Feldspar geothermometry of the Hell Canyon Pluton,Boulder Batholith,Montana

The bulk compositions of the groundmass alkali feldspar from the Hell Canyon Pluton is 0.146mole% albite. The composition of the outermost zone of the oscillatory zoned plagioclase is 0.686 mole% albite, whereas the most calcic cores have a composition of 0.43 mole% albite. The structural state of the alkali feldspar is near orthoclase. Both composition of coexisting feldspars and structural state of the alkali feldspar are nearly constant throughout the pluton.Exsolved albite in the alkali feldspar have a composition of 0.965 mole% albite and the orthoclase host has a composition of 0.032 mole%. Singe crystal X-ray studies indicate that the albite intergrowths are coherent with the host.Equilibrium temperatures derived from the coexisting feldspar average 554 ° C; about 150 ° C, too low for the minimum solidus temperatures for reasonable emplacement pressures (2 kb). If this minimum solidus temperature is assumed, then the alkali feldspar has lost about 0.15 mole% albite. This loss was most likely caused by hydrothermal solutions associated with the crystallizing magma and equilibrated at about 550 ° C. However, based on the coherent albite intergrowths and the orthoclase structure state it can be inferred that the system was relatively free of volatiles below 500 ° C. Final equilibirium between orthoclase host and albite intergrowths occurred at about 311 ° C.     

Textural evolution of the rapakivi granites,south Greenland —Sr,O and H isotopic investigations

The development of rapakivi texture in feldspars from the Ketilidian granitoids of south Greenland has been investigated using Sr, O and H isotopes. A low temperature signature is found in the Sr and O data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the K-feldspar ovoids. An origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed, which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar. This mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of K-feldspar to microcline. Rapakivi granites with both white and dark green feldspar occur in south Greenland but show no major isotopic differences, although the dark alkali feldspars contain significantly more fluid. Equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution.     

二长石地质温度计在估算乌拉山金矿碱性长石形成温度中的应用

内蒙古乌拉山金矿田内主要出露晚太古代乌拉山群区域变质岩和规模不一的花岗岩体以及不同时代、不同种类的脉状地质体。含金矿脉中主要矿物共生组合为碱性长石、石英、斜长石、碳酸盐矿物(方解石、白云石)和少量金属硫化物。矿床的显著特征为碱性长石交代作用强烈，碱性长石也广泛产于该地区其他各种类型的岩石中。本文采用电子显微探针分析了共生碱性长石和斜长石的化学成分，并采用三元二长石温度模型估计了碱性长石的平衡温度。结果表明，第一成矿阶段的碱性长石一石英含金矿脉中碱性长石的形成温度为353℃，第二成矿阶段石英含金矿脉中碱性长石的形成温度为281℃，矿脉碱性长石形成压力约为5kbar。这些结果与同类矿石中平衡共生的碳酸盐矿物和云母类矿物的地质温度计估计的形成温度以及共生石英中流体包裹体的均一温度非常一致。因此，乌拉山金矿床形成和富集的温度可估测为260～380℃，压力约为5kbar。此外，应用二长石温度计计算了本地区区域变质片麻岩和花岗岩中碱性长石的平衡温度，所得温度比采用共生铁铝榴石和黑云母温度计估计的温度要低约250℃。这表明共生的铁铝榴石和黑云母的平衡温度可能代表其寄主变质岩变质期温度及寄主花岗岩原生温度，而区域变质岩和花岗岩中的碱性长石在经历了随后多次热液作用后，可能重新平衡再生，这也与前人对乌拉山金矿的矿床地质和同位素研究的结果一致。     

Activity-composition relations in feldspars

Seck's (1971a) compositional data on coexisting feldspars in the Or-Ab-An ternary at 650° C and 1 kb were used to calculate the activity-composition relations in binary alkali feldspar and binary plagioclase. The energy constants in Guggenheim's expression for excess free energy of mixing are A ₀=3920 and A ₁=657 cal/mole for alkali feldspar, in excellent agreement with values obtained by Thompson and Waldbaum (1969), and 1320 and 373 cal/ mole for plagioclase. Using Orville's (1972) data from ion-exchange experiments between plagioclase and Na—Ca chloride solutions at 700° C and 2 kb, we obtained 967 cal/mole for A ₀ and 715 cal/mole for A ₁ in the plagioclase crystalline solution.Activity-composition relations for plagioclase are interpreted in terms of a continuous, random substitution of CaAl for NaSi across the high structural state plagioclase series. This interpretation is consistent with that obtained from a consideration of lattice parameters.     

Low-temperature coherent exsolution in alkali feldspars from high-grade metamorphic rocks of Sri Lanka

In the alkali feldspars of the amphibolite- and granulite-facies rocks of Sri Lanka, a late-stage, final exsolution event is observed which produced film lamellae and fine-scale spindles. These were investigated by optical, microprobe, single-crystal, transmission electron microscopy and atomic resolution microscopy techniques. The lamellae and spindles exsolved below the coherent solvus at temperatures as low as 300 to 350° C. Precession photographs and ARM micrographs show that the intergrowth is perfectly coherent. In sections (010) the rhombic section of the Pericline twins corresponds to analbite or high albite. The albite lamellae and spindles nucleated and grew at low temperatures in a metastable disordered structural state within a tweed-orthoclase matrix and became periodically twinned analbite or high albite, which subsequently developed only a slight increase in Al, Si order. The relationship between twin periodicity and lamellar width, predicted for coherent intergrowths by Willaime and Gandais (1972), is obeyed. In Or-rich grains, in which coherent exsolution is the only exsolution event, the film lamellae tend to be restricted to the rim, the fine-scale spindles to the centre of the grains. The films nucleated heterogeneously at grain boundaries and grew towards the grain centres. Fine-scale spindles probably nucleated homogeneously in the interior part of grains. Heterogeneous nucleation and coherent growth are not mutually exclusive.     

桐庐杂岩体中共存的两种碱性长石研究

桐庐同熔型火山－侵入杂岩中出现两种碱性长石，一种为无色透明，另一种为微红色，本文利用X射线衍射仪，电子探针，透射电镜对其结构态，成分及出溶微结构进行了对比研究。结果表明，两者成分差别不大，而结构差别较大，无色透明的为透长石，出溶叶片发育；微红色的为正长石或低透长石，出溶结构不发育。而且随着冷却速率的变慢，无色透明碱性长石的Al/Si有序度增大，微红色碱性长石的有序度降低，分析后认为这种共存现象是岩浆混和造成的，从而为桐庐杂岩体的岩浆混合成因提供了有力的证据。     

Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging

Charge contrast imaging in the scanning electron microscope can provide new insights into the scale and composition of alkali feldspar microtextures, and such information helps considerably with the interpretation of their geological histories and results of argon isotope thermochronological analyses. The effectiveness of this technique has been illustrated using potassium-rich alkali feldspars from the Dartmoor granite (UK). These feldspars contain strain-controlled lamellar crypto- and microperthites that are cross-cut by strain-free deuteric microperthites. The constituent albite- and orthoclase-rich phases of both microperthite generations can be readily distinguished by atomic number contrast imaging. The charge contrast results additionally show that sub-micrometre-sized albite ‘platelets’ are commonplace between coarser exsolution lamellae and occur together to make cryptoperthites. Furthermore, charge contrast imaging reveals that the orthoclase-rich feldspar is an intergrowth of two phases, one that is featureless with uniform contrast and another that occurs as cross-cutting veins and grains with the {110} adularia habit. Transmission electron microscopy shows that the featureless feldspar is tweed orthoclase, whereas the veins and euhedral grains are composed of irregular microcline that has formed from orthoclase by ‘unzipping’ during deuteric or hydrothermal alteration. The charge contrast imaging results are especially important in demonstrating that deuteric perthites are far more abundant in alkali feldspars than would be concluded from investigations using conventional microscopy techniques. The unexpected presence of such a high volume of replacement products has significant implications for understanding the origins and geological histories of crustal rocks and the use of alkali feldspars in geo- and thermochronology. Whilst the precise properties of feldspars that generate contrast remain unclear, the similarity between charge contrast images and corresponding cathodoluminescence images of deuteric microperthites indicates that trace element chemistry and possibly also elastic strain within the crystal play a major role.     

Retrograde metamorphic reactions in deforming granites and the origin of flame perthite

Abstract Microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram: orthoclase (Or_92–97), albite and oligoclase-andesine. The albite occurs as grains in the matrix, as flame-shaped lamellae in orthoclase, and in patches within plagioclase grains. We present a metamorphic model for albite flame growth in the K-feldspar in these rocks that is related to reactions in plagioclase, rather than alkali feldspar exsolution. Flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions. Reaction 1 is a continuous or discontinuous (across the peristerite solvus) reaction in plagioclase, in which the An component forms epidote or zoisite. Most of the albite component liberated by Reaction 1 stays to form albite in the host plagioclase, but some Na migrates to form the flames within the K-feldspar. Reaction 2 is the exchange of K for Na in K-feldspar. Reaction 3 is the retrograde formation of muscovite (as ‘sericite’) and has all of the chemical components of a hydration reaction of K-feldspar. The Si and Al made available in the plagioclase from Reaction 1 are combined with the K liberated from the K-feldspar, to produce muscovite in Reaction 3. The muscovite forms in the plagioclase, rather than the K-feldspar, as a result of the greater mobility of K relative to Al. The composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction. Using an initial plagioclase composition of An₂₀, the total reaction can be summarized as: 20 oligoclase + 1 K-feldspar + 2 H₂O = 2 zoisite + muscovite + 2 quartz + 15 albite_plagioclase+ 1 albite_flame. This model does not require that any additional feldspar framework be accreted at replacement sites: Na and K are the only components that must migrate a significant distance (e.g. from one grain to the next), allowing Al to remain within the altering plagioclase grain. The resulting saussuritization is isovolumetric. The temperature and extent of replacement depends on when, and how much, water infiltrates the rock. The fugacity of the water, and therefore the pressure of the fluid, may have been significantly lower than lithostatic during flame growth.     

Modeling H,Na, and K diffusion in plagioclase feldspar by relating point defect parameters to bulk properties

Hydrogen and alkali ion diffusion in plagioclase feldspars is important to study the evolution of the crust and the kinetics of exsolution and ion-exchange reactions in feldspars. Using the available PVT equation of state of feldspars, we show that the diffusivities of H and alkali in plagioclase feldspars as a function of temperature can be successfully reproduced in terms of the bulk elastic and expansivity data through a thermodynamic model that interconnects point defect parameters with bulk properties. Our calculated diffusion coefficients of H, Na, and K well agree with experimental ones when uncertainties are considered. Additional point defect parameters such as activation enthalpy, activation entropy, and activation volume are also predicted. Furthermore, the electrical conductivity of feldspars inferred from our predicted diffusivities of H, Na, and K through the Nernst–Einstein equation is compared with previous experimental data.     

Ternary feldspar thermometry in granulites from the Oaxacan Complex,Mexico

Coexisting feldspars from across 2,000 km² of the granulite facies Oaxacan Complex, southern Mexico exhibit variable amounts of solid solution from nearly binary (Ab-An and Ab-Or) to substantially ternary (Ab-An-Or). Reintegrated analyses of 21 coarsely exsolved perthite (AF)-plagioclase (PL) pairs yield AF=Or_30–63 Ab_30–56An_2–15 and PL=Or_1–2Ab_70–84An_11–28. These data have been used to test existing two feldspar geothermometers for this extended composition range.For all compositions, temperature estimates show relatively little spread in value (660° to 795° C, 7 kbar) using the Haselton et al. (1983) calibration (HHHR). These temperatures are in fair agreement with estimates of 750±40° C for feldspar pairs with nearly binary compositions using the Stormer (1975) thermometer (STO). However, STO temperatures increase significantly (to 990° C) with increasing ternary solid solution in AF, suggesting that thermometers derived for binary systems are inaccurate for ternary compositions. Isotherms drawn from HHHR which take into account variable anorthite solution in alkali feldspar show that estimated temperature decreased by 50–100° C for each 5 mole percent anorthite in alkali feldspar.Experimentally determined solvus relations (Seck 1971) require feldspars with significant ternary solid solution to have crystallized or to have equilibrated at higher temperature than feldspars with more binary compositions. However, petrographic and field relations of ternary and binary feldspars in the Oaxacan Complex suggest they were all equilibrated at similar metamorphic pressures and temperatures and do not support a model where ternary feldspars have preserved higher premetamorphic temperatures. The composition of coexisting feldspars from other Precambrian granulite-facies terranes are also inconsistent with Seck's (1971) results. Hence, thermometers which fit Seck's solvus relations may not yield accurate temperatures in high grade metamorphic terranes. Parallel tie-lines for ternary and binary feldspars in the Oaxacan Complex and the consistency of inferred temperatures (HHHR) for many granulite terranes suggest that estimation of temperature using tie-line slopes rather than solvus width may yield more accurate results for these samples.Peak metamorphic conditions in the Oaxacan Complex are inferred to have been 730±50° C, 7±1 kbar. Pressure estimates from four garnet-plagioclase barometers show good agreement. Results of feldspar thermometry are consistent with diopside-forsterite equilibria in marbles which restrict T=720–765° C at P=7 kbar.     

Myrmekite — one hundred years later

Myrmekite, first detected by Michel-Lévy in 1875 and named by Sederholm in 1899, is an intergrowth between vermicular quartz and (sodic) plagioclase situated next to potash feldspar. In felsic plutonic rocks it occurs as: rims bordering granular plagioclase, intergranular blebs set between adjacent microperthite crystals, lobes associated with muscovite in deformed alkali feldspar megacrysts or as bulbous growths at their margins, and rims on plagioclase inclusions held within orthocalse megacrysts. A literature review based largely on papers published in the past quarter century shows that hypotheses for myrmekite genesis fall mainly into five categories: simultaneous or direct crystallization, replacement of potash feldspar by plagioclase, replacement of plagioclase by potash feldspar, solid-state exsolution, and recrystallizing quartz involved with blastic plagioclase.     

Exsolution in ternary feldspars

The equilibrium precipitate formed by exsolution in alkali feldspar containing calcium in solid solution is a two-phase intergrowth of plagioclase and quartz. It is the kinetics of the precipitation, however, which decides whether or not this reaction will take place. It is shown that lamellar growth of plagioclase and quartz is only accomplished by the advancement of an incoherent boundary originating at an alkali feldspar — alkali feldspar boundary or at an interface between alkali feldspar and plagioclase.     

Experimental silicate mineral/melt partition coefficients for beryllium and the crustal Be cycle from migmatite to pegmatite

Partition coefficients (D_Be^mineral/melt) for beryllium between hydrous granitic melt and alkali feldspars, plagioclase feldspars, quartz, dark mica, and white mica were determined by experiment at 200 MPa H₂O as a function of temperature (650-900°C), activity of Be in melt (trace levels to beryl saturation), bulk composition, and thermal run direction. At trace levels, Be is compatible in plagioclase of An₃₁ (1.84 at 700°C) and muscovite (1.35 at 700°C) but incompatible in biotite (0.39-0.54 from 650-800°C), alkali feldspar (0.38-0.19 from 680-850°C), quartz (0.24 at 800°C), and albite (0.10 at 750°C). The partition coefficients are different at saturation of the melt in beryl: lower in the case of plagioclase of An₃₁ (0.89 at 700°C), muscovite (0.87 at 700°C), biotite (0.18-0.08 from 675-800°C), alkali feldspar (0.18-0.14 from 680-700°C), and quartz (0.17-0.08 from 750-800°C), but higher in the case of albite (0.37 at 750°C).With other data sources, these new partition coefficients were utilized to track, first, the distribution of Be between aluminous quartzofeldspathic source rocks and their anatectic melts, and second, the dispersion or concentration of Be in melt through igneous crystal fractionation of different magma types (e.g., S-type, I-type) up to beryl-saturated granitic pegmatites and, finally, into their hydrothermal aureoles. Among the rock-forming minerals, cordierite, calcic oligoclase, and muscovite (in this order) control the fate of Be because of the compatibility of Be in these phases. In general, beryl-bearing pegmatites can arise only after extended crystal fractionation of large magma batches (to F, fraction of melt remaining, ≤0.05); granitic magmas that originate from cordierite-bearing protoliths or that contain large modal quantities of calcic oligoclase will not achieve beryl saturation at any point in their evolution.     

Ternary feldspars: Kinetics and possible equilibria at 800° C

The ternary feldspar system KAlSi₃O₈ - NaAlSi₃O₈ - CaAl₂Si₂O₈ was reinvestigated at 650 ° C and 800 ° C (P _H2O = 1 kb) using mixtures of crystalline plagioclases and alkali feldspars as starting materials. The compositions of plagioclases and alkali feldspars of the run products were determined by X-ray means. The Or-content of the feldspar phases was determined by measuring the position of the (201) X-ray peak of the unexchanged feldspars, whereas the An-content was determined by measuring the same X-ray peak of the K-exchanged feldspars. The reaction rate of a reaction leading to a more An-rich plagioclase (type II reaction) is much faster than a reaction producing a more Ab-rich plagioclase (type I). In a type II reaction run times of approximately 20 days are needed to reach new constant plagioclase and alkali feldspar compositions at 650 ° C, and 10 days are needed to reach constant compositions at 800 ° C. In a reaction of type I only the outer zone of the plagioclases reacts to more Abrich compositions. A diffuse zone with a wide range of compositions was observed in 650 ° C runs. Equilibrium could not be reached in these experiments within 45 days. At 800 ° C a new zone having a specific composition develops in 42 days. This new zone is believed to be in equilibrium with the coexisting alkali feldspar. The depth of reaction is calculated as 0.03 μm after 42 days (800 ° C, P _f= 1 kb). The reaction between the two feldspar phases could be reversed at 800 ° C. The following compositions are considered to represent equilibrium data at 800 ° C and P _t = 1 kb:

An 43 Ab 51 Or 6 coexisting with Or 79 Ab 20 An 1, and

An 40 Ab 54 Or 6 coexisting with Or 75 Ab 24 An 1.

Recent data obtained with gels of ternary feldspar composition as starting materials do not agree with the results presented in this paper. Gels obviously crystallize spontaneously forming coexisting feldspars of non - equilibrium composition - alkali feldspars too rich in Ab and plagioclases too rich in An.     

Mantled feldspars from the Golden Horn batholith,Washington

Mantled feldspars that formed by resorption, development of skeletal plagioclase crystals, and filling with alkali feldspar are common in the Golden Horn batholith, Washington. Subhedral plagioclase mantles have weak normal zoning from An₁₇ to An₁₀. Plagioclase zoning and twinning are crosscut by resorption channels. Resorption cavities and channels are coated with albite (An₁₀). Anhedral, perthitic orthoclase within the plagioclase is optically continuous with orthoclase in channels and on the mantle exterior.This texture resulted from resorption of calcic cores of plagioclase as pressure decreased when water-undersaturated granite magma intruded to a shallow crustal level. At shallow level, only alkali feldspar and quartz crystallized and were available to fill the skeletal plagioclase.     

Activity/composition relations in the ternary feldspars

Activity/composition relations are presented for high-structural state feldspars whose bulk compositions lie within the ternary system NaAlSi₃O₈ CaAl₂Si₂O₈-KAlSi₃O₈. The expressions are parameterized from the data for coexisting feldspars of Seck (1971a) using an asymmetric regular solution approximation for the excess Gibbs free energy of mixing and an Al-avoidance model for the configurational entropy of solution. The solution properties of the plagioclase and alkali-feldspar binaries have been made to conform to the recent work of Thompson and Hovis (1979) and Newton et al. (1980). Using the proposed model the ternary feldspar solvus is extrapolated in temperature (up to 1,500° C) and pressure (up to 5kbars). A new two-feldspar geothermometer is presented which provides somewhat more reasonable estimates of crystallization temperatures than the equations and graphs of Stornier (1975), Powell and Powell (1977), Brown and Parsons (1981) and Haselton et al. (1983). In conjunction, some criteria are suggested for establishing the existence of equilibrium tie-lines between coexisting ternary feldspars in rhyolites and trachytes. Calculated values of the activity of KAlSi₃O₈ in plagioclase are examined in some detail. These compare favorably with independent estimates obtained from experimentally grown plagioclases precipitating at liquidus temperatures from igneous rocks of widely varying alkali contents.     

Sidewall crystallization in the Klokken intrusion: zoned ternary feldspars and coexisting minerals

The syenitic layered series in the Klokken intrusion is surrounded by a zone (500 m thick) of nearly structureless unlaminated syenite followed outwards by a zone of vertically banded gabbro (200 m thick) at the outer rim. The unlaminated syenite is intrusive into the gabbro and develops a thin (2 m) transition zone of syenodiorite at the contact. A traverse across the vertical transition zone and inwards towards the layered series was sampled with a portable drill. Mafic silicates (olivine, clinopyroxene, biotite) show inward evolution in Fe/(Fe+Mg) across the syenodiorite-unlaminated syenite zones. Feldspars change rapidly across the syenodiorite zone from rocks dominated by plagioclase, in some cases together with two alkali feldspars, one a mesoperthite or cryptomesoperthite, the other a cryptoperthite, to rocks in which plagioclase is seen only rarely as cores to cryptomesoperthitic alkali feldspar crystals. Plagioclase is absent from the layered series.Alkali feldspars occurring in pairs have bulk compositions on solvus isotherms in the Or-Ab-An ternary system, estimated at 950° C in a syenogabbro and 910° C in a syenodiorite, at 1 kbar. The more calcic liquids from which they crystallized fractionated on paths that intersected the two- feldspar surface, whereas the more syenitic members crystallized from liquids which terminated crystallization in the one- feldspar field at 900° C. Plagioclases evolve from calcic andesine in syenodiorites, to very rare sodic oligoclase in the most evolved unlaminated syenites. The boundaries between plagioclase cores and alkali feldspar rims, which are usually optically abrupt, involve complex mixed zones on the m -scale, consistent with arrested reaction between plagioclase primocrysts and crystallizing syenitic liquid. Ternary liquidus-solidus relationships are in qualitative agreement with this interpretation. The syenodiorites are cumulates produced during sidewall crystallization of a trachytic magma against a gabbroic chamberlining. This magma changed little in bulk composition as it evolved, giving rise to the unlaminated syenites by further sidewall crystallization. Water build- up in this liquid probably caused a change in style of chamber filling, giving rise to the layered series by bottom accumulation. Microtextures in the zoned feldspars are described in an accompanying paper.CRPG contribution 729     

A ternary feldspar-mixing model based on calorimetric data: development and application

A mixing model for high structural state ternary feldspars in the NaAlSi₃O₈–KAlSi₃O₈–CaAl₂Si₂O₈ system is presented based exclusively on calorimetric and volumetric measurements. Comparisons with existing mixing models, which are based on phase-equilibrium experiments, reveal distinct differences. The incorporation of K into Ca-rich plagioclase and of Ca into K-rich alkali feldspar is more strongly limited by our calorimetry-based model, whereas the stability field of Na-rich feldspars is broadened. Natural feldspar assemblages from well-studied magmatic and high-grade metamorphic rocks (i.e. a teschenite sill in Scotland, the Klokken syenogabbro in Greenland, and a granulite-facies metapelite in Sri Lanka) were used to test the mixing models. It was found that the new model largely eliminates discrepancies between observed and predicted feldspar compositions that were present in earlier attempts. The reasons for the problems associated with phase-equilibrium based mixing models are discussed.     

Rate control in dissolution of alkali feldspars—I. Study of residual feldspar grains by X-ray photoelectron spectroscopy

Sanidine grains (100–600 μm in diameter) were subjected to dissolution at 82°C in aqueous electrolyte solutions of pH ranging from 4 to 8 for 293 or 377 hr. Dissolution equivalent to the removal of silica from the outer 300–900 A of these grains was accomplished. The shallow subsurfaces of feldspar grains were then analyzed for K, Al, and Si by X-ray photoelectron spectroscopy. The results rule out any continuous precipitate layer; if an alkali-depleted subsurface zone (leached layer) was present in the feldspar, the thickness of such a zone approximated by linear increase of alkali concentration with depth was not more than about 17 Å.It is concluded that in the absence of a compact precipitate layer, dissolution of feldspars in the temperature range corresponding to deep diagenesis is controlled by the processes at the feldspar-solution interface and a leached layer more than one feldspar unit cell thick does not form. Whether the same applies at the temperatures of shallow diagenesis and weathering cannot be judged with certainty, but parallels with leached layers on alkali silicate glasses suggest that it does.     

Graphic intergrowths of feldspars and quartz in some Czechoslovak pegmatites

Graphic intergrowths of alkali feldspar+quartz, and plagioclase+quartz, occur together in pegmatites in the eastern part of the Czechoslovak Moldanubicum. They form zones between the finer-grained wall zone and the central blocky feldspar+quartz core. The normative Or-Pig-Q compositions of the graphic intergrowths and the Or-Ab-An contents of their feldspars show broad variations generally, but have a restricted range within individual pegmatites. At two localities studied in more detail, coexisting feldspars show gradual changes in composition, from the margins up to the innermost graphic pegmatite, compatible with fractional crystallization along the feldspar solidus-solvus intersection in the Or-Ab-An-Q-H₂O system, at different vapour pressures in different pegmatites. Two models are demonstrated for low and high pressure cases. The feldspar compositions from central blocky zones deviate from the magmatic fractional crystallizations paths; this corresponds with the general assumption that they crystallized from supercritical gaseous fluids.In these and in similar pegmatites, coexisting alkali feldspar+quartz and plagioclase + quartz intergrowths are interpreted as the last products of cotectic crystallization from an ultimately fractionated granitic magma. Positive correlation of Ab solid solution and quartz content in the potassic intergrowths suggests that these characteristics may be indicative of the relative pressure and temperature effective during their crystallization, when compared in pegmatites of the same bulk composition.