Enthalpies of ordering in the plagioclase feldspar solid solution

Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A “low” series contained the natural ordered material and a “high” series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole “e” structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il? structure →- Cl? high albite structure: ~0.7-1.9 kcal/mole Il? structure equilibrated at low temperature → Il? structure equilibrated at high temperature: ~ 1.8?0.8 kcal/mole.ΔH_soln data for the high series overlap with the data of Newtonet al. (1980) for synthetic high structural state plagioclases except in the composition range ~An₉₀–An₁₀₀. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl? symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il? and “e” structures.Values of the enthalpy change due to disordering (ΔH_ord) also show a number of systematic trends. Firstly, the values for e → Cl? are larger than for Il? → Cl? in the composition range where both e and Il? structures are observed (~An₆₅-An₇₂). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔH_ord, which occurs at ~An₅₀, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il? anorthites and bytownites to a Cl? structure have been estimated by assuming that the Cl? solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.     

Comparison of low-temperature with high-temperature diffusion of sodium in albite

Diffusion of sodium in albite from Amelia County, Virginia, was examined at high and intermediate temperatures by means of the sectioning technique and at low temperatures by means of an exchange technique. These studies indicate that diffusion is heterogeneous at low and intermediate temperatures, but goes to volume diffusion at high temperatures. Apparent diffusion coefficients of the order of magnitude of 10⁻¹⁸ cm²/sec and an activation energy of approximately 3000 cal/mole sodium are determined for the low-temperature movement. An activation energy of approximately 35 kcal/mole sodium is determined for the high-temperature movement.     

Hydrolytic weakening of experimentally deformed Westerly granite and Hale albite rock

In order to determine the effect of water on deformation in the brittle-ductile transition region of crustal rocks, experiments have been conducted on Westerly granite and a polycrystalline albite rock, comparing samples dried at 160°C for 12 h (‘dry’) and samples with about 0.2 wt% water added (‘wet’). The deformation mechanisms and style of deformation of the wet and dry samples, determined using optical and transmission electron microscopy, have been found to depend on temperature, pressure, strain rate, and strain. At 15 kb and 10⁻⁶, the added water reduces the temperature of the transition between microcracking and dislocation glide and climb by about 150–200°C for both quartz and feldspar. However, the penetration of ‘water’ into the grains is slow compared with the time of the experiments and many of the wet samples show evidence of initial microcracking and later dislocation creep. Wet samples deformed at 10 kb show less hydrolytic weakening than wet samples deformed at 15 kb. Because the deformation mechanism and strength of silicates depend so sensitively on trace amounts of water, and because the water content of experimental samples varies with temperature and pressure and thus with time, flow laws for any samples are only meaningful if the water content has been carefully controlled or characterized.     

Kinetics of intracrystalline order-disorder reactions in tremolite

The temperature dependent Fe-Mg distribution in tremolite from Zillertal, Austria was investigated using Mössbauer spectroscopy. The standard free energy change for the exchange reaction Fe²⁺(M4)+Mg(M2)=Mg(M4)+Fe²⁺(M2) decreases with increasing temperature, corresponding to an enthalpy term of 9.2±1.5 kcal/mole and an entropy term of 1.9±1.7 cal/mole K. Kinetic experiments performed as hydrothermal runs, yield an activation energy of 70±7 kcal/mole for the disordering reaction. Difficulties in analysing the very small amount of Mg(M4) in the natural sample introduce large errors in the calculation of cooling rates for the natural rock. The estimated error in the Mg(M4) site occupancy results in a shift of the obtained cooling rate of about three orders of magnitude, making tremolite less useful for estimations of rock cooling rates.     

Experimental deformation of sintered albite above and below the order-disorder transition

Abstract

Purified albite powder (44-53 μm) has been sintered to form an albite polycrystal suitable for deformation studies close to the melting temperature. Experiments have been carried out in Griggs solid medium deformation apparatus at 800, 1020 °C and 700 MPa pressure in a dehydrating pyrophyllite confining medium at constant strain rates of 10^?4, 10^?5, 10^?6 and 10^?7/s. At 800 °C the samples were brittle-ductile whereas at 1020 °C they were ductile with a rheology well described by a power law with a stress exponent of 3. The transition from brittle-ductile to ductile also coincided with the order-disorder or low-high albite transition, as indicated by the marked increase in mechanical twinning on the albite law at high temperature. At 1020 °C high dislocation densities (10^?10 - 10^?11/cm² and mechanical twinning characterised the original high albite grains, whereas fine recrystallised grains ( < 5 цm) had low dislocation densities (10⁷ - 10⁸/cm²) and often contained polysynthetic albite and pericline (M-type) twins. It is suggested that the recrystallized grains were monalbite (monoclinic) under test conditions which have inverted to high albite (triclinic) and in so doing produced M-twins, and that the recrystallization mechanism involved grain-boundary bulging to nucleate new high angle boundaries. The implications of the order-disorder transition for twinning and grain boundary migration are discussed and it is suggested that the data cannot be simply extrapolated to natural deformation in the low albite field.     

A re-examination of the role of hydrogen in Al−Si interdiffusion in feldspars

Recent experimental studies have shown that the rates of Al–Si order-disorder and interdiffusion in alkali feldspars at high pressures under dry conditions increase dramatically in the approximate pressure range 7–14 kb, depending on temperature and feldspar composition (Goldsmith 1987, 1988). Enhancement of Al–Si interdiffusion rates is ascribed to the involvement of hydrogen, but the species of hydrogen involved is undetermined.A simple kinetic analysis of the data of Goldsmith (1987) on disordering of dry albite at 800°–950° C and 6–24 kb in the solid media press is consistent with the NaCl pressure cell acting as a proton donor by enhancing dissociation of water in the pressure medium, generating a high in the experimental environment. The rate constant for disordering of albite is found to increase linearly with the estimated experimental and with the density of aqueous salt solution, implicating H⁺ as the rate-enhancing species.Further experimental studies confirm the importance of . At 16 kb and 850° C, dry albite in sealed Pt capsules in a NaCl cell containing tantalum powder (which reduces H₂O to H₂) remains highly ordered over the same time that complete disordering would occur in the absence of Ta. H₂ cannot therefore be the rate-enhancing species. At 1 kb and 850° C, the extent of Al–Si disorder in albite in direct contact with various NaCl–H₂O solutions increases from partially disordered for pure H₂O to completely disordered for saturated aqueous NaCl solution, giving strong support to the proton model. SIMS scanning ion imaging of albite run products demonstrates conclusively that solution-reprecipitation is not responsible for enhanced disordering rates.Results of disordering experiments in the solid media apparatus cannot be duplicated in Ar gas media internally-heated pressure vessels, even with the same experimental configuration around the albite-bearing capsules, due to the different proton-buffering capacities of the solid and gas media apparatus.     

Diffusion-controlled growth of albite and pyroxene reaction rims

The growth rates of albite and pyroxene (enstatite + diopside + spinel) reaction rims were measured at 1000°C and ˜700 MPa and found to be parabolic indicating diffusion-controlled growth. The parabolic rate constants for the pyroxene (+ spinel) rims in samples with 0.5 wt% H₂O added or initially vacuum dried at 25°C and 250°C are 1.68 ± 0.09, 0.54 ± 0.05 and 0.25 ± 0.06 μm²/h, respectively. The values for albite rim growth in samples initially dried at 60°C and with 0.1 wt% H₂O added are 0.25 ± 0.04 and 0.33 ± 0.03 μm²/h, respectively. The latter values were used to derive the product of the grain boundary diffusion coefficient D′_A, where A = SiO₂, NaAlO₂, or NaAlSi₋₁, and the grain boundary thickness δ in albite. The calculated D′_SIO2δ in the albite aggregate for the situations of two different water contents are about 9.9 × 10⁻²³ and 1.4 × 10⁻²² m³ s⁻¹, respectively. Both the rate constants and the calculated D′_Aδ demonstrate that the effect of water content on the grain boundary diffusion rate in monomineralic albite and polymineralic pyroxene (+ spinel) aggregates is small, consistent with recent studies of monomineralic enstatite and forsterite rims. Received: 1 July 1995 / Accepted: 1 August 1996     

Sillimanite—disordering enthalpy by calorimetry

Heat of solution measurements in an oxide-melt were performed on samples of natural sillimanite heat-treated in the range 1200–1700°C at pressures of 16–23 kbar. A distinct enthalpy of solution decrement relative to the unheated sillimanite of about 1.3 kc/mole is shown by samples run at 1400–1550°C. Pressure variations in the range 16–23 kbar cause little change in the heat of solution in this temperature range. This pressure-independent ‘plateau’ in heat of solution is interpreted to be due to Al-Si disordering on tetrahedral sites in the sillimanite structure. A simple temperature-dependent disordering model developed by navrotsky and Kleppa (1967) for spinels leads to an Al-Si interchange enthalpy, Δ_Hnt, of16 ± 1 kcal/mole, in good agreement with the value derived by Holdaway(1971) on entirely different grounds.Above 1550°C, larger heat of solution decrements were observed. Microprobe analyses of quenched samples indicate that the sillimanite has not deviated significantly from the ideal formula. Some unknown profound disordering process may account for the heat effects in the very high temperature range.Unit cell volumes of quenched samples also describe a ‘plateau’ region in the temperature range 1400–1550°C. This plateau consists of an increase of the 6 crystallographic axis beginning at 1350°C without much change in the other axes in the range 1350–1550°C. A sudden expansion of the α-axis occurs between 1550 and 1630°C.We conclude that Al-Si disorder of the type postulated by Beger et al. (1970), and Holdaway (1971) has been confirmed calorimetrically for samples heated under pressure in the temperature range 1400–1550°C.     

Dislocation-assisted diffusion of oxygen in albite

Oxygen diffusion in albite has been determined by the integrating (bulk ¹⁸O) method between 750° and 450° C, for a P _H2O of 2 kb. The original material has a low dislocation density (<10⁶ cm^?2), and its lattice diffusion coefficient (D ₁), given below, agrees well with previous determinations. A sample was deformed at high temperature and pressure to produce a uniform dislocation density of 5 × 10⁹ cm^?2. The diffusion coefficient (D _a) for this deformed material, given below, is about 0.5 and 0.7 orders of magnitude larger than D ₁ at 700° and 450° C, respectively. This enhancement is believed due to faster diffusion along the cores of dislocations. Assuming a dislocation core radius of 4 Å, the calculated pipe diffusion coefficient (D _p), given below, is about 5 orders of magnitude larger than D ₁. These results suggest that volume diffusion at metamorphic conditions may be only slightly enhanced by the presence of dislocations. $$\begin{gathered} D_1 = 9.8 \pm 6.9 \times 10^{ - 6} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 33.4 \pm 0.6(kcal/mole)/RT] \hfill \\ \end{gathered} $$ $$\begin{gathered} D_a = 7.6 \pm 4.0 \times 10^{ - 6} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 30.9 \pm 1.1(kcal/mole)/RT] \hfill \\ \end{gathered} $$ $$\begin{gathered} D_p \approx 1.2 \times 10^{ - 1} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 29.8(kcal/mole)/RT]. \hfill \\ \end{gathered} $$      

Chemical and physical studies of type 3 chondrites—IV: Annealing studies of a type 3.4 ordinary chondrite and the metamorphic history of meteorites

Samples of a type 3.4 chondrite have been annealed at 400–1000°C for 1–200 hours, their thermoluminescence properties determined and analyzed for K, Na, Mn, Sc and Ca by instrumental neutron activation analysis. After annealing at ?900°C, the samples showed a 50% decrease in TL sensitivity, while after annealing at 1000°C it fell to 0.1-0.01 times its unannealed value and loss of Na and K occurred. The TL and compositional changes resemble those observed for the equilibrated Kernouve chondrite after similar annealing treatments, except that the sharp TL decrease, and element loss, occurred at ~ 1100°C; this difference is presumably due to petrographic differences in the feldspar of the two meteorites. The temperature and the width of the TL peak showed a discontinuous increase after annealing at 800°C; peak temperature jumped from 130 to 200°C and peak width increased from 90 to 150°C. The activation energies for these TL changes are 7–10 kcal/mole. Similar increases in the TL peak temperature have been reported in TL studies of Amelia, VA, albite, where they were associated with the low to high-temperature transformation. However, the activation energy for the transformation is ~80 kcal/mole. These changes in TL emission characteristics resemble trends observed in type 3 ordinary chondrites and it is suggested that type 3.3–3.5 chondrites have a low-feldspar as TL phosphor and > 3.5 have high-feldspar as the phosphor. Thermoluminescence therefore provides a means of palaeothermometry for type 3 ordinary chondrites.     

Potassium and sodium self-diffusion in alkali feldspar

The rate of potassium self-diffusion in pure microcline was measured between 600° and 800° C using K⁴⁰ as a tracer. Transport of K⁴⁰ by processes other than volume diffusion was insignificant or minimal. Isotropic diffusion coefficients were calculated assuming spherical grains. The data are well fit by the Arrhenius relation and yield a pre-exponential factor (D₀) of 133.8 cm²/sec and an activation energy (Q) of 70 kcal/mole. Similar experiments on the self-diffusion of Na²² in a pure low-albite (exchanged microcline) yield D₀ of 2.31×10^–6 cm²/sec and Q of approximately 19 kcal/mole for the temperature interval from 200° to 600° C. The large difference in these activation energies suggests that the atomic mechanisms for sodium and potassium diffusion are different.     

Chemical diffusion of fluorine in jadeite melt at high pressure

The chemical diffusion of fluorine in jadeite melt has been investigated from 10 to 15 kbars and 1200 to 1400°C using diffusion couples of Jadeite melt and fluorine-bearing jadeite melt (6.3 wt.% F). The diffusion profile data indicate that the diffusion process is concentration-independent, binary, F-O interdiffusion. The F-O interdiffusion coefficient ranges from 1.3 × 10^?7 to 7.1 × 10^?7 cm²/sec and is much larger than those obtained by Kushiro (1983) for Si-Ge and Al-Ga interdimision in jadeitic melts. The Arrhenius activation energy of diffusion is in the range of 36 to 39 kcal/mole as compared with 19 kcal/mole for fluorine tracer diffusion in a lime-aluminosilicate melt. The diffusivity and activation energy of F-O interdiffusion vary slightly with pressure, but the pressure dependence of F-O, Al-Ga and Si-Ge interdiffusion may be related to the relative volumes of the interdiffusing species for each pair. The magnitude of chemical diffusivity of fluorine is comparable to that of the chemical diffusivity of water in obsidian melts. The diffusivities of various cations are significantly increased by the addition of fluorine or water to a silicate melt. This fact, combined with the high diffusivity of fluorine, suggests that the F^? ion is the principal diffusing species in dry aluminosilicate melts and that dissolved fluorine will accelerate chemical equilibration in dry igneous melts.     

Thermal metamorphism of primitive meteorites—I. Variation of six trace elements in Allende carbonaceous chondrite heated at 400–1000°C

Determination by neutron activation of 6 trace elements retained in Allende (C3) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~10^?5 atm H₂) atmosphere reveals loss of small proportions of Ga and Se and large proportions of Bi, In and Tl-Co being unaffected. The retentivity patterns for the 5 volatile elements differ and in no way duplicate a step-function. In contrast to these trace elements, sulfur is initially present in discrete mineral(s) and visually it appears to be released over a narrow temperature range. Elements are lost more easily from powder than from chips but the difference is ≤35 per cent. Above 600°C, the process of loss appears due to process(es) with apparent activation energies of 2 kcal/mole (Bi, Tl), 4 kcal/mole (Se) and 22 kcal/mole (In). Loss of Bi, Se and Tl below 600°C involves higher apparent activation energies. Two-element correlation diagrams involving Bi, In and Tl are consistent with the idea that trends among highly-volatile elements in enstatite chondrites arise from metamorphism.     

Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants,effective surface area,and the hydrolysis of feldspar

Analysis of experimental data reported by Lagache (1965, 1976), Evans (1965), Busenberg (1975), Busenberg and Clemency (1976), Holdren and Berner (1979), Siegel and Pfannkuch (1984), and Chou and Wollast (1984) with the aid of irreversible thermodynamics and transition state theory (Aagaard and Helgeson, 1977, 1982) suggests that at temperatures at least up to 650°C, the rate of both congruent and incongruent feldspar hydrolysis in aqueous solutions far from equilibrium at pH ? 10.6 ? (2300/T), where T stands for temperature in kelvins, is a function solely of effective surface area and pH at constant pressure and temperature. At higher pH, the rate is apparently pH-independent up to ~pH 8 at 25°C, where it again becomes pH-dependent at higher pH. Observations of scanning electron micrographs indicate that the cross-sectional area of etch pits on hydrolyzed feldspar grains is of the order of 10^?9 to 10^?8 cm² and that the ratio of the effective to total surface area (which may or may not change with reaction progress) ranges from <0.01 to 1, depending on the grain size, dislocation density, and the extent of comminution damage on the surfaces of the grains. Apparent rate constants retrieved from experimental data reported in the literature for feldspar hydrolysis in the lower pH-dependent range extend from ~10^?13 to ~10^?7 moles cm^?2 sec^?1 at temperatures from 25° to 200°C, which is consistent with activation enthalpies for albite and adularia of the order of 20 kcal mole^?1. In contrast, the apparent rate constants for the pH-independent rate law range from ~10^?16 to ~10^?11 moles cm^?2 sec^?1 at temperatures from 25° to 650°C, which requires an activation enthalpy for adularia of ~ 9 kcal mole^?1. These observations are consistent with surface control of reaction rates among minerals and aqueous solutions. The rate-limiting step in the pH-dependent case apparently corresponds at the lower end of the pH scale to breakdown of a protonated configuration of atoms on the surface of the reactant feldspar, but at higher pH the rate is limited by decomposition of an activated surface complex corresponding in stoichiometry to hydrous feldspar. In highly alkaline solutions, an activated complex containing hydroxyl ions apparently controls the rate of feldspar hydrolysis. Nevertheless, near equilibrium, regardless of pH the rate is proportional to the chemical affinity of the overall hydrolysis reaction.     

Concentrations and behavior of oxygen and oxide ion in melts of composition CaO · MgO · xSiO2

The concentrations and behavior of oxygen and oxide ion were studied in silicate melts of composition CaO · MgO · xSiO₂ (1.25 ≤ x ≤ 3) in the temperature range 1425 to 1575°C by cyclic voltammetry and chronopotentiometry. Electroreduction of oxygen is a reversible, 2 electron process involving dissociated oxygen atoms. The Henry's Law constant for O₂ in molten diopside (CaO · MgO · 2SiO₂) is 0.023 ± 0.004 mole/l atm at 1450°C. The diffusion coefficient for molecular oxygen in diopside melt is 4.5 ± .5 × 10^?6 cm²/sec at 1450°C and the activation energy of diffusion is 80 ± 2 kcal/mole. Oxide ions produced by electroreduction of oxygen, rapidly dissociate silicate polymers, causing the concentration of free oxide ions in diopside melt to be buffered at a low level (4.7 ± .8 × 10^?5 mole/l). The concentration of free oxide ion increases at higher proportions of metal oxides but remains at this value in more silicic melts. The rate of formation of oxide ions by polymerization in diopside melt is 0.021 ± .007 mole/l sec. Thermodynamic parameters (the standard free energy, enthalpy and entropy) for the oxidation of Ni, Co, and Zn in diopside melt in equilibrium with gaseous oxygen agree with those for solid oxide systems. The platinum reference electrode in molten diopside is a reversible, oxygen electrode.     

A mechanism of shape-transformation of quartz inclusions in albite of regional metamorphic rocks

The process of shape-transformation of quartz inclusions from polyhedral to spherical grains in albite single crystals during metamorphism is mainly controlled by the grain boundary diffusion of oxygen along the quartz/albite interface to reduce the interfacial free energy. The rate of the process, which is represented by the growth rate of the curvature of the edge surface of the grain, depends significantly on temperature and on the grain size of the quartz inclusion. The relations between temperature, T, the time, t_r, and the critical radius, R_c, which is equal to the radius of maximum spherical grains, are given by log R_c = −0.11E_b/RT + 0.25log t_r + C, in which E_b is the activation energy of the grain boundary diffusion of oxygen along the quartz/albite interface and C is a material constant.

The mean critical radius of spherical quartz inclusions in albite is 5 μm for the upper chlorite zone and garnet zone, 10 μm for the lower biotite zone, and 20 μm for the upper biotite zone in the Sambagawa metamorphic terrain. The mean values of the critical radii of spherical quartz inclusions in oligoclase of the Ryoke metamorphic rocks is about 5 μm for the chlorite zone and about 10–20 μm for the sillimanite zone.

Assuming temperatures of about 350°C for the upper chlorite and garnet zones, 400°C for the lower biotite zone, 550°C for the upper biotite zone, and 700°C for the sillimanite zone, the activation energy for the grain boundary diffusion of oxygen along the quartz/plagioclase interfase is estimated to be about 30 kcal/mol.     

Thermochemistry of the high structural state plagioclases

The enthalpies of solution of a suite of 19 high-structural state synthetic plagioclases were measured in a Pb₂B₂O₅ melt at 970 K. The samples were crystallized from analyzed glasses at 1200°C and 20 kbar pressure in a piston-cylinder apparatus. A number of runs were also made on Amelia albite and Amelia albite synthetically disordered at 1050–1080°C and one bar for one month and at 1200°C and 20 kbar for 10 hr. The component oxides of anorthite, CaO, Al₂O₃ and SiO₂, were remeasured.The ΔH of disorder of albite inferred in the present study from albite crystallized from glass is 3.23 kcal, which agrees with the 3.4 found by Holm and Kleppa (1968). It is not certain whether this value includes the ΔH of a reversible displacive transition to monoclinic symmetry, as suggested by Helgesonet al. (1978) for the Holm-Kleppa results. The enthalpy of solution value for albite accepted for the solid solution series is based on the heat-treated Amelia albite and is 2.86 kcal less than for untreated Amelia albite.The enthalpy of formation from the oxides at 970 K of synthetic anorthite is ?24.06 ± 0.31 kcal, significantly higher than the ?23.16 kcal found by Charluet al. (1978), and in good agreement with the value of ?23.89 ± 0.82 given by Robieet al. (1979), based on acid calorimetry.The excess enthalpy of mixing in high plagioclase can be represented by the expression, valid at 970 K: ΔH^ex(±0.16 kcal) = 6.7461 X_abX²_An + 2.0247 X_AnX²_Ab where X_Ab and X_An are, respectively, the mole fractions of NaAlSi₃O₈ and CaAl₂Si₂O₈. This ΔH^ex, together with the mixing entropy of Kerrick and Darken's (1975) Al-avoidance model, reproduces almost perfectly the free energy of mixing found by Orville (1972) in aqueous cation-exchange experiments at 700°C. It is likely that Al-avoidance is the significant stabilizing factor in the high plagioclase series, at least for X_An≥ 0.3. At high temperatures the plagioclases have nearly the free energies of ideal one-site solid solutions. The Al-avoidance model leads to the following Gibbs energy of mixing for the high plagioclase series: $\text{ΔG}{}^{\mathrm{mix}}\text{= ΔH}{}^{\mathrm{ex}}\text{+ RT X}{}_{\mathrm{Ab}}\text{ln[X}{}^2{}_{\mathrm{Ab}}\text{(2 ? X}{}_{\mathrm{Ab}}\text{)]+ X}{}_{\mathrm{An}}\text{ln}\text{[X}{}_{\mathrm{An}}\text{(1+X}{}_{\mathrm{An}}\text{)}{}^2\text{]}\text{4}$. The entropy and enthalpy of mixing should be very nearly independent of temperature because of the unlikelihood of excess heat capacity in the albite-anorthite join.     

Diffusion of Na,K, Rb and Cs in glasses of albite and orthoclase composition

The measurement of diffusion coefficients for Na, K, Rb and Cs has been realized by the technique of active salt deposits on glasses of albite and orthoclase composition, at normal pressure and in the temperature range 300–1000°C. The values of D are between 10^?6 and 10^?12 cm² s^?1 and, for every type of run, they vary with temperature according to Arrhenius laws, with activation energies ranging from 13 to 68 kcal mole^?1. These important variations are related to the size of the diffusing element (at 700°C in albite glass D_Na/D_K/D_RbD_Cs ～- 10⁷/10⁵/10³/1) and to the size of the major alkali element (for rubidium at 800°C D_or·gl/D_ab·gl ～- 20). By comparison with available data on diffusion in feldspars, we emphasize the influence of the defect density on the diffusion process.     

Characteristics of Rare Earth Elements,Zr, and Hf in Ore‐Bearing Porphyries from the Western Awulale Metallogenic Belt,Northwestern China and their Application in Determining Metal Fertility of Granitic Magma

The western Awulale metallogenic belt in northwestern China hosts a number of small‐ to medium‐sized porphyry Cu deposits that are associated with albite porphyry. The common presence of plagioclase (albite) as phenocrysts and the absence of hydrous minerals (amphibole and biotite) indicate that the water content of albite porphyry is low. Trace‐element compositions of whole rocks and zircon grains from these ore‐bearing porphyries were measured. Zircon grains from albite porphyries exhibit Ce⁴⁺/Ce³⁺ ratios ranging from 7.75 to 95.1, which indicate that these porphyries have a low oxygen fugacity. Trace element compositions of ore‐bearing porphyries exhibit (La/Yb)_N ratios ranging from 1.09 to 11.1 and Eu/Eu^* ratios ranging from 0.10 to 0.66. These ore‐bearing porphyries have Zr values ranging from 171 to 707 ppm and Hf values ranging from 8.30 to 18.9 ppm. Combining these porphyries with other ore‐bearing porphyries that formed in the Central Asian Orogenic Belt (CAOB) and the Pacific Rim metallogenic belt, we found that the (La/Yb)_N and Eu/Eu^* ratios of ore‐bearing porphyries in western Awulale are low, while the Zr and Hf values are high. Specifically, REEs can be used to evaluate the degree of differentiation and degree of partial melting, and Zr and Hf can be used to evaluate the redox conditions and water content of magmatic rocks. Our findings indicate that ore‐bearing porphyries in western Awulale have a lower oxygen fugacity, degree of differentiation, and water content than do others in the CAOB and the Pacific Rim metallogenic belt. Compared to those of ore‐bearing porphyries with lower zircon Ce⁴⁺/Ce³⁺ ratios, the (La/Yb)_N ratios of ore‐bearing porphyries in our study are low, and the Zr and Hf values are high. This finding indicates that, under reducing conditions, the degree of evolution and water content may have an important influence on the metal abundance in magmas. There is also a clear relationship between (La/Yb)_N, Eu/Eu^*, Zr, Hf, and the size of the deposits. Large‐ (>4 Mt Cu) and intermediate (1.5–4 Mt Cu)‐sized porphyry Cu deposits are associated with granitic intrusions that have higher (La/Yb)_N and Eu/Eu^* ratios and lower Zr and Hf values. This finding indicates that, in addition to oxygen fugacity, the degree of evolution and water content are controlling parameters for metal abundance in magmas, especially in low oxygen fugacity porphyry Cu deposits. Such a conclusion may be useful in the exploration for other concealed porphyry Cu deposits.     

Silica Springs,Tongariro National Park,New Zealand—analyses of the spring water and characterisation of the alumino-silicate deposit

The creamy-white deposit in the stream bed below Silica Springs outlet on Mount Ruapehu, Tongariro National Park, New Zealand, has been identified as a hydrous, X-ray-amorphous, aluminosilicate (allophane). The SiO₂/Al₂O₃ mole ratio varies from close to one, to close to two. The elements K, Ca, Mn and Fe are present in low concentrations relative to those in allophanic soil clays, and tend to increase in concentration downstream from where the deposit first occurs. The concentration of S decreases downstream from 0.5% to 0.1%. Surface areas of samples, measured by the ethylene glycol desorption method, are about 200–300 m²/g. The outlet water at Silica Springs contains low dissolved solids and is undersaturated with respect to amorphous silica, but is supersaturated with respect to several alumino-silicate minerals (of which allophane may be considered the precursor) and with respect to CO₂. Gas bubbling at the outlet contains about 10% CO₂ which has a δC¹³_PDB value of ?7.5%..Silica Springs water is derived from the addition of geothermal CO₂ (and possibly H₂S) to near-surface meteoric water from the lava flow above the outlet, and the chemical attack of this water on the andesitic rocks and soil through which it passes. The pH of water at Silica Springs increases from 5.45 at the outlet, to 5.90 where deposition first occurs, to 6.80 below the region of maximum deposition. This rise in pH correlates with loss of excess CO₂ in turbulent regions of the stream, and, through surface charge effects, is probably an important influence on the site of deposition, which begins approx. 100m downstream from the outlet.