Low Albite Research Articles

40Ar/39Ar Data on Primary and Secondary Minerals of a Volcanic Series Affected by Burial Metamorphism: Example of a Cretaceous Andean Extensional Basin

Precise time constraints on both the formation of magmatic rocks and subsequent alteration or very low-grade metamorphism are difficult to obtain on the same rocks, mainly because of the difficulty to separate uncontaminated mineralogical components relative to these two events. We initiated a 4~ study on a thick Lower Cretaceous basaltic sequence in a regional subsiding basin affected by a very low to low-grade metamorphism due to burial, with the aim to obtain reliable ages on primary minerals (volcanism) and secondary phases (metamorphism) permitting to define the time interval between volcanism and burial metamorphism. The 9 km thick series, located at the latitude of Santiago, is part of a.c. 1000 km long volcanic belt of Early Cretaceous age exposed along the Coastal Range of central Chile (see more details in Aguirre et al., 1989). Most of the basic lavas belong to the highK calc-alkaline and shoshonite series. The rocks have been affected by very lowto low-grade, non deformational metamorphism, which preserved the primary structures and textures. Its grade increases with stratigraphic depth ranging from zeolite facies at the top to lower greenschist facies at the very bottom of the pile. Characteristic metamorphic assemblages in zeolite and prehnite-pumpellyite facies are: (a) laumontite + chlorite + pumpellyite + prehnite + epidote + K-feldspar and (b) pumpellyite + epidote + chlorite + prehnite + K-feldspar. The various characteristics of the series and of its geologic framework have been interpreted by different authors as representing an extensional ensialic setting either of a marginal basin or of an island arc type. Transparent plagioclases in bulk samples and single grains of phenocrysts, and the freshest whole rocks were carefully selected to provide the best estimate of the age of the volcanism. Adularia single grains, filling amygdales (together with pumpellyite, chlorite and low albite), and cloudy single grains of plagioclases (contaminated by secondary sericite, Kfeldspar and albite) were analysed to obtain the age of the metamorphism. Previous dating by K/Ar, 4~ and Rb/Sr displayed ages mainly ranging from 100 to 134 Ma (Aberg et al., 1984; Munizaga et al., 1988; Boric and Munizaga, 1994). The plagioclase bulk sample displayed a disturbed age spectrum because we failed to obtain uncontaminated phases on several hundreds of grains. Concordant plateau age (119.4 41.2 Ma) and high temperature apparent ages were obtained on a restricted population (29 and 37 grains, 96BU6 on Fig. 1) and single phenocrysts of transparent plagioclases. This represents the best estimate of the age of the volcanism. Adularia single grains gave a well defined plateau age at 93.1 _+ 0.3 Ma on one grain (96BU10 in Fig. 1) and slightly increasing apparent ages (probably due to a slight alteration) converging towards a value of 94.2 _+ 0.6 Ma on another grain. This corresponds to the age of the very lowgrade metamorphism. Between these extreme values characterizing the two events, intermediate apparent ages were obtained

Reinvestgation of Tweed Microstructures in Annealed Albite

Abstract Previous TEM study of natural Amelia albite (Ab99.3An0.1Or0.6) isothermally annealed at 1073°C for various periods revealed distinct tweed microstructures from the intermediately disordered albite samples (intermediate albite), but not from both the natural sample (low albite) and the highly disordered samples (high albite).1 These tweed microstructures seemed to correlate well with the degree of peak broadening and distortion in corresponding conventional powder X-ray diffractograms. Previous XRD, synchrotron radiation and TEM studies on the annealed albite samples at 1080°C for various periods gave rise to similar results. Reinvestigation of the annealed albite samples with distinct tweed microstructures by TEM confirmed their existence in wide areas, giving two-directional streaking in the corresponding SADP’s (Fig. 1 and 2). Tweed directions are roughly perpendicular to and parallel to the trace of albite twin plane, {010}, as shown in Fig. 3. This tweed microstructure is apparently unaffected by the twinning and probably formed after it.

Feldspar-fluid interactions in braid microperthites: pleated rims and vein microperthites

Braid microperthitic alkali feldspars in the Klokken, South Greenland and Coldwell, Ontario syenite intrusions have bulk-compositional variations along grain boundaries called pleated rims. These, together with vein microperthites in aplites which cross-cut the syenites, have been investigated by SEM and TEM. We distinguish two main types of pleated rims, “arched ” and “parallel-sided ”, consisting of alternating Ab- and Or-rich areas on (001), which are 0.5–300 μm in length normal to (010) and 0.2–20 μm in width along (010). The smallest pleats, which occur on intracrystalline boundaries in Klokken feldspars, are fully coherent and composed of low albite and low microcline. Above the heads of some of the coarser pleats, braid microperthite grades into a film crypto- and micro-perthite and antiperthite microtexture called a “transitional zone” containing roughly planar lamellae of low albite and tweed orthoclase. During pleat development, local alternating volumes form in which the proportions of the phases differ ( phase separation) and the morphology of the intergrowths changes from braided to straight in response to this change in local bulk composition. Straightening is also accompanied by transformation of low microcline to tweed orthoclase. The coarsest pleats, which occur along grain boundaries in feldspars from the Coldwell syenite, are semi- or in-coherent and have a thick coherent and semicoherent transitional zone. Coarsening of pleats and development of the transitional zone has been facilitated by diffusion of “water” into grain interiors. In many cases, pleated rims have suffered deuteric alteration, by dissolution–reprecipitation processes, through the action of a water-rich fluid from the grain boundary, in which tweed orthoclase was transformed into irregular microcline and micropores developed. Vein microperthites in aplites from Klokken, and by extension the vein microperthites almost universal in most alkali granites, are interpreted to have formed by propagation of pleat heads across entire crystals during pervasive interaction with water.

Solid-state nuclear magnetic resonance and infrared spectroscopy of alkali feldspars

29Si,27Al MAS NMR and IR spectra of rnonophase K-feldspars (sanidine, orthoclase and microcline) and Na-feldspars (monalbite, anorthoclase and low albite) in different structural states have been studied. The NMR and IR spectra of K-feldspars and Na-feldspars vary regularly along with their degrees of Si/Al ordering evolution. Si in orthoclase occupiesT 2m,T2o andT1m, and the high-temperature Na-feldspar (monalbite and anorthoclase) coincides in29Si,27Al NMR and IR spectra. Moreover, all the high-temperature Na-feldspars and sanidine have the same27Al NMR and IR spectra.

Cross-Polarization from Quadrupolar Nuclei to Silicon Using Low-Radio-Frequency Amplitudes during Magic-Angle Spinning

The dynamics of cross-polarization from the central transition of a quadrupolar nucleus (27Al or 23Na) to a spin-1/2 nucleus (29Si) during magic-angle spinning and using low-radio-frequency field strengths are analyzed for the mineral low albite. Under these conditions additional complications in the spin-lock behavior of the quadrupolar nucleus and in the cross-polarization process were found experimentally and are examined in detail. A step-by-step procedure for optimizing cross-polarization from the central transition of a quadrupolar nucleus to a spin-1/2 nucleus is described. Significant enhancement of 29Si NMR sensitivity and several applications are demonstrated.

Stability of the IRSL Spectra of Alkali Feldspars

Measurements of infrared stimulated luminescence (IRSL) emission spectra are presented for detrital potassium-rich feldspars separated from sands and for museum specimens of low albite and intermediate microcline. The samples show common emission bands at 335, 400 and 550 nm and a thermally unstable emission band at 290 nm which is associated with sodic phases in the crystal structure. Rapid preheating and room temperature storage causes a decrease in emission from the 290 nm centre with a corresponding increase in the other emission centres, with potential problems for pulse annealing routines. This may be due to charge transfer or structural transformations in the defect. A sustained preheat regime is essential if detrital potassium-rich feldspars are to be used for IRSL dating.

The coordination numbers of Na and K atoms in low albite and microcline as determined from a procrystal electron-density distribution

Procrystal models for the electron-density distributions of low albite and microcline were constructed by placing spherically averaged Clementi-Roetti atomic wave functions at experimentally observed atomic positions. The topography of the model electron-density distribution was analyzed to locate (3, - 1) critical points between the Na or K positions and the positions of the surrounding framework 0 atoms. Because the number of (3, -1) critical points associated with a given atom corresponds with its coordination number, the analysis indicated that Na is fivefold and K is sevenfold coordinated. In particular, the results indicate that the Oco atom in low albite is not coordinated with Na, whereas in microcline it is coordinated with K. This suggests that in low albite the Oco atom is underbonded and thus a possible hydrophilic site, whereas in microcline the bonding requirements of Oco are satisfied. Details of the local bonding explain the effects of pressure, H, and H20 on ordering of Al and Si, as well as compressibility systematics.

Crystal structure of KBSi3O8 isostructural with danburite

AbstractThe crystal structure of KBSi3O8 (orthorhombic, Pnam, with a = 8.683(1), b = 9.253(1), c = 8.272(1) Å,, V = 664.4(1) Å3, Z = 4) has been determined by the direct method applied to 3- dimensional rcflection data. The structure of a microcrystal with the dimensions 20 × 29 × 37 μm was refined to an unweightcd residual of R = 0.031 using 386 non-zero structure amplitudes. KBSi3O8 adopts a structure essentially different from recdmergneritc NaBSi3O8, with the low albite (NaAlSi3O8) structure, and isotypic with danburite CaB2Si2Os which has the same topology as paracelsian BaAl2Si2O8. The chenfical relationship between this sample and danburitc gives insight into a new coupled substitution; K+ + Si4+ = Ca2+ + B3+ in the extraframework and tetrahedral sites. The present occupancy refinement revealed partial disordering of B and Si atoms which jointly reside in two kinds of general equivalent points, T(1) and T(2) sites. Thus the expanded crystal-chemical formula can be written in the form K(B0.44Si0.56)2(B0.06Si0.94)2O8The systematic trend among crystalline compounds with the M+T3+T4+3O8 formula suggests that they exist in one of four structural types; the feldspar structures with T3+/T4+ ordered and/or disordered forms, and the paracelsian and the hollandite structures.

Experimental mineral - fluid interaction in the Na-Ca-(Sr)-Al-Si system

To investigate the factors controlling the composition of geothermal fluids, we have studied the hydrothermal transformation of plagioclase → neogenic minerals at 180°C. At equilibrium this transformation is predicted thermodynamically to produce the assemblage quartz, kaolinite, laumontite, low temperature albite. Synthetic anorthite (An100) and labradorite (An50) were spiked with strontium and reacted with two different aqueous solutions: one near-equilibrium with the secondary minerals, the other far-from-equilibrium. The following results were obtained: 1) At initial condition near-equilibrium, the first neogenic minerals formed during labradorite dissolution were quartz - prehnite - kaolinite - low temperature albite and, during anorthite dissolution, quartz - prehnite - kaolinite. A later transformation of prehnite → smectite occurred after 6 months of reaction (for both An50 and An100). 2) At initial conditions far-from-equilibrium, smectite formed after 1 day of reaction. Strontium and calcium are released stoichiometrically during plagioclase dissolution, and the Sr/Ca partition coefficient between the solution and the combined neogenic phases is 1 at 180°C

Petrographic and geochemical evolution of pervasively altered Bushveld granites at the Zaaiplaats tin mine

Tin with associated tungsten and rare earth element mineralization at the Zaaiplaats mine is hosted by alkali feldspar granites of the Lebowa Granite Suite which intrude granophyric granites of the Rashoop Granophyre Suite. From the upper contact downward the alkali feldspar granites include a marginal pegmatite (stockscheider), the fine-grained Lease Granite, and the coarse-grained Bobbejaankop Granite. The major mineralization styles consist of subhorizontal, tabular zones (low-grade orebodies) containing disseminated cassiterite in the Lease Granite, a zone of disseminated cassiterite mineralization (ZDM) in the Bobbejaankop Granite approximately 50 m below the Lease Granite, and a series of shallowly plunging and branching pipe orebodies which occur in both granite types and which show a close spatial relationship to zones of disseminated mineralization.The granites contain miarolitic cavities generally less than 5 mm in diameter which range from more than 10 vol percent in the upper facies of the Lease Granite to less than 1 vol percent in the deeper part of the Bobbejaankop Granite. Miarolitic cavities are filled or partially filled by hydrothermal minerals including quartz, albite, chlorite, sericite, fluorite, hematite, and calcite. The zones of disseminated mineralization contain high abundances of miarolitic cavities and appear to reflect zones of hydrothermal fluid accumulation during crystallization. In addition to the small cavities within the disseminated cassiterite mineralization zone, the Bobbejaankop Granite contains a series of large cavities (to approx 1 m) and patches of strongly miarolitic granite. Additional infill phases in mineralized zones include cassiterite, scheelite, synchisite, arsenopyrite, chalcopyrite, galena, and other sulfide phases. Pervasive alteration of the granites is expressed mainly by the replacement of primary biotite by chlorite, sericite, and hematite, by ordering of alkali feldspar (maximum microcline, low albite), and by substantial replacement of alkali feldspar by chlorite, sericite, hematite, and other hydrothermal phases.In the Bobbejaankop Granite below the disseminated cassiterite mineralization zone amounts of Al 2 O 3 , K 2 O, Na 2 O, Ba, Rb, Nb, Ta, and U increase with increasing elevation whereas SiO 2 and Ti decrease. This is compatible with upward crystallization in an evolving F-rich magma, but it may partly reflect upward transport of alkalies and incompatible elements via a fluid phase. The ZDM is a zone of enrichment in SiO 2 , Ba, Cu, Pb, Zn, and Sn, whereas it is depleted in Al 2 O 3 , K 2 O, Na 2 O, Rb, Ta, and U compared to the underlying granite.The increasing importance of sericite as an infilling and replacement mineral as the Alpha pipe is approached suggests that part of the alteration and cavity filling in the Lease Granite occurred at the same time as alteration and mineral precipitation in the Alpha pipe. This increasing importance of sericitic alteration is reflected in the geochemical data by increasing CO 2 , Rb, and Cs contents and decreasing Na 2 O, F, and Ba contents with proximity to the pipe.The distribution of disseminated and pipe-style mineralization at Zaaiplaats reflects a combination of processes including (1) early separation of an H 2 O-NaCl-CO 2 fluid enriched in boron to produce pipelike conduits by collection and upward flow beneath crystallization fronts, supplemented by later dissolution of granite, (2) accumulation of fluorine-rich fluids evolved at a more advanced stage of crystallization beneath downward-advancing crystallization fronts to produce strongly miarolitic zones of the Lease Granite containing disseminated cassiterite (low-grade orebodies), (3) accumulation of fluorine-rich hydrothermal fluids, possibly between upward- and downward-advancing crystallization fronts to produce the disseminated cassiterite mineralization zone, and (4) pervasive alteration of the granites accompanied by filling of miarolitic cavities and pipes. The lack of significant hydraulic or tectonic fracturing during crystallization of the granites at Zaaiplaats was probably the most crucial factor in the development of these unusual styles of mineralization.

Development of microporosity, diffusion channels and deuteric coarsening in perthitic alkali feldspars

Turbidity is an almost universal feature of alkali feldspars in plutonic rocks and has been investigated by us in alkali feldspars from the Klokken syenite using SEM and TEM. It is caused by the presence of myriads of tubular micro-inclusions, either fluid-filled micropores or sites of previous fluid inclusions, and is associated with coarsening of microperthite and development of sub-grains. Micropores are abundant in coarsened areas, in which porosities may reach 4.5%, but are almost absent from uncoarsened, pristine braind-microperthite areas. The coarsening is patchy, and involves a scale increase of up to 103 without change in the composition of the phases, low albite and low microcline, or in the bulk composition of the crystal. It occurs abruptly along an irregular front within individual crystals, which retain their original shapes. The coherent braid microperthite gives way across the front to an irregular semi-coherent film perthite over a few μm and then to a highly coarsened irregular patch perthite containing numerous small sub-grains on scales of a few hundred nm, in both phases. The coarsening and micropore formation occured at a T≤400°–450° C and it is inferred to have been driven by the release of coherent strain energy, low-angle grain-boundary migration being favoured by a fluid. The patchy nature of the coarsening and the absence of a relationship with initial grain boundaries suggest that the fluid was of local origin, possibly arising in part through exsolution of water from the feldspar. The sub-grain texture and microporosity modify profoundly the permeability of the rock, and greatly enhance the subsequent reactivity of the feldspars.

Glide twinning and pseudotwinning in peristerite: twin morphology and propagation

Optically visible Albite glide “twins” in a peristerite (∼An9Or1.6), identified from their tapering shape and relationship to grain boundaries, were studied by transmission electron microscopy. Near the tips in sections ⊥a, the microstructure consists of small (∼400 nm long) lensshaped Albite twins centred exclusively on the oligoclase lamellae. The lenses extend partly outwards into the two adjacent low albite lamellae and induce strong inhomogeneous strain. Where the lenses are closer together, they form, depending on the sense of shear, nearly linear left or right-stepping en echelon arrays, with overlap of the strain fields. Slightly farther in from the tip, the twin domains coalesce to form continuous pinch-and-swell lamellae, being always thicker in the oligoclase. Because of Si,Al order, only elastic glide pseudotwins are possible in low albite. In oligoclase glide pseudotwins may be mechanically stable (metastable relative to Si,Al order) and may deviate only slightly from true twins. Pseudotwins develop first in the oligoclase, propagate dynamically by jumping across the intervening albite lamellae, extend lengthways and thicken sideways and finally coalesce. They are stabilized by diffusion-controlled inversion of Si,Al order giving rise to true twins described in a companion paper.

Raman microprobe study of synthetic diaplectic plagioclase feldspars

Raman microprobe spectra were made on three post shock, diaplectic plagioclase feldspars. Optical and X-ray diffraction studies indicated that feldspars maintained a partially or totally crystalline state after having passed through the mixed phase zone of Hugoniot response to shock waves (15–38 GPa). The appearance of uniquely glass-type spectra occurs at different shock pressures for each specimen according to its atomic structural arrangement, below 38 GPa for mosaic structured labradorite, near 40 GPa for anorthite and above 50 GPa for the highly ordered low albite. The diaplectic anorthite and labradorite glasses give spectra which indicate the presence of two glass types. Shifts in the band envelope frequencies compared to spectra of fused glass and statically pressure densified glass suggest that these glasses have specific structural arrangements. These differences suggest that the shock and fusion glass-forming processes are not exactly identical.

Hydrothermal reaction of albite and a sodium aluminosilicate glass: A solid-state NMR study

We present here a solid-state NMR study of the structure and chemical composition of the products and mechanisms of the reaction of crystalline low albite and a glass of nearly albite composition with aqueous solutions of pH from 1 to 11 at 250°C. For the crystalline albite, there are no detectable bulk or surface structural changes due to aqueous attack, consistent with the idea that both cation exchange and disruption of the aluminosilicate framework occur only near the mineral/solution interface and that the hydrated surface layer, if it exists, is not more than about 30 Å thick. This reaction occurs by solution/reprecipitation, and its rate decreases with increasing solution pH, supporting the idea that the dissolution of feldspar is initiated by cation-exchange. For the glass, the reaction proceeds by cation exchange of protons for Na +, incorporation of molecular water into the bulk glass, and a small amount of depolymerization of the aluminosilicate framework in the interior of the glass. Cation exchange becomes less important with increasing solution pH. The incorporation of molecular water and cation-exchange cause structural changes in the glass via solidstate adjustment without dissolution/reprecipitation. The large cations in the hydrated glass (Na and K) probably have a shell of water molecules around them, with a maximum average coordination number of six. The secondary phases formed from both albite and the glass are often amorphous and can be well characterized by NMR. The compositional and structural variations of the amorphous phases are important factors in these reactions and cannot be ignored in theoretical models of aluminosilicate dissolution. As expected, the aluminum coordination in the secondary phases changes from six-fold to four-fold as the solution pH increases.

Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks

AbstractHomogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. Al,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500°C in albite and possibly to ∼ 200°C in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ∼ 75–150°C depending on pressure, low albite being stable up to about 620–650°C and high albite above about 725°C at low pressure. Symmetry is broken at ∼ 980°C mainly by a cooperative shearing of the whole framework and not by Al,Si ordering alone; there is a thermal crossover near 700°C shearing being dominant above (high albite) and ordering dominant below (intermediate albite).In potassium feldspar symmetry is broken by Al,Si ordering at a temperature of about 500°C The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ∼ 1075 to ∼ 550°C and to a lesser extent in microcline from 450 to 200°C. Ordering rates in sanidine down to 500°C and ordering rates in microcline between 450 and 200°C are almostas fast as in albite. Ordering in sanidine at 500°C and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of Al,Si diffusion, but because the totaldriving forceis very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite.The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Ab80-75at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition andTare given, as well as two extreme phase andbehaviourdiagrams for complete coherent and complete incoherent (strain-free) relationships. These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline.

27Al, 29Si, and 23Na MAS NMR study of an Al, Si ordered alkali feldspar solid solution series

Solid-state 27Al, 29Si and 23Na MAS NMR spectra have been obtained for an Al,Si ordered low albite to low microcline ion exchange series for which unit-cell parameters and 29Si NMR data have previously been reported. 27Al δi vary continuously with composition from 63.4 (±0.5) ppm for albite to 58.9 (±0.5) ppm for microcline, and parallel the 29Si chemical shifts assigned to the T2m-site. The 27Al and 29Si chemical shifts for this series correlate well with composition-dependent lattice parameters, most notably cell volume and the angle [201]1b. The linewidths of the 29Si and 27Al resonances indicate a significant amount of structural disorder in the intermediate compositions due to Na, K substitution. The 1 σ width of the distribution of average Si-O-T angles for each T-site is estimated to be about 1° for the Or33 sample. The average 23Na δi varies monotonically from -8.5 (±1) ppm for albite to -24.3 (±1)ppm for Or83. Similarly, the average 23Na nuclear quadrupole coupling constant decreases from 2.60 to 1.15 (±0.05) MHz and the asymmetry parameter of the electric field gradient increases from 0.25 to 0.6 with increasing K-content from albite to Or83. The observed variations in the quadrupole coupling parameters are consistent with simple electrostatic calculations. Higher resolution 23Na spectra of the intermediate compositions obtained at 11.7 T indicate the presence of an inhomogeneous linebroadening which is related to the distribution of Na-environments. A model based on a random distribution of local compositions does not simulate the spectra, suggesting that the distribution of Na is skewed toward Na-rich clusters. Observation of the 23Na NMR lineshape of Or49 after short periods of heat treatment indicate that 23Na NMR is very sensitive to the changes in the Na, K distribution accompanying the early stages of exsolution. Reversible changes occur after heating at 530° C for 3 h, whereas heating at 600° C produces no changes, possibly bracketing the position of the coherent spinodal for Al, Si ordered alkali feldspars at this composition.

Enthalpies and Volumes Related to K--Na Mixing and Al-Si Order/Disorder in Alkali Feldspars

In order to investigate the thermodynamic properties of alkali feldspars, three new feldspar ion-exchange series have been synthesized, two based on monoclinic parent materials having intermediate degrees of Al—Si order, the other on Amelia low albite. Acid solution calorimetric measurements have been carried out in 20·1% HF at 50°C under isoperibolic conditions on 30 members of the three series, and compared with revised values for a previously reported sanidine—analbite series. Molar volumes have been determined for all feldspars, and for an additional series based on Eifel sanidine. Enthalpies of K—Na mixing (Aex) calculated from the 50°C heats of solution are dependent on Al—Si distribution for both topochemically monoclinic and triclinic alkali feldspars, and in general can be expressed as where NOr and NAb are mole fractions of KAlSi3O8 and NaAlSi3O8, respectively, and Z is an ordering parameter defined as twice the difference in the mole fraction of Al in the T1 vs the T2 tetrahedral sites. Aex values for all but the most disordered series are maximized toward sodic compositions, and increase both in magnitude and asymmetry as ordering increases. For topochemically monoclinic alkali feldspar series, volumes of K—Na mixing(Vex) are asymmetric with NOr, but within the precision of present data do not depend on Al—Si distribution: Microcline-low albite feldspars appear to have volumes of mixing with the opposite asymmetry, but expressions of V¯ex for these differ somewhat among various investigators. Since no single thermodynamic mixing property is markedly asymmetric with respect to composition, the excess Gibbs energies implied from solvus data for alkali feldspars, and maximized at sodic compositions, are apparently the result of additive effects of subtle asymmetries in the volumes, enthalpies, and entropies of K—Na mixing in these minerals. The thermodynamic properties of an alkali feldspar at any composition are significantly affected by the distribution of Al and Si between T1 and T2 tetrahedral sites. The enthalpy of formation at 50°C of a monoclinic potassium feldspar with perfect order (Z=1) differs by 2·19 kcal/mol from one with a completely random Al—Si distribution (Z=0), while a value of 2·86 kcal/mol applies to analagous sodium end members. ConverselyY-ordering (between T1O andT1m sites) seems to have little or no effect on the enthalpy of formation of either end member, evidenced by the fact that most of the enthalpy differences for the low microcline to sanidine and corresponding low albite to analbite transitions (1·73 and 2·79 kcal/mol, respectively) can be attributed to Al—Si exchanges between T1 and T2 sites. Observed enthalpy differences in alkali feldspars are probably related to strain at domain boundaries, whether the domains are extremely small, or somewhat larger as in modulated structures. Neither Z-nor Y-ordering has a substantial effect on the molar volumes of alkali feldspars.

Comparative compressibility of end-member feldspars

The compressibilities of the three end-member feldspars have been determined between 1 bar and 50 kbar by single crystal X-ray diffraction techniques, using a Merrill-Bassett type diamond anvil cell with three crystals loaded simultaneously. Low albite (ordered aluminium-silicon distribution) and high sanidine (disordered Al-Si) show similar behaviour on compression, with bulk moduli (linear fit to volume-pressure data) of 0.70 and 0.67 Mbar respectively. The most compressible cell axis of all three feldspars studied is a, indicating that the major change in the feldspar framework with pressure is a shortening of the overall length of the “crankshaft chains” by reduction of T-O-T angles.

Geochemistry and hydrodynamics of deep-basin brines, Palo Duro Basin, Texas, U.S.A.

Permeable strata of the Deep-Basin Brine aquifer system underlie bedded Permian evaporites in the Palo Duro Basin, Texas Panhandle. Formation water samples collected from four U.S. Department f of Energy test wells and two independent oil and gas company wells in the basin were analyzed for chemical and isotopic compositions to characterize the geochemical environment and to determine the origin and compositional evolution of the water. Formation waters are Na Cl brines that contain 140 to 290 g/l total dissolved solids (TDS). Chemical and isotopic compositions suggest grouping the brine samples into one of two types. Samples from western Palo Duro Basin have high Cl:Br ratios, Sr isotopic ratios that are significantly more radiogenic than Permian seawater or precipitates from Permian seawater, and oxygen isotopic compositions that are depleted with respect to values predicted for equilibrium with calcite under in situ conditions. Samples from central and eastern Palo Duro Basin have low Cl:Br ratios, Sr isotopic ratios only slightly radiogenic relative to Permian marine Sr, and oxygen isotopic compositions that suggest equilibration with calcite. The concentrations of major and minor ions appear to be controlled by equilibrium with calcite, dolomite, anhydrite, celestite, low albite, microcline, and Na-smectite. High Na activities drive ion exchange reactions, which, coupled with maintenance of chemical equilibrium, elevate the concentrations of divalent cations and lower the concentrations of dissolved SO 4 and carbonate. The origin and evolution of deep-basin brines are interpreted from chemical and isotopic compositions integrated with results of previous geological and hydrogeological investigations. Brines from western Palo Duro Basin acquired salinity by dissolving halite along a lateral flow path from the recharge zone; radiogenic Sr was derived from detrital silicate minerals and oxygen isotopic equilibrium with aquifer minerals has not been attained. Brines from central and eastern Palo Duro Basin originated as either recharge that achieved chemical and isotopic equilibrium with the host rock during long residence times or as Permian seawater concentrated by evaporation. Regional chemical and isotopic variations augment the results of hydrogeological modeling: brines in the western part of the basin flow from the west and southwest where siliciclastic sedimentary rocks predominate; brines in the central and eastern parts of the basin flow from the south and southwest where carbonate strata predominate.

Al/Si interdiffusion in albite: effect of pressure and the role of hydrogen

The effect of pressure on the rate of Al/Si disorder in albite has been determined at temperatures from 800° C to 1050° C and at pressures up to 24 kbar, using dried samples in welded Pt containers, in piston-cylinder devices and internally-heated gas apparatus. In the piston-cylinder device with NaCl medium, the effect of pressure is profound. A pure low albite from Clear Creek, California reaches the equilibrium state of disorder at 850° C and 22 kbar in 10 h, whereas at 6 kbar it has not equilibrated in three weeks, and at one bar it probably cannot be disordered at 850° C in the laboratory. The enhancement of Al/Si interdiffusion takes place under dry conditions: any H2O penetrating the samples would have produced melting, and none was observed. Hydrogen, however, is produced by dissociation of moisture in the pressure medium and can penetrate the Pt sample capsules. If the samples are deprived of hydrogen by replacing NaCl with glass or by embedding the samples in a hydrogen getter such as Fe2O3 or ZnO, the order-disorder reaction is greatly inhibited.