,ihre stofflichen Äquivalente und Uniwandlungsprodukte in Nordcalabrien (Süditalien)

In the southern Apennin (= northern part of the region dealt with) and the Coasta Chain (= southern part) there are metabasalts wich are classified in the northern part as:

1. Glaucophane rocks of the albite-lawsonite-glaucophane-subfacies with the assemblage glaucophane + pumpellyite + lawsonite ±albite ±aragonite ±muscovite (7 rock analyses, 8 mineral analyses). These rocks are conceived as relics of an older burial metamorphism.
2. Rocks with pumpellyite and chlorite or also chlorite alone, that are interpreted as reaction rims between the metastable glaucophane rocks and the country rock (phyllites, quartzites). The assemblages pumpellyite + chlorite and chlorite alone are to be found (2 rock analyses and 2 mineral analyses).
3. Rocks with lawsonite and/or epidote belong to the same mineral facies as the country rock: a facies similar to the greenschist facies (called “lawsonite-albite-chlorite-subfacies”) which is characterized by the assemblages lawsonite + albite + chlorite ±calcite and also epidote ±lawsonite + albite + chlorite ± muscovite. These types are attributed to a younger dynamo-metamorphism (2 rock analyses).

In the southern part, the metabasalts can be found only as rocks with epidote and/or lawsonite, a metamorphism with more than one event cannot be proved petrologically (3 rock analyses). Equations of the observed mineral reactions are given. The transitions of one facies into another are represented in the pseudo-quaternary system Al₂O₃-CaO-Na₂O · Al₂O₃-2 Fe₂O₃ + FeO + MnO + MgO-(H₂O). The pressure-temperature conditions are estimated on the basis of published experimental data (300° C and 6–7 kb for the glaucophane rocks; 400° C and about 6 kb for the rocks with lawsonite and/or epidote) and are compared with geologic facts.     

P–T–X controls on phase stability and composition in LTMP metabasite rocks – a thermodynamic evaluation

The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H₂O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe₂O₃ bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe₂O₃ contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe₂O₃‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe₂O₃ (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe₂O₃‐saturated rocks, compositional isopleths for sodic amphibole (Al³⁺ and Fe³⁺ on the M2 site), epidote (Fe³⁺/Fe³⁺ + Al³⁺) and chlorite (Fe²⁺/Fe²⁺ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al³⁺ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe²⁺‐Mg ratios in chlorite may be dependent on pressure. The Fe³⁺/Al and Fe²⁺/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.     

Compositions and phase relations of calcic amphiboles in epidote- and clinopyroxene-bearing rocks of the amphibolite and lower granulite facies,central Massachusetts,USA

Calcic amphiboles coexisting with epidotegroup minerals (zoisite, clinozoisite, epidote) and/or clinopyroxene±plagioclase±quartz±garnet occur in amphibolites and calc-silicate rocks that underwent amphibolite to lower granulite-facies metamorphism in the Acadian metamorphic high of central Massachusetts, USA. Across the region, peak metamorphic conditions range from about 580° C and 6.2 kbar to 730° C and 6.3 kbar. The coexistence of most Ca-amphiboles with Fe³⁺-rich epidote-group minerals suggests the presence of Fe³⁺ in most of these amphiboles. An empirical Fe³⁺ estimation for the microprobe analyses is based on two constraints: the Na?Ca content of the M4 sites of Ca-saturated, gravimetrically analyzed hornblendes gives the relation: Ca(M4) ^c =-1.479 Na(M4) ^c +2 (c=corrected). The second constraint is the stoichiometric equation Ca(M4)+Na(M4)+FM=15, where FM is the sum of all cations exclusive of Ca, Na, and K. Solving the two equations simultaneously gives: 20.185=0.479 Ca(M4)+1.479 ΣFM. Starting with the uncorrected values of Ca(M4) ^u and ΣFM(M4) ^u (u = uncorrected) of the all ferrous formula, the normalization factor NF for calculating the corrected cations of the ferric formulas is: 20.185/(0.478 Ca(M4) ^u +1.479 ΣFM ^u ). From the deficient oxygen the Fe³⁺ content which is equal to 2(23-ΣOX) can be calculated. Determinations of Fe³⁺ contents of four hornblende separates by Mössbauer spectroscopy are in agreement with the calculated values. The Ca-amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite-facies zones: increasing edenite and tschermakite substitution, increasing Ti content, and increasing Fe²⁺/(Fe²⁺+Mg) ratio. In addition, the coexisting clinopyroxenes are also characterized by an increase in Fe²⁺/(Fe²⁺+Mg) ratio. In quartz-free rocks with coexisting Ca-amphibole and plagioclase there is an increase in the ratio X _Ab/X _Ed, where X _Ab=Na/(Na+Ca) in plagioclase and X _Ed=Na in the amphibole A-site. These chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite-facies metamorphic conditions are attributed to the continuous reaction: albite+epidote+Fe-Mg hornblende→Fe?Mg clinopyroxene+anorthite+(NaAlSi_-1)^Hbl+H₂O.     

Sedimentmikrobioiogie und ihre geologischen Aspekte

The influence of bacteria on recent sediments was first discussed in 1885, whenFischer andGazert were discussing the cycle of substances in the sea as well as in sediments. The influence of bacteria on the cycling of C, N, S, P in recent sediments and the open sea was soon accepted by marine geologists. Nevertheless, only very few experiments have, so far, shown more than qualitative and quantitative data collection in various restricted areas. This is due to the extensive and complicated chain of reactions on the surface of sediments and in the sediment itself. Biologists are asking for the amount of organic and inorganic matter which is reworked and released to the sea. Geologists usually emphasize the amount of substances which are sedimentated. For biologists the sediment is only part of their dominant ecosystem (the sea). While, for geologists the “sea” is only furnishing and influencing their first range system sediment. How much then, are bacteria involved in the slow process of conversion from a recent sediment to sedimentary rocks? Bacteria influence more or less strongly and to a more or less advanced degree of diagenesis:

1. The organic matter in sediments and the final form in which it is found.
2. The anions CO₃ ^2?, NO₃ ^?, OH-, SO₄ ^2?, PO₄ ^3? as well as their intermediate stages and the resulting minerals.
3. The cations H⁺, NH₄ ⁺, Ca²⁺, Fe²⁺, Fe³⁺, and a series of metals which are dissolved or precipitated by microbial activities as for example Fe, Mn, Cu, Ag, V, Co, Mo, Ni, U, Se, Zn.
4. The equilibrium of silicium. At least diatoms and radiolarians are precipitating silica, while other reactions which have been proved are not yet shown to influence marine sediments.
5. p_H-values and oxidation-reduction potentials of the sediment.
6. The composition of interstitial waters.
7. The surface activity of minerals, since bacteria are growing especially on particle surfaces.
8. The energy content and temperature of sediments.
9. The texture of fine grained sediments.
10. The fossilization of microfauna, macrofauna and trace fossils.

Sedimentology and mineralogy may also influence the bacterial activities and the composition of the microflora within sediments. Methods and problems of sediment microbiology are demonstrated by some investigations in the German Bay (North Sea) in connection with the first German Underwater Station (UWL). Ecological work proves to be difficult in various directions. The main cause of difficulties in microbiological work on sediments are the great variety of different factors influencing the environment (microbial, chemical, physical, mineralogical), the difficulty of taking representative samples, and the small amount of data which has been collected so far.     

Actinolite-hornblende pairs in metamorphosed gabbros,Hidaka mountains,Hokkaido

Actinolite-actinolitic hornblende and actinolitic hornblende-hornblende pairs are described from gabbroic amphibolites and epidote amphibolites formed by dynamic metamorphism during uplift of gabbroic rooks in the Hidaka Metamorphic Belt, Hokkaido. Electron microprobe analyses indicate that coupled substitutions involved in the transition from actinolite to hornblende are essentially those of edenite and tschermakite-ferritschermakite together with smaller amounts of glaucophane-riebeckite, i.e. Al^IV, Al^VI, Fe³⁺, A-site occupancy and Na^M4 increase with replacement of Mg by Fe²⁺+ Mn and Si by Al^IV. During metamorphism the amount of deformation due to shearing has affected the degree of compositional discontinuity in the actinolite-hornblende series and the compositional gap is most pronounced in the epidote amphibolite. The coexisting actinolite-hornblende do not represent an equilibrium pair as textural relations indicate that the actinolitic amphiboles are relics. It is suggested that shearing deformation during uplift has caused an overstepping of the changing physico-chemical conditions of metamorphism so that compositional readjustment of amphiboles was not achieved. Equilibrium-disequilibrium actinolite-hornblende pairs are discussed from other localities where rocks of basaltic composition have been metamorphosed.     

Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer

There is currently a dearth of reliable thermobarometers for many hornblende and plagioclase-bearing rocks such as granitoids and amphibolites. A semi-empirical thermodynamic evaluation of the available experimental data on amphibole+plagioclase assemblages leads to a new thermometer based on the Al^iv content of amphibole coexisting with plagioclase in silica saturated rocks. The principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is \(\left( {Na\square _{ - 1} } \right)^A \left( {AlSi_{ - 1} } \right)^{T1}\) . We have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1. $$1. Edenite + 4 Quartz = Tremolite + Albite$$ 2. $$2. Pargasite + 4 Quartz = Hornblende + Albite.$$ The equilibrium relation for both (1) and (2) leads to the proposed new thermometer $$T = \frac{{0.677P - 48.98 + Y}}{{ - 0.0429 - 0.008314 ln K}} and K = \left( {\frac{{Si - 4}}{{8 - Si}}} \right)X_{Ab}^{Plag} ,$$ where Si is the number of atoms per formula unit in amphiboles, with P in kbar and T in K; the term Y represents plagioclase non-ideality, RTlnγ_ab, from Darken's Quadratic formalism (DQF) with Y=0 for X _ab>0.5 and Y=-8.06+25.5(1-X _ab)² for X _ab<0.5. The best fits to the data were obtained by assuming complete coupling between Al on the T1 site and Na in the A site of amphibole, and the standard deviation of residuals in the fit is ±38°C. The thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende-plagioclase assemblages with uncertainties of around ±75° C for rocks equilibrated at temperatures in the range 500°–1100° C. The thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than An₉₂ and with amphiboles containing less than 7.8 Si atoms pfu. Good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks. Our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry.     

Laihunite—A new iron silicate mineral

Laihuite reported in the present paper is a new iron silicate mineral found in China with the following characteristics:

1. This mineral occurs in a metamorphic iron deposit, associated with fayalite, hypersthene, quartz, magnetitc, etc.
2. The mineral is opaque, black in colour, thickly tabular in shape with luster metallic to sub-metallic, two perfect cleavages and specific gravity of 3.92.
3. Its main chemical components are Fe and Si with Fe³⁺>Fe²⁺. The analysis gave the formula of Fe Fe _1.00 ³⁺ ·Fe _0.58 ²⁺ ·Mg _0.03 ²⁺ ·Si_0.96O₄.
4. Its DTA curve shows an exothermic peak at 713°C.
5. The mineral has its own infrared spectrum distinctive from that of other minerals.
6. This mineral is of orthorhombic system; space group:C _2h ^/5 ?P2₁/c; unit cell:α=5.813ű0.005,b=4.812ű0.005,c=10.211ű0.005,β=90.87°.
7. The Mössbauer spectrum of this mineral is given, too.

     

Origin of biotite-hornblende-garnet coronas between oxides and plagioclase in olivine metagabbros,Adirondack region,New York

Complex multivariant reactions involving Fe-Ti oxide minerals, plagioclase and olivine have produced coronas of biotite, hornblende and garnet between ilmenite and plagioclase in Adirondack olivine metagabbros. Both the biotite (6–10% TiO₂) and the hornblende (3–6% TiO₂) are exceptionally Titanium-rich. The garnet is nearly identical in composition to the garnet in coronas around olivine in the same rocks. The coronas form in two stages:

1. Plagioclase+Fe-Ti Oxides+Olivine+water =Hornblende+Spinel+Orthopyroxene±Biotite +more-sodic Plagioclase
2. Hornblende+Orthopyroxene±Spinel+Plagioclase =Garnet+Clinopyroxene+more-sodic Plagioclase

The Orthopyroxene and part of the clinopyroxene form adjacent to olivine. Both reactions are linked by exchange of Mg²⁺ and Fe²⁺ with the reactions forming pyroxene and garnet coronas around olivine in the same rocks. The reactions occur under granulite fades metamorphic conditions, either during isobaric cooling or with increasing pressure at high temperature.     

Kyanite eclogites

Prior experimental work has shown that in the laboratory the mineralogy of eclogites is sensitive to the ratio of CaO ∶ MgO ∶ FeO and that the reaction pyroxene + kyanite?garnet + quartz proceeds to the right at high pressures in rocks rich in magnesium and to the left in rocks rich in calcium and iron. Typical basalts crystallized at high pressure never contain kyanite. The chemistry and mineralogy of a large number of naturally occurring eclogites show they belong to three classes.

1. Kyanite-free magmatic eclogites, rich in magnesium, from:
2. kimberlites
3. dunites and serpentinites.
4. Kyanite-bearing eclogites and grosspydites rich in CaO and low in FeO with intermediate MgO from:
5. kimberlites
6. gneisses.
7. Kyanite-free eclogites of metamorphic origin rich in iron with low magnesium and intermediate amounts of calcium from:
8. glaucophane schists
9. gneisses.

     

Assessment of the Garnet-Clinopyroxene Thermometer

A thermometer based on the MgFe_?1 exchange equilibrium between garnet and clinopyroxene is formulated by using new experimental data measured at 600° to 950°C, 0.8 to 3.0 GPa, and f(O₂) defined by the fayalite-quartz-magnetite buffer in the basalt-H₂O system. The new formulation is T = 3820 / 1.828 + lnK_D (1 + a(2.2 ? p)), where T is temperature (K), P is pressure (GPa), KD is the Fe-Mg partition coefficient between garnet and clino-pyroxene, defined as KD = (Fe²+/Mg)garnet/(Fe²+/Mg) clinopyroxene, and a = 132/T. Application of the thermometer to rocks in amphibolite, granulite, and eclogite terranes yields temperatures that are in reasonable agreement with other well-calibrated thermometers and the experimental calibrations by Ellis and Green (1979) and Pattison and Newton (1989).     

Origin of coronas in metagabbros of the Adirondack mts., N. Y.

Metagabbros from two widely separated areas in the Adirondacks show development of coronas. In the Southern Adirondacks, these are cored by olivine which is enclosed in a shell of orthopyroxene that is partially, or completely, rimmed by symplectites consisting of clinopyroxene and spinel. Compositions of the corona phases have been determined by electron probe and are consistent with a mechanism involving three partial reactions, thus:

1. Olivine=Orthopyroxene+(Mg, Fe)⁺⁺.
2. Plagioclase+(Mg, Fe)⁺⁺+Ca⁺⁺=Clinopyroxene+Spinel+Na⁺.
3. Plagioclase+(Mg, Fe)⁺⁺+Na⁺=Spinel+more sodic plagioclase+Ca⁺⁺.

Reaction (a) occurs in the inner shell of the corona adjacent to olivine; reaction (b) in the outer shell; and (c) in the surrounding plagioclase, giving rise to the spinel clouding which is characteristic of the plagioclase in these rocks. Alumina and silica remain relatively immobile. These reactions, when balanced, can be generalized to account for the aluminous nature of the pyroxenes and for changing plagioclase composition. Summed together, the partial reactions are equivalent to:

1. Olivine + Anorthite = Aluminous orthopyroxene + Aluminous Clinopyroxene + Spinel (Kushiro and Yoder, 1966).

In the Adirondack Highlands, coronas between olivine and plagioclase commonly have an outer shell of garnet replacing the clinopyroxene/spinel shell. The origin of the garnet can also be explained in terms of three partial reactions:

1. Orthopyroxene+Ca⁺⁺=Clinopyroxene+(Mg, Fe)⁺⁺.
2. Clinopyroxene+Spinel+Plagioclase+(Mg, Fe)⁺⁺=Garnet+Ca⁺⁺+Na⁺.
3. Plagioclase+(Mg, Fe)⁺⁺+Na⁺=Spinel + more sodic plagioclase+Ca⁺⁺.

These occur in the inner and outer corona shell and the surrounding plagioclase, respectively, and involve the products of reactions (a)-(d). Alumina and silica are again relatively immobile. Balanced, and generalized to account for aluminous pyroxenes and variable An content of plagioclase, they are equivalent to:

1. Orthopyroxene+Anorthite+Spinel=Garnet (Green and Ringwood, 1967).

Amphibole coronas about opaque oxides in rocks of both areas are the result of oxide/plagioclase reactions with addition of magnesium from coexisting olivine. Based on published experimental data, pressure and temperature at the time of corona formation were on the order of 8 kb and 800° C for the garnet bearing coronas, with somewhat lower pressures indicated for the clinopyroxene/spinel coronas.     

A comment on “Calcic amphibole equilibria and a new amphibole — plagioclase geothermometer” by J.D. Blundy and T.J.B. Holland (Contrib Mineral Petrol (1990) 104: 208–224)

Conclusions The calibration by Blundy and Holland is not a calibration of the reaction albite + tremolite = edenite + 4quartz, because the Al^IV content of amphiboles is a combined result of substitutions.The requirements for a calibration of any of these substitutions are: (1) an amphibole-activity model unequivocally accounting for each substitution and (2) a data-set, wherein all amphiboles are buffered by the same assemblages.     

Stabilitätsbeziehungen zwischen Chlorit,Cordierit und Almandin bei der Metamorphose

The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe²⁺/Fe²⁺+Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture. Cordierite is formed according to the following reaction: $${\text{Chlorite + muscovite + quartz}} \rightleftharpoons {\text{cordierite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}}$$ . At low pressures this reaction characterizes the facies boundary between the albite-epidotehornfels facies and the hornblende-hornfels facies, at medium pressures the beginning of the cordierite-amphibolite facies. Experiments were carried out reversibly and gave the following equilibrium data: 505±10°C at 500 bars H₂O pressure, 513±10°C at 1000 bars H₂O pressure, 527±10°C at 2000 bars H₂O pressure, and 557±10°C at 4000 bars H₂O pressure. These equilibrium data are valid for the Fe-rich starting material, using thuringite as the chlorite, as well as for the Mg-rich starting mixture with ripidolite. At 6000 bars the equilibrium temperature for the Mg-rich mixture is 587±10°C. In the Fe-rich mixture almandite was formed instead of cordierite at 6000 bars. The following reaction was observed: $${\text{Thuringite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Experiments with the Fe-rich mixture, containing Fe²⁺/Fe²⁺+Mg in the ratio 8∶10, yielded three stability fields in a P,T-diagram (Fig.1):

1. Above 600°C/5.25 kb and 700°C/6.5 kb almandite+biotite+Al₂SiO₅ coexist stably, cordierite being unstable.
2. The field, in which almandite, biotite and Al₂SiO₅ are stable together with cordierite, is restricted by two curves, passing through the following points:
   1. 625°C/5.5 kb and 700°C/6.5 kb,
   2. 625°C/5.5 kb and 700°C/4.0 kb.
3. At conditions below curves 1 and 2b, cordierite, biotite, and Al₂SiO₅ are formed, but no garnet.

An appreciable MnO-content in the system lowers the pressures needed for the formation of almandite garnet, but the quantitative influence of the spessartite-component on the formation of almandite could not yet be determined. the Mg-rich system with Fe²⁺/Fe²⁺+Mg=0.4 garnet did not form at pressures up to 7 kb in the temperature range investigated. Experiments at unspecified higher pressures (in a simple squeezer-type apparatus) yielded the reaction: $${\text{Ripidolite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Further experiments are needed to determine the equilibrium data. The occurence of garnet in metamorphic rocks is discussed in the light of the experimental results.     

Phase relations to 3 kbar in the systems edenite + H2O and edenite + excess quartz + H2O

Amphiboles approached edenite (NaCa₂Mg₅Si₇AlO₂₂(OH)₂), richterite (Na₂CaMg₅Si₈O₂₂(OH)₂), tremolite (□Ca₂Mg₅Si₈O₂₂(OH)₂) solid solutions were studied by conventional hydrothermal techniques employing the bulk compositions edenite, and edenite + additional quartz, all with excess H₂O. For the stoichiometric edenite bulk composition + excess H₂O, the equilibrium phase assemblage is diopside + Na-phlogopite + forsterite + fluid at, and just above the amphibole high-temperature limit at 850 ± 5°C, 500 bar, and 880 ± 5°C, 1000 bar. The breakdown temperature of sodic phlogopite is 855 ± 3°C at 500 bar, and 890 ± 5°C at 700 bar, producing nepheline + plagioclase (or melt), additional forsterite and fluid. Diopside and Na-phlogopite solid solution coexist over a broad P_fluid-T region, even within the amphibole field, where they are associated with an edenite-richterite (-tremolite) solid solution of approximate composition Ed₃₅Rc₅₀Tr₁₅.In the system edenite + 4 quartz + excess H₂O, nearly pure tremolite and albite coexist stably between 670° and 830°C at 1000 bar and give way to the possibly metastable assemblage diopside + talc + albite below 670°C. In the presence of albite, tremolite reacts to produce diopside + quartz + enstatite + fluid above 830°C at 1000 bar. For the investigated silica-rich bulk composition, amphibole P_fluid-T stability is divided by the albite melting curve into a tremolite + albite field, and a tremolite + aqueous melt field. Substantial equilibrium solid solution of tremolite towards edenite or richterite was not observed for silica-excess bulk compositions. Metastable edenite-rich amphiboles initially synthesized change to tremolite with increasing run length in the presence of free SiO₂.Edenitic amphibole is stable only over a very limited temperature range in silica-undersaturated environments, thus accounting for its rarity in nature. Na-phlogopite solid solutions are also disfavored by high a_SiO2; even for nepheline-normative lithologies, a hypothesized rapid low-temperature conversion to vermiculite or smectite could partly explain the scarcity of sodic phlogopite in rocks.     

The compositional variation of synthetic sodic amphiboles at high and ultra-high pressures

Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30?kbar and 700–800?°C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc?±?quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite?+?4quartz?+?4H₂O=7talc, nyböite?+?3quartz=3jadeite?+?talc, 3Al–Na-cummingtonite + 11quartz + 2H₂O=6jadeite + 5talc, and 3 sodium anthophyllite?+?13quartz?+?4H₂O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of Al^IV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [Na^AAl^IV]<=>[□^ASi] (edenite) toward nyböite, and [Na^(M4)Al^VI]<=>[Mg^(M4)Mg^VI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.     

Zersetzung von Kalk-Alkali-Gesteinen im rezenten Fumarolengebiet auf Nea Kameni/Santorin/Griechenland

In the present phase of the volcanic activity on Nea Kameni / Santorini / Greece the calc-alkalic volcanic rocks are decomposed by H₂O, CO₂ and SO₂ gases of about 100 °C. Using a method ofGresens (1967) for the determination of gains and losses of compounds five different processes could be distinguished:

1. leaching of compounds
2. enrichment of Ca as gypsum
3. increase of Fe₂O₃ (6%–11%), TiO₂ (0,8%–3%) and Zr
4. enrichment of Al₂O₃ (15%–29%), TiO₂ (0,8%–1,5%) and K
5. increase of Al₂O₃ (15%–26%), Fe₂O₃ (6%–9%), TiO₂ (0,8%–1,3%), Sr, Ba, Pb and Zr

Finally a model for the process of decomposition of calc-alkalic volcanic rocks by sulfuric acid was developed.     

Chemistry of calcic amphiboles from the area around Terakanambi,southern Karnataka

Chemial analyses of twelve amphiboles from the area around Terakanambi are presented. Results indicate that they are ferro-hornblende and ferro-pargasitic hornblende types in banded iron formations; magnesio hornblende in ultramafic rocks and edenite; and ferroan pargasite and ferroan pargasitic hornblende types in calcamphibolites. Titanium content in the amphiboles of the present study is relatively low compared to results from similar zones elsewhere. The lower titanium content of the amphiboles may be attributed to either bulk chemical composition or to low oxygen fugacity. Mg/Fe ratios vary considerably and it is mainly controlled by host rock composition. The plots of calcic amphiboles on (lOONa/Ca + Na)/(100 Al/Si + Al) and Al^iv/Al^vi diagrams indicate that they are of medium to low pressure type.