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11

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The metamorphic paragenesis of cordierite in pelitic rocks (1969)

Hess, Paul C.

Springer

Contributions to mineralogy and petrology 24 (1969), S. 191-207

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract A petrogenetic grid is constructed for mineral assemblages occurring in metapelitic rocks, particularly those involved in the paragenesis of cordierite. The most useful assemblages for estimating pressures and temperatures are staurolite-cordierite, cordierite-biotite-Al2SiO5 and cordierite-hypersthene. Cordierite is stable with kyanite, sillimanite or andalusite. At high pressures cordierite is Mg-rich so that pelitic rocks typically do not contain the phase. Cordierite is stable at temperatures less than 500° C but does not commonly appear in metapelitic rocks until the garnet-chlorite, chlorite-staurolite or chlorite-Al2SiO5 tie-lines are broken. At high metamorphic grades, the assemblage garnet-hypersthene-cordierite indicates relatively low pressures, and the assemblage hypersthene-cordierite-sillimanite relatively high pressures. It is clear however, that the absence of cordierite is of little use in characterizing a metamorphic facies unless an alternate mineral assemblage can be shown to be more stable.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00376047

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12

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Solution behavior of +4 cations in high silica melts: petrologic and geochemical implications (1986)

Ellison, Adam J. ; Hess, Paul C.

Springer

Contributions to mineralogy and petrology 94 (1986), S. 343-351

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The saturation surfaces of rutile (TiO2), zircon (ZrSiO4), and hafnon (HfSiO4) were determined in anhydrous, peraluminous, high silica liquids of the system SiO2-Al2O3-Na2O-K2O as functions of silica concentration at 1,400° C in air. The saturation concentrations of TiO2, ZrO2, and HfO2 in rutile, zircon, and hafnon-saturated liquids, respectively, decrease smoothly and gradually as functions of increasing silica concentration. Thermodynamic analyses of the data demonstrate that the activity coefficients of TiO2, ZrO2, and HfO2 increase smoothly and gradually as silica concentration is increased from 67 wt-% to 80 wt-%, and that changes in SiO2 of 1 or 2 wt-% result in small changes in the saturation concentrations and activity coefficients of +4 cations. Because the solution behavior of +4 cations in highly siliceous liquids (〉75 wt-% SiO2) is predictably different than in less siliceous liquids (70 to 75 wt-% SiO2), classification of highly-siliceous igneous rocks on the basis of silica concentration alone should not be interpreted to mean that their solution chemistry differs significantly from that of less siliceous rocks. The results of this study are compared with other studies of +4 cation solution behavior. From this it is concluded that variations in liquid compositions observed in cogenetic suites of high silica rhyolites cannot cause the observed changes in +4 cation concentrations. Thus, even if a large change in solution behavior of +4 cations is inferred from the large variations in their concentrations, it cannot be due to changes in bulk composition of the parental liquid. In addition, the similarity in the solution behavior of Zr and Hf seen in this study suggests that their solution mechanisms are similar. It is thus unlikely that liquid-state processes can fractionate one with respect to another, and variations in Zr/Hf ratios in suites of extrusive rocks are likely due to crystal-liquid equilibria, e.g., zircon fractionation.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00371443

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The role of phosphorus in rhyolitic liquids as determined from the homogeneous iron redox equilibrium (1993)

Gwinn, Rosa ; Hess, Paul C.

Springer

Contributions to mineralogy and petrology 113 (1993), S. 424-435

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The redox ratio of iron is used as an indicator of solution properties of silicate liquids in the system (SiO−Al2O3−K2O−FeO−Fe2O3−P2O5). Glasses containing 80–85 mol% SiO2 with 1 mol% Fe2O3 and compositions covering a range of K2O/Al2O3 were synthesized at 1400°C in air (fixed fO2). Variations in the ratio FeO/FeO1.5 resulting from the addition of P2O5 are used to determine the solution behavior of phosphorus and its interactions with other cations in the silicate melt. In 80 mol% SiO2 peralkaline melts the redox ratio, expressed as FeO/FeO1.5, is unchanged relative to the reference curve with the addition of 3 mol% P2O5. Yet, the iron redox ratio in the 85 mol% SiO2 potassium aluminosilicate melts is decreased relative to phosphorus-free liquids even for small amounts of P2O5 (0.5 mol%). The redox ratio in peraluminous melts is decreased relative to phosphorus- free liquids at P2O5 concentrations of 3 mol%. In peraluminous liquids, complexing of both Fe+3−O−P+5 and Al+3−O−P+5 occur. The activity coefficient of Fe+3 is decreased because more ferric iron can be accommodated than in phosphorus-free liquids. In peralkaline melts, there is no evidence that P+5 is removing K+ from either Al+3 or Fe+3 species. In chargebalanced melts with 3 mol% Fe2O3 and very high P2O5 concentrations, phosphorus removes K+ from K−O−Fe+3 complexes resulting in a redox increase. P2O5 should be accommodated easily in peraluminous rhyolitic liquids and phosphate saturation may be suppressed relative to metaluminous rhyolites. In peralkaline melts, phosphate solubility may increase as a result of phosphorus complexing with alkalis. The complexing stoichiometry may be variable, however, and the relative influence of peralkalinity versus temperature on phosphate solubility in rhyolitic melts deserves greater attention.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00286932

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The structural role of Al2O3 and TiO2 in immiscible silicate liquids in the system SiO2-MgO-CaO-FeO-TiO2-Al2O3 (1980)

Wood, Marcus I. ; Hess, Paul C.

Springer

Contributions to mineralogy and petrology 72 (1980), S. 319-328

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The effects of the addition of Al2O3 on the large stable two liquid field in the SiO2-TiO2-CaO-MgO-FeO system were experimentally determined. The increase of Al2O3 content in the starting composition results in the decrease of critical temperature, phase separation and liquidus temperature of the two liquid field until it is rendered completely metastable. The shrinkage of the two liquid field indicates that Al2O3 is acting in the role of a network former and homogenizes the structure of the two melts. In this alkali-free system Al+3 utilizes the divalent cations, Ca+2 and Mg+2, for local charge balance with a preference for Ca+2 over Mg+2. Thus, AlO4 tetrahedra combine with SiO4 tetrahedra to form an aluminosilicate framework which polymerizes the SiO2-poor melt and makes it structurally more similar to the SiO2-rich melt. However, Ca+2 and Mg+2 are not as efficient in a charge balancing capacity as the monovalent K+ and Na+ cations. The lack of alkalis in this system limits the stability of AlO4 tetrahedra in the highly polymerized SiO2-rich melt and results in the preference of Al2O3 for the SiO2-poor melt. The partitioning systematics of Ti are virtually identical to those of Al. It is concluded that Ti occurs in tetrahedral coordination as a network forming species in both the high — and low — SiO immiscible melts.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00376151

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15

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Prograde and retrograde equilibria in garnet-cordierite gneisses in south-central Massachusetts (1971)

Hess, Paul C.

Springer

Contributions to mineralogy and petrology 30 (1971), S. 177-195

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Partial electron microprobe analyses of garnet, biotite and cordierite in sillimanite-K feldspar gneisses of the Brimfield Formation in south-central Massachusetts indicate that the compositions of these minerals are not constant in a thin section. The FeO/MgO mol ratio of biotite is sensitive to the nature of other FeO-MgO minerals occurring in close proximity. The most iron-rich biotites are those that do not contact either cordierite or garnet. The most iron-poor biotites occur as inclusions in garnet. Biotites in direct contact with either cordierite or garnet have intermediate FeO/MgO ratios. The bulk of a given grain of garnet or cordierite is homogeneous in composition. Chemical zoning is absent. All grains of garnet and cordierite in a thin section are constant in composition. However, where garnet and cordierite abut biotite, the FeO/MgO ratio of the garnet rim is increased and that of cordierite is decreased. The FeO/MgO ratios of garnet, cordierite and biotite bare a regular relation to each other indicating a possible equilibrium state. However the distribution coefficient defined by the compositions of minerals in direct contact are greater than those defined by the compositions of the interiors of garnet and cordierite matched with the compositions of biotites removed from these phases. This pattern is believed to be the result of two thermal events. The first event produced the mineral assemblages and widespread equilibrium was obtained. A subsequent retrograde event left the mineralogy intact but caused cation exchange reactions at immediate contacts between garnet, cordierite and biotite. The physical conditions of the first event are estimated at P=5–6 kb, T=700–750° C. The retrograde event occurred at lower temperatures and very low activities of H2O since no muscovite is developed at microcline-sillimanite contacts.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00455670

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16

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Aluminum coordination in metaaluminous and peralkaline silicate melts (1982)

Hess, Paul C. ; Wood, Marcus I.

Springer

Contributions to mineralogy and petrology 81 (1982), S. 103-112

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Incremental amounts of Na2O and K2O added to immiscible melts in the MgO-CaO-TiO2-Al2O3 SiO2 system cause a decrease in critical temperature, phase separation and change in the pattern of Al2O3 partitioning. Al2O3, which is concentrated in the low SiO2 immiscible melts in the alkali-free system, is increasingly partitioned into the high-SiO2 immiscible melt as the alkali/aluminium ratio is increased. However, K2O is more effective than Na2O in stabilizing Al2O2 in the SiO2-rich melt. The coordination changes occurring in the aluminosilicate melts upon the addition of the alkali oxides are described by CaAl2O4+2SiOK=2KAlO2+SiOCaOSi where K (or Na) displaces Ca as the charge-balancing cation for the networkforming AlO4 tetrahedra. The increased stability of the AlO4 species in the highly polymerized SiO2-rich melt and the consequent shrinkage of the miscibility gap is ascribed to positive configurational entropy and negative enthalpy changes associated with the formation of K, Na-AlO4 species. Element partition systematics indicate that (Na, K)AlO2 species favor the more polymerized, CaAl2O4 and TiO2 species, the less polymerized silicate structure in the melt.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00372047

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