Characterization of NH4-phlogopite (NH4) (Mg3) [AlSi3O10] (OH)2 and ND4-phlogopite (ND4) (Mg3) [AlSi3O10] (OD)2 using IR spectroscopy and Rietveld refinement of XRD spectra

 A synthesis technique is described which results in >99% pure NH₄-phlogopite (NH₄) (Mg₃) [AlSi₃O₁₀] (OH)₂ and its deuterium analogue ND₄-phlogopite (ND₄) (Mg₃) [AlSi₃O₁₀] (OD)₂. Both phases are characterised using both IR spectroscopy at 298 and 77 K as well as Rietveld refinement of their X-ray powder diffraction pattern. Both NH₄ ⁺ and ND₄ ⁺ are found to occupy the interlayer site in the phlogopite structure. Absorption bands in the IR caused by either NH₄ ⁺ or ND₄ ⁺ can be explained to a good approximation using T_d symmetry as a basis. Rietveld refinement indicates that either phlogopite synthesis contains several mica polytypes. The principle polytype is the one-layer monoclinic polytype (1M), which possesses the space group symmetry C2/m. The next most common polytype is the two-layer polytype (2M ₁ ) with space group symmetry C2/c. Minor amounts of the trigonal polytype 3T with the space group symmetry P3₁12 were found only in the synthesis run for the ND₄-phlogopite. Electron microprobe analyses indicate that NH₄-phlogopite deviates from the ideal phlogopite composition with respect to variable Si/Al and Mg/Al on both the tetrahedral and octahedral sites, respectively, due to the Tschermaks substitution ^VIMg²⁺+^IVSi⁴⁺↔^VIAl³⁺+^IVAl³⁺ and with respect to vacancies on the interlayer site due to the exchange vector ^XII(NH₄)⁺+^IVAl³⁺↔^XII□+^IVSi⁴⁺. Received: 30 August 1999 / Accepted: 2 October 2000     

Processes of regional metamorphism and ultrametamorphism and some problems of the geology of the deep-lying sections of folded zones

On the basis of 135 pairs of chemical analyses of coexisting hornblendes and biotites, we have established a relationship between the contents of Al^IV, Al^VI, Fe³⁺, Mg, Ti, Na, and K and the overall iron index in the hornblendes and the depth of granitoid formation. This relationship has been emphasized by the R-method of factor analysis. We have examined the strength and nature of the correlations between the elements in the hornblendes and have considered the types of Isomorphism in the amphiboles according to depth, from the viewpoint of crystal chemistry. A regular increase in the amounts of A^IV in hornblende from <0.8 to > 1.6 formula units; of (Al^VI + Fe³⁺ + Ti) from <0.5 to >1. 0 formula units; of (K + Na) from <0.35 to >0. 64 formula units; and of Group A from <0.24 to >0.51 formula units has been recorded from the near-surface granitoids to the ultra-abyssal types. Biotites In this respect display no adequately clear and reliable information.—Authors.     

The Blosseville Coast basalts of East Greenland: Their occurrence,composition and temporal variations

In the system CaO-MgO-A1₂O₃-SiO₂ the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatite_ss+sillimanite pyrope-grossular_ss+quartz (3), anorthite+enstatite_ss pyrope-grossular_ss+diopside_ss+quartz (2), anorthite+pyrope-grossular_ss+ quartz diopside_ss+kyanite (4) and anorthite+diopside_ss grossular-pyrope_ss +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En_0.5Fs_0.5). Enstatite and plagioclase (An_0.5Ab_0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.     

Determination of Gibbs free energies of formation for the silicates MnSiO3, Mn2SiO4 and Mn7SiO12 in the temperature range 1000–1350 K by solid state emf measurements

Standard Gibbs free energies of formation (A _f G⁰) for the minerals rhodonite (MnSiO₃), tephroite (Mn₂SiO₄) and braunite (Mn₇SiO₁₂) have been determined by measuring the oxygen fugacities of the following redox equilibria: 3Mn₂SiO₄(s)+1/2O₂(g)3MnSiO₃(s)+Mn₃O₄(s) MnSi0₃(s)+2Mn₃O₄(s)+1/2₂O₂(g)Mn₇SiO₁₂(s) and 7MnSiO₃(s)+3/2O₂(gMn₇SiO₁₂)(s)+6SiO₂(s) The measurements were performed by means of the solid-state emf technique involving calcia-stabilized zirconia as electrolyte material. The temperature range covered was 1000–1350 K.The results obtained were used to construct a diagram at 1200 K with the axes lgf(O₂)-molar ratio Si/(Si+Mn) to indicate the stability ranges of the manganese silicates.     

Experiments on the phase transition calcite+wollastonite+ +epidote=grossular-andraditess+CO2+H2O

The reaction 2 epidote+2 calcite+3 wollastonite3 grossular-andradite_ss+ 2 CO₂+1 H₂O has been explored by hydrothermal experiments at a total fluid pressure of 1000 bars. For a grossular-andradite_ss of andradite ₂₅ composition, the isobaric univariant curve passes through the points 458°C: X_CO2=0.00; 521°C: X_CO2=0.026; 523°C: X_CO2=0.052; 526°C: 0.088; 528°C: X_CO2=0.104. This curve intersects the isobaric univariant curve of the reaction calcite+quartz+[H₂O] wollastonite+CO₂+[H₂O] at the isobaric invariant point around 528°C and X_CO2=0.12. At higher values of X_CO2, this reaction is replaced by another one, namely: 2 epidote+5 calcite+3 quartz3 grossular-andradite_ss+5 CO₂+ 1 H₂O. It is demonstrated that both the reactions do actually take place during the metamorphism of calcareous rocks. The petrologic significance of contrasted sequence of reactions within this system observed by various workers is also discussed.     

The relation between c dimension and exchange components in micas

Summary A number of micas of varying compositions and polytypism have been selected from the literature for multiple linear regression analysis. The c dimension in micas is found to depend on the sizes of the interlayer cation, d_i, and tetrahedral cation, d_t, as well as on the hydroxyl content, [OH]. The regression equation obtained: c_r = 5.415 + 0.071[OH] + 2.098d_i + 2.335d_t with R² = 90.5%, shows that the three variables affect the c-axis dimension in the order d_t > d_i [OH]. Addition of 2- and 3-layer polytypes to the regression analysis reduces R² to 87.2%. Application of the regression analysis to synthetic Al-rich biotites from the literature shows that the amount of [A1^IVA1^VI]_1Y[Fe²⁺, MgSi]_–1y in solid solution is limited and always less than [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (i.e. 0.35 > y z). The maximum value of the vector y in solution is slightly higher than that reported for natural Al-rich biotites.

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Die Beziehung zwischen der Gitterkonstante c und den Austauschkomponenten in Glimmern

Zusammenfassung Eine Anzahl von Glimmern unterschiedlicher Zusammensetzung und Polytypie wurde aus der Literatur für eine multiple lineare Regressionsanalyse ausgewählt. Es stellte sich heraus, dass in Glimmern die Gitterkonstante c von den Grössen des Zwischenschicht-Kations d_i und des tetraedrischen Kations d_t abhängt, ferner vorn Hydroxylgehalt, [OH]. Die erhaltene Regressionsgleichung c_r = 5,415 + 0,071 [OH] + 2,098d_i + 2,335d_t mit R² = 90,5% zeigt, dass die drei Variablen die Grösse der c-Achse in der Reihenfolge d_t > d_i [OH] beeinflussen. Der Einschluss von 2- und 3-Schicht Polytypen in die Regressionsanalysen verkleinert R² auf 87,2%. Die Anwendung der Regressionsanalysen auf synthetische Al-reiche Biotite aus der Literatur zeigt, der Betrag von [A1^IVA1^VI]_1y[Fe²⁺, MgSi]_–1y beschränkt und immer kleiner als [A1^VIO]_1z[Fe²⁺, MgOH]_–1z (mit 0,35 < y z) ist. Der Maximalwert des Vektors y in Lösung ist etwas grösser als jener, der für natürliche Al-reiche Biotite angegeben wurde.

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With 1 Figure     

Statistical chemistry of calcic amphiboles

Principal components analysis is used to study the chemistry of 639 calcic amphiboles. Eigenvectors representing multiple partial correlation coefficients give various sets of substitutional relationships. The relative significance of each set can be noted by the percent variation of the data it represents. The highest percent variation (36%) is associated with the substitutions $$Si + Mg \rightleftharpoons Al^{IV} + Al^{VI} + Ti + Fe^{3 + } + Fe^{2 + } + Na + K$$ . Other expected substitutions among the ions such as Al^IV + Na ? Si, the positive correlation between Al^IV and Al^VI etc. are shown statistically. The substitution of Al in T ₁ and T ₂ positions imposes an ordering in the M ₁, M ₂ and M ₃ sites. Variability of OH in the amphiboles is found to be significant. There is no definite correlation between OH and Fe³⁺ but OH and Ti are positively correlated. Under certain conditions and provided the concentration of Al^IV does not change significantly, Fe and Mg may be assumed to mix ideally in the amphibole solid solution.     

Corrensite and mixed-layer chlorite/corrensite in metabasalt from northern Taiwan: TEM/AEM,EMPA, XRD,and optical studies

Many chloritic minerals in low-grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties, expand slightly upon glycolation (expandable chlorite) and/or have excess Al^VI relative to Al^IV, as well as significant Ca, K and Na contents. Chloritic minerals with these properties fill vesicles and interstitial void space in low-grade metabasalt from northern Taiwan and have been studied with a combination of TEM/AEM, EMPA, XRD, and optical microscopy. The chloritic minerals include corrensite, which is an ordered 1:1 mixed-layer chlorite/smectite, and expandable chlorite, which is shown to be a mixed-layer chlorite/corrensite. Corrensite and some mixed-layer chlorite/corrensite occur as rims of vesicles and other cavities, while later-formed mixed-layer chlorite/corrensite occupies the vesicle cores. The TEM observations show that the mixed-layer chlorite/corrensite has ca. 20%, and the corrensite has ca. 50% expandable smectite-like layers, consistent with XRD observations and with their abnormal optical properties. The AEM analyses show that high Si and Ca contents, high Al^VI/Al^IV and low Fe^VI/(Fe+Mg)^VI ratios of chlorites are correlated with interstratification of corrensite (or smectite-like) layers in chlorite. The AEM analyses obtained from 200–500 Å thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low Al^IV/Si^IV and low Fe^VI/(Fe+Mg)^VI, while chlorite has high Al^IV/Si^IV and high Fe^VI/(Fe+Mg)^VI. This implies that the trioctahedral smectite-like component of corrensite has significantly lower Al^IV/Si^IV and Fe^VI/(Fe+Mg)^VI. The ratios of Fe^VI/(Fe+Mg)^VI and Al^IV/Si^IV thus decrease in the order chlorite, corrensite, smectite. The proportions of corrensite (or smectite-like) layers relative to chlorite layers in low-grade rocks are inferred to be controlled principally by Fe/Mg ratio in the fluid or the bulk rock and by temperature. Compositional variations of chlorites in low-grade rocks, which appear to correlate with temperature or metamorphic grade, more likely reflect variable proportions of mixed-layered components. The assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases, such as chlorite-corrensite, corrensite-smectite, or chlorite-corrensite-smectite. A model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1:1 ordered mixed-layer chlorite/smectite.     

Pumpellyites in two low grade metamorphic terranes North of Newcastle,NSW Australia

Microprobe analyses of pumpellyites from rocks of variable chemistry formed under similar metamorphic conditions in two Palaeozoic, low grade metamorphic terranes show that they have an extreme range in composition (FeO^*=0.9–22.96) and that Fe²⁺Mg²⁺ and Fe³⁺Al³⁺ are the dominant substitutions. A less extreme variation in composition of pumpellyites has been noted in samples taken from a metamorphosed differentiated metadolerite. On an A1-Fe^*-Mg diagram, these pumpellyites extend through the fields of high pressure to low pressure terranes, indicating that pumpellyite compositions should be used with caution when determining metamorphic conditions.Bulk chemical composition of the host rock does not appear to be a controlling factor in determining pumpellyite compositions. Rather, intensity of alteration, particularly of opaque mineral phases, fluid chemistry and variation in oxidation potential are considered to be more important variables. Coexisting epidote and composition of the precursor mineral also appear to be important in some rocks.     

Piemontite from the manganiferous hematite ore deposits in the Tokoro Belt,Hokkaido, Japan

Summary Piemontites occur in manganiferous hematite ore deposits and radiolarian chest in the Nikoro Group, Tokoro Belt, eastern Hokkaido, Japan. The piemontite-bearing chest and ore bodies have suffered low-grade metamorphism of high pressure intermediate type. In ore bodies, piemontite forms veinlets with quartz and/or pumpellyite-(Mn²⁺) containing Mn³⁺ in Y site. In chest, piemontite occurs not only in veinlets but also in radiolarian tests with pumpellyite-(Mn²⁺). The mineral assemblages characterized by piemontite, pumpellyite-(Mn²⁺), okhotskite, hematite and bixbyite indicate that chest and ore deposits were metamorphosed under extremely highfO₂ condition. Some piemontites in ores contain as much as 1.12 Mn³⁺, and the sum of Mn³⁺ and Fe³⁺ attains 1.46 per formula unit, whereas piemontites in chest contain less (Mn³⁺ + Fe³⁺). This difference in compositions may essentially be ascribed to the difference in the host rock compositions. On the other hand, Mn³⁺ and Fe³⁺ contents of piemontites in ores vary considerably by Al (Mn³⁺, Fe³⁺) and Mn³⁺ Fe³⁺ substitutions. This phenomenon may be interpreted in terms of the local availability of Mn³⁺ and Fe³⁺ in the host rocks.The low-temperature stability limit of piemontite is evaluated from the relations between piemontite and pumpellyite and from the estimated P-T conditions of piemontite crystallization in chert and ore deposits.

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Piemontit aus den manganreichen Hematit-Lagerstätten des Tokoro-Gürtels, Hokkaido, Japan

Zusammenfassung Piemontite treten in manganführenden Hämatitlagerstätten und Radiolariten in der Nikoro-Gruppe des Tokoro-Gürtels, Ost-Hokkaido, Japan; auf. Die Piemontit-füh-renden Radiolarite und Erzkörper zeigen eine niedrig temperierte (Low-grade Bereich), Hochdruck (intermediate-type)-Metamorphose. In den Erzkörpern bildet Piemontit Gänge zusammen mit Quarz und/oder Mn³⁺ (in der Y-Position)-führendem Pumpellyit-(Mn²⁺). In den Radiolariten tritt Piemontit nicht nur in Gängen, sondern auch zusammen mit Pumpellyit-(Mn²⁺) in Radiolarien auf. Die Mineralparagenese Piemontit, Pumpellyit-(Mn²⁺), Okhotskit, Hämatit und Bixbyit deutet darauf hin, daß die Radiolarite und Erzlagerstätten unter hohenfO₂-Bedingungen metamorphisiert worden sind.In den Erzkörpern enthalten einige Piemontite bis zu 1.12 Mn³⁺ und die Summe von Mn³⁺ und Fe³⁺ erreicht 1.46 pro Formeleinheit. Die Piemontite in den Radiolariten zeigen geringere Mn³⁺ + Fe³⁺ Gehalte. Diese Unterschiede in der Zusammensetzung sind auf die unterschiedlichen Trägergesteine zurückzuführen. Außerdem variieren die Mn³⁺ und Fe³⁺-Gehalte der Piemontite in den Erzkörpern deutlich auf Grund der Substitution von Al (Mn³⁺, Fe³⁺) und Mn³⁺ Fe³⁺. Dieses Phänomen kann durch die lokale Verfügbarkeit von Mn³⁺ und Fe³⁺ im Trägergestein interpretiert werden.Die niedrige Temperatur-Stabilität von Piemontit kann durch die Assoziation Piemontit-Pumpellyit und durch die bestimmten P-T-Bedingungen der Piemontit-Kristallisation in den Radiolariten und Erzlagerstätten abgeschätzt werden.

     

Experimentelle Untersuchung der Metamorphose von kieselig dolomitischen Sedimenten

The equilibrium conditions of the following diospide forming reactions have been determined: 1 tremolite+3 calcite+2 quartz 5 diopside+3 CO₂+1 H₂O (6) 1 Ca₂Mg₅[(OH)₂Si₈O₂₂]+3 CaCO₃+2 SiO₂ 5 CaMg[Si₂O₆]+3 CO₂+1 H₂O 1 tremolite+3 calcite 4 diopside+1 dolomite+1 CO₂+1 H₂O (7) 1 Ca₂Mg₅[(OH)₂Si₈O₂₂]+3 CaCO₃ 4 CaMg[Si₂O₆]+1 CaMg(CO₃)₂+1 CO₂+1 H₂O 1 dolomite+2 quartz 1 diopside+2 CO₂ (8) 1 CaMg(CO₃)₂+2 SiO₂ 1 CaMg[Si₂O₆]+2 CO₂ The experimentally determined equilibrium data of the heterogeneous bivariant reaction (6) are shown in the temperature- -diagram of Fig. 6. For the total fluid pressures of 500 and 1,000 bars the equilibrium temperatures valid for -values >0.75 were calculated using equilibrium constants derived from experimental equilibrium data at smaller values of , and fugacities of CO₂ and H₂O (see Fig. 7). The equilibrium data of reaction (6) were also calculated thermodynamically for the total fluid pressure of 500 and 1,000 bars (see Fig. 3). The results found by this method agree well with those of the experimental investigation (see Figs. 8 and 9). The equilibrium data of the heterogeneous univariant reaction (8) were calculated thermodynamically only (see Fig. 10), because an experimental determination has not been successful so far. The calculated decrease of the equilibrium temperature of reaction (8) caused by a decrease of the CO₂-concentration and a corresponding increase of the H₂O-concentration is shown in Fig. 11. Combining this curve and the equilibrium curve of reaction (6) determined at a fluid pressure of 1,000 bars, an isobaric invariant point of intersection results [see Fig. 12 and also point (II) in Fig. 2 of Metz and Trommsdorff (1968)]. From this point of intersection two further equilibrium curves radiate, i.e. the isobaric univariant equilibrium curves of the reaction (4) (not investigated in this paper) and (7) (see Fig. 14); this follows from phase theory. The equilibrium data of the heterogeneous bivariant reaction (7) were calculated thermodynamically for the total fluid pressure of 1,000 bars. The calculated -curve, since precisely meeting the isobaric invariant point (II), is very well consistent with the equilibrium data of reactions (6) and (8).During metamorphism of siliceous dolomites diopside is formed almost exclusively by reaction (6). In Fig. 15 the equilibrium data of this reaction are shown in a P _f -temperature-diagram (P _f is the total fluid pressure). In this diagram the maximum temperatures at which diopside is formed correspond to the temperature-maxima of the -equilibrium curves of Fig. 6. The temperature range shown in Fig. 15 covers all equilibrium temperatures for -values from 0.1 to approx. 1.0; the extremely low temperatures of diopside formation valid for <0.1 are not included. Generally, however, such low CO₂ concentrations are not to be expected in the process of diopside formation during the progressive metamorphism of siliceous dolomites, because the formation of tremolite consuming H₂O and liberating CO₂ is the preceeding reaction at lower temperatures. Apart from the conditions of diopside formation, Fig. 15 includes the pressure-temperature data of staurolite formation (Hoschek, 1965–1969; Richardson, 1968; Ganguly and Newton, 1968). The equilibrium data of both reactions show that the formation of staurolite in FeO-rich pelitic sediments coincides with diopside formation in siliceous dolomites only at pressures below approx. 1 kb. This is realized during shallow contact metamorphism; while at higher pressures the formation of diopside takes place at higher temperatures than the formation of staurolite. These statements which are results of equilibrium data determined experimentally very well agree with petrographic observations, e.g. in the contact aureole of Bergell granite, and in the regional metamorphism of the Lepontine Alps (E. Niggli, 1960; E. Niggli and C. E. Niggli, 1965; Trommsdorff, 1966).     

Partition of Mg and Fe2+ in coexisting pyroxenes

A general theory for the partition of elements between coexisting, multicomponent phases is outlined and applied to data for Ca-rich pyroxene (Cap) — Ca-poor pyroxene (Op) assemblages from the Skaergaard and Bushveld intrusions and from charnockites. The intercrystalline partition of Mg and Fe²⁺ are studied separately rather than through the exchange reaction, MgSiO ₃ ^Cap +FeSiO ₃ ^Op FeSiO ₃ ^Cap +MgSiO ₃ ^Op .The separate distributions for x_MgSiO3> and x_FeSiO3> are quite distinct and demonstrate directly that solutions of both Mg and Fe²⁺ in the two pyroxenes are nonideal.     

Electronic absorption spectroscopy of natural (Fe2+, Fe3+)-bearing spinels of spinel s.s.-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures

Natural Fe²⁺, Fe³⁺-bearing spinel solid solutions from the spinel s.s.-hercynite and gahnite-hercynite series were analyzed and studied by electronic absorption spectroscopy in the spectral range 30000–3500 cm^–1 in the temperature and pressure ranges 77 T_K 600 and 10^–4 P_GPa 11.0. Two crystals were light-violet in color (type I) and six green or bluish-green (type II). The spectra of both types of spinels are dominated by an UV-absorption edge near 28000 to 24000 cm^–1, depending on the iron contents, and a very intense band system in the NIR centered around 5000 cm^–1, which is caused by spin-allowed dd-transition of tetrahedral Fe²⁺, derived from ⁵ E⁵ T₂. The strong band is in all spinels studied, split into four sub-bands, which can only be observed in very thin platelets. Between the UV-edge and the high-energy wing of the NIR-band there occur a number of very weak bands in type I spinels while the green type II spinels show some of these with significantly enhanced intensity. The intensity of the very weak bands is nearly independent from temperature. Such bands are attributed to spin-forbidden electronic transitions of ^IVFe²⁺. Temperature and pressure dependence of the intensity enhanced bands of spinels type II indicate that they are caused by ^IVFe²⁺ and ^VIFe³⁺. They are attributed to spin-forbidden transitions ⁶A_1g⁴A_1g, ⁴E_g, ⁴T_2g and ⁴T_1g of ^VIFe³⁺, the two latter being strongly intensified by exchange-coupling interaction with adjacent ^IVFe²⁺. The pressure dependence of ^IVFe²⁺ dd-band system in the NIR caused by spin-allowed ⁵ E⁵ T₂ transition noticeably differs from that of octahedral Fe²⁺, an effect which is attributed to a dynamic Jahn-Teller effect of ^IVFe²⁺ in the spinel structure.

Miscibility and compatibility of braunite,Mn2+Mn 6 3+ O8/SiO4, in the system Mn-Si-O at 1 atm in air

Experimental investigations between 800 ° to 1,100 ° C yielded no evidence for extensive substitution of Mn²⁺+Si⁴⁺2Mn³⁺ in braunite, leading to a complete solid solution series between partridgeite (Mn₂O₃) and braunites with silica contents up to 40 wt. % as proposed by Muan (1959a, b). In the presence of excess manganese braunite of nearly ideal composition coexists at 800 ° C with partridgeite and at T1,000 ° C with hausmannite (Mn₃O₄). At 800 ° C and 1,000 ° C braunite coexists, in the presence of excess silica, with a SiO₂-polymorph and at 1,100 ° C with rhodonite (MnSiO₃). Quantitative analysis of the X-ray patterns of coexisting cristobalite and braunite confirms a maximum silica-excess in braunite of only about 2 wt.% over the ideal composition, Mn²⁺Mn ₆ ³⁺ SiO₁₂.     

Clinopyroxenes from plagioclase peridotites (Zabargad Island,Red Sea) and comparison between high- and low-pressure mantle clinopyroxenes

Summary A detailed crystal-chemical study of clinopyroxenes from the peridotite-pyroxenite association from Zabargad Island (Red Sea) has been carried out to decline the intercrystalline relationships in mantle-derived clinopyroxenes equilibrated at low pressure conditions (plagioclase facies:Pl-Cpx).Pl-Cpx typically show larger cell volume (>437 °A3) compared with those from spinel and garnet-spinel peridotite nodules (Sp-Cpx). The larger cell volume is mainly achieved through higher Mgm, and louver Al^VI occupancies, which strongly increase the M l volume. Concurrently, overcharging on the O3 oxygens due te, the high Ca_M2 ( 0.828 atoms per formula unit, a.f.u.) and low Na_M2 (< 0.037 a.f.u.) occupancies requires lengthening of T-0 distances and increase of the T volume. Consequently: i) for the saine M 1 volume,Pl-Cpx have larger cell volume compared withSp-Cpx; ii) for a given trivalent cations (R³⁺) content in M1, Al^IV is higher inPl-Cpx than inSp-Cpx. Plots of cell volume vs Ml volume and of M1-O2 vs T-O_nbr bond lengths are a simple way to illustrate the complex intracrystalline relationships which control (Ca Na)_M2, (Si Al^IV)_T and (Mg R³⁺)_M1 substitutions, and thus permit sensitive qualitative discrimination of the pressure regimes of equilibration of mantle clinopyroxenes.

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Klinopyroxene aus Plagioklas-Peridotiten der Insel Zabargad (Rotes Meer) und Verqleich zwischen Hoch- und Tiefdruck-Klinopyroxenen des Mantels

Zusammenfassung Klinopyroxene aus der Peridotit-Pyroxenit-Assoziation der Insel Zabargad (Rotes Meer) wurden eingehend kristallchemisch untersucht, um die inter-kristallinen Bezie-hungen in aus dem Mantel stammenden Klinopyroxenen zu definieren, die unter niedrigem Druck (Plagioklas-Fazies,PI-Cpx) equilibriert wurden.PI-Cpx zeigen typisch ein größeres Zellvolumen (>437 ų) im Vergleich zu jenen aus Knollen von Spinell- und Granat-Spinell-Peridotiten (Sp-Cpx). Das größere Zellvolumen wird hauptsächlich durch größere Mg_M1- und kleinere Al^VI-Besetzungen erreicht, welche das M1-Volumen stark erhöhen. Gleichlaufend verlangt ein Ladungsüberschuß an den O3-Sauerstoffen durch hohe Ca_M2-Besetzungen (> 0,828 Atome pro Formeleinheit) und niedrige Na_M2-Besetzungen ( 0,037 Atome pro Formeleinheit) eine Verlängerung der T-O-Abstände und ein Anwachsen des T-Volumens. Folglich haben für das gleiche M1-Volumen diePl-Cpx ein größeres Zellvolumen gegenüber denSp-Cpx, und ferner ist für einen gegebenen Gehalt an dreiwertigen Kationen (R³⁺) auf M1 das Al^IV inPI-Cpx höher als inSp-Cpx. Diagramme von Zellvolumen gegen Volumen von M1, sowie von M1-O2 gegen T-O_nbr sind ein einfaches Mittel, um die komplexen interkristallinen Beziehungen aufzuzeigen, welche die Substitutionen (Ca Na)_M2, (Si AI_IV)_T und (Mg R³⁺)_M1 beherrschen, sie erlauben eine empfindliche qualitative Unterscheidung der Druckverhältnisse bei der Equilibrierung von Klinopyroxenen aus dem Mantel.

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With 6 Figures     

Distribution of Al between coexisting micas in metamorphic rocks from the Central Alps

In 61 pairs of coexisting biotites and muscovites from the Central Alps total Al scatters considerably, but in both series a gradual increase is noticed with increasing metamorphic grade. The ratio Al _Mu ^tot /Al _Bi ^tot remains virtually constant (1.61 average for greenschist facies, 1.57 for amphibolite facies). Tetrahedral Al varies little in biotites and increases in muscovites-phengites with rising metamorphic grade; accordingly the ratio Al _Mu ^IV /Al _Bi ^IV increases slightly with grade. Far the best control of metamorphism is evidenced by octahedral Al. In the muscovite series, and still more pronounced in the biotite series, Al^VI increases with increasing metamorphic grade. Consequently 1 $$K_D = \frac{{Al_{Mu}^{VI} }}{{Al_{Bl}^{VI} }}$$ decreases from 14 to 3. A map (Fig. 6) representing the regional distribution of the K_D values locates a 100 km long and 23 km broad central zone with low K_D. The outline of this central core almost coincides with the isograds anorthite-diopside-calcite and labradorite-pyroxene-hornblende of the Tertiary regional metamorphism; with some deviations this core also agrees with the zone in which phenomena of partial anatexis are observed. The K_D values of micas from anateotic pegmatites agree with those of associated gneisses and schists. The study demonstrates that in the course of progressive regional metamorphism equilibrium has been approached to an unexpected extent and that the two micas coexisted in a strict sense.     

Anthophyllit und Hornblende in einigen metamorphen Reaktionen

Reactions which occur at the lower boundary of the hornblende-hornfels facies and in the so-called pyroxene-hornfels facies were experimentally investigated for an ultrabasic rock at 500, 1000 and 2000 bars H₂O pressure.The starting material used was a mixture of natural chlorite, talc, tremolite and quartz such that its composition, except for surplus quartz, corresponded to that of an ultrabasic rock. The atomic ratio Fe²⁺+Fe²⁺/Mg+Fe³⁺+Fe³⁺ in the system was 0.16.The lower boundary of the hornblende-hornfels facies was defined by the formation of the orthorhombic amphibole anthophyllite and hornblende according to the following idealized reaction: chlorite+talc+tremolite+quartz hornblende+anthophyllite+H₂O In effect, this reaction consists of the two bivariant reactions: chlorite+tremolite+quartz hornblende+anthophyllite+H₂O talc+chlorite anthophyllite+quartz+H₂OThe equilibrium temperatures obtained for the two reactions in the given system are practically the same and are as follows: 535±10°C at 500 bars H₂O pressure 550±20°C at 1000 bars H₂O pressure 560±10°C at 2000 bars H₂O pressure 580±10°C at 4000 bars H₂O pressureAt 2000 bars and higher temperatures within the hornblende-hornfels facies, anorthite is formed in addition to hornblende and anthophyllite, probably according to the following reaction: hornblende₁+quartz hornblende₂+anthophyllite+anorthite+H₂O; because of the formation of anorthite it is to be expected that the hornblende in this case is poorer in aluminium than the hornblende at 500 and 1000 bars. Winkler (1967) suggests renaming the pyroxene-hornfels facies as K-feldspar-cordierite-hornfels facies which, in turn, is subdivided into a lower-temperature orthoamphibole subfacies without orthopyroxene and a higher-temperature orthopyroxene subfacies without orthoamphibole. The orthopyroxene subfacies itself may in its lower temperature part still carry hornblende which finally disappears in the higher temperature part.The appearance of orthopyroxene characterizes the transition from the orthoamphibole to the orthopyroxene subfacies of the K-feldspar-cordierite hornfels facies. The following reaction takes place at pressures lower than 2000 bars: hornblende₁+anthophyllite hornblende₂+enstatite+anorthite+H₂OSince at 2000 bars an Al-poor hornblende already exists in the hornblende-hornfels facies, it is very likely that here only anthophyllite breaks down to give enstatite+quartz+H₂O.The equilibrium temperatures for these reactions which give rise to enstatite are: 650±10°C at 250 bars H₂O pressure 690±10°C at 500 bars H₂O pressure 715±10°C at 1000 bars H₂O pressure 770±10°C at 2000 bars H₂O pressureOnly after an increase in temperature to about 710°C at 500 bars and about 770°C at 1000 bars does hornblende in the system investigated here break down completely according to the reaction: hornblende = enstatite+anorthite+diopside+H₂OExcept at very small H₂O-pressures (see Fig. 3), there exists, therefore, a region within the orthopyroxene subfacies where hornblende, enstatite and anorthite coexist. As a result we have, as mentioned above, a lower-temperature and a higher-temperature part of the orthopyroxene subfacies, and it is only in the latter part that the parageneses correspond to the pyroxene-hornfels facies as stated by Eskola (1939).Summing up, the starting material consisting of chlorite, talc, tremolite plus quartz remains unchanged in the albite-epidote-hornfels facies; this gives rise in the hornblende-hornfels facies to the paragenesis hornblende+anthophyllite, or — at higher pressures — to hornblende+anthophyllite+anorthite. For the particular composition of the starting material, however, no reactions take place at the transition of the hornblende-hornfels facies to the orthoamphibole subfacies of the K-feldspar-cordierite-hornfels facies as this transition is typified by the breakdown of muscovite in the presence of quartz. However, at the end of the orthoamphibole subfacies the breakdown of anthophyllite, by which orthopyroxene is formed, heralds the onset of the orthopyroxene subfacies. In this subfacies — at greater than about 300 bars — hornblende is still present and coexists with enstatite and anorthite, but with rising temperature hornblende breaks down to give way to the paragenesis enstatite+anorthite+diopside. The experimentally determined parageneses confirm known petrographic occurrences.

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Für die Förderung dieser Arbeit danken wir der Deutschen Forschungsgemeinschaft vielmals. Der Dank von Choudhuri gilt dem Akademischen Auslandsamt der Universität Göttingen für ein Stipendium, das ihm den Abschluß seiner Studien an der Universität Göttingen ermöglichte.     

An experimental investigation into the metastable formation of phosphoran olivine and pyroxene

The formation of phosphoran olivine by crystallization from a melt was investigated experimentally using a one atmosphere furnace, using San Carlos olivine [(Mg,Fe)₂SiO₄] mixed with either iron phosphide (FeP) or magnesium pyrophosphate (Mg₂P₂O₇). Both dynamic crystallization and isothermal experiments produced phosphoran olivine as zoned single crystals and as overgrowths surrounding normal, phosphorus-free olivine grains. The crystallization pathways that form phosphoran olivine were traced and confirm that it is a metastable phase that can crystallize from a phosphorus-rich melt over timescales of hours to days. Removal of the P and equilibration of the olivine however requires weeks to months in the presence of silicate melt. Phosphoran olivine with up to 27 wt% P₂O₅ was generated and up to 69% of the Si tetrahedral sites were replaced by P. The substitution of Si by P into olivine was confirmed as 4^VIM⁺² + 2^IVSi⁺⁴ ↔ 3^VIM⁺² + 2^IVP⁺⁵ + ^VI[]. Phosphoran olivine compositions that vary from (Mg,Fe)₂SiO₄ to (Mg,Fe)_1.65[]_0.35Si_0.3P_0.7O₄ have been produced in these experiments.Phosphoran pyroxene was also generated in a few experiments and forms when phosphoran olivine reacts with either tridymite or melt. It has compositions compatible with protopyroxene, orthopyroxene, pigeonite and sub-calcic augite, and can contain up to 31.5 wt% P₂O₅. Like phosphoran olivine, it is also a metastable phase. Phosphorus replaces Si in pyroxene by the following substitution methods: 8^IVSi⁺⁴ ↔ 3^IVSi⁺⁴ + 4^IVP⁺⁵ + ^IV[] with Al entering the structure by the exchange 2^IVSi⁺⁴ ↔ ^IVAl⁺³ + ^IVP⁺⁵. Phosphoran pyroxene compositions vary from (Mg,Fe)₈Si₈O₂₄ to (Mg,Fe)₈Si₃P₄[]O₂₄.     

ELNES spectroscopy of mixed Si coordination minerals

 Si K- and L-edge ELNES spectroscopy and multiple-scattering (MS) calculations are used to examine mixed Si coordination compounds varying in Si^VI:Si^IV ratio. As in previous studies, the edges are influenced mainly by silicon coordination (tetrahedral vs. octahedral), as supported by the MS calculations. We demonstrate two methods semi-quantitatively to extract the value of Si^VI/(Si^VI+Si^IV): (1) A linear relationship between the L_2,3-L₁ splitting and Si^VI/(Si^VI+Si^IV) is observed, (2) a fitting method based on the coaddition of reference tetrahedral and octahedral Si spectra is applied to both Si K- and L-edge ELNES spectra. Received: February 10, 1997 / Revised, accepted: May 23, 1997     

Low-temperature compatibility relations of the assemblage quartz-paragonite and the thermodynamic status of the phase rectorite

The reaction 3 Na-montmorillonite + 2 albite 3 paragonite + 8 quartz has been studied experimentally using starting materials composed of natural low albite, kaolinite and quartz. Rate studies at 2, 4 and 7 kb demonstrate that the reaction takes place at 335–315° C from lower to higher pressures. Attempts to reverse this reaction with runs lasting several months were without success. Comparison with pertinent data from natural mineral assemblages indicate that despite non-reversal, the data presented here may be very near to the true lower thermal compatibility limit of the assemblage quartz-paragonite.The above reaction becomes metastable beyond the upper pressure stability limit of the phase Na-montmorillonite; it is replaced here by another reaction 1 albite + 1 kaolinite 1 paragonite + 2 quartz + 1 H₂O, as suggested originally by Zen (1960). A P-T-grid showing possible compatibility relations of the assemblage quartz-paragonite is provided (Fig. 4). Perusal of natural assemblages belonging to the subsystem Na₂O-Al₂O₃-SiO₂-H₂O lends credence to this grid.In course of the rate studies reported here, various regular paragonite-sodium montmorillonite mixed-layer phases were encountered (Fig. 2); the 11 regular mixed-layer phase represents the synthetic analogue of the mineral rectorite (sometimes called allevardite), widely recorded from deep diagenetic and anchimetamorphic environments. Results of rate-studies (Fig. 3) suggest that the mixed-layer phases are all transient, metastable products obtained during the transformation of the albite-Na-montmorillonite assemblage to paragonite-quartz. As such, rectorite and related mixed-layer phases on the join montmorillonite-paragonite, are always less stable relative to the assemblage Na-montmorillonite-paragonite.