The superstructure of plagioclase feldspars

A modulation function representing the position and density of (Na, Ca) atoms in the superstructure of the e-plagioclase has been derived from the average structures of different plagioclase and a general modulation theory. Based on this function the superstructure of bytownite (An₇₃) has been studied with the single crystal X-ray method. The cell dimensions by Megaw's axes are a=7.946(3)A, b=67.09(2)A, c=12.236(4)A, α=39.03(1)°, β=45.63(1)° and γ=59.63(1)°. Z=18(Na, Ca) Al(Al, Si)Si₂O₈. The initial phase factor of the modulation function for bytownite has been obtained from the intensity data of the satellite reflections. This modulation function indicates a coherent small-scale alternation of the Na-rich and Ca-rich bands in the superstructure. This superstructure has been refined by applying the albite and anorthite structures to the Na-rich and Ca-rich bands, respectively. The change of the superstructure of the e-plagioclase due to the compositional change has been described based on the movements of the satellites in reciprocal space. The direction of the coherent small-scale intergrowth of the anorthite-like and albite-like bands is perpendicular to the t vector. The thickness of the intergrowth is 1/|t|. Both direction and thickness change regularly from An₇₅ to An₂₅.     

Molecular dynamics simulation of the effect of the Al-O-Al linkage on the P&1macr;-I&1macr; phase transition of anorthite

Molecular dynamics (MD) simulations of the influence of the Al-O-Al linkage on the P&1macr;-I&1macr; phase transition of pure anorthite (An) were carried out using two different types of structures with fully ordered (FO) and partially disordered (PD) arrangements of Al/Si in tetrahedral sites. Discontinuous changes in unit cell volumes and structure factors at the transition temperature were observed in FO-An but not in PD-An. These results show that the orders of the transitions of FO-An and PD-An are first and non-first, respectively. In both structures, the motions of the Ca atoms and the framework are strongly correlated with each other during the transition and Ca atoms dominate the system. Since high-temperature X-ray studies have shown that the transition of natural anorthite is non-first order, it is suggested that the natural anorthite has a partially disordered arrangement of Al and Si atoms. Received: 22 October 1998 / Revised, accepted: 12 March 1999     

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.     

Theoretical approach to evaluating plagioclase dissolution mechanisms

The more rapid dissolution of Ca-rich feldspars relative to Na, K-rich feldspars has been attributed to the preferential leaching of Al deep within the feldspar structure. Evidence from surface microanalysis (e.g., Hellmann et al., 2003), however, shows that preferential dissolution of Al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution. It is thus critical to examine the extent of preferential Al removal. Here we present a theoretical study of plagioclase dissolution behavior using parameterized Monte Carlo simulations. Two different dissolution mechanisms, a mechanism involving preferential leaching of Al and an interfacial dissolution-reprecipitation mechanism, are tested using compositions representing the entire plagioclase solid solution series. Our modeling results indicate that under the control of the preferential Al leaching mechanism, the influence of (Al, Si) disorder on the dissolution rate is significant. At a fixed composition, an increase in the degree of (Al, Si) disorder yields an increased dissolution rate, with an 8-fold increase in dissolution rate observed for highly disordered albite (An₀) compared to low albite. Increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder. The difference in the dissolution rate of 293 tested oligoclase configurations with a composition of An₂₀ is 3-fold, and the difference is reduced to 2-fold among 107 andesine configurations of An₃₀. Furthermore, feldspar configurations with completely disordered (Al, Si) distributions yield a consistent log-linear dependence of dissolution rate on the anorthite content (An), while other feldspar configurations with modest degrees of (Al, Si) disorder exhibit rates less than this trend. In contrast, when Al removal is confined to the top surface layers, a variety of feldspar configurations with different (Al, Si) disorder but a single fixed composition have similar dissolution rates; and the dissolution rate of Ca-rich feldspars departs positively from its log-linear relationship with anorthite content. This departure occurs around An₈₀ and is in good agreement with previous experimental studies. Subsequent modeling results of aluminum inhibition, ΔG dependence, and formation of altered surface layers in the framework of the interfacial dissolution-reprecipitation mechanism are all comparable with experimental investigations, and these results suggest that an interfacial dissolution-reprecipitation mechanism governs the dissolution of plagioclase feldspars.     

苏鲁造山带东部荣成榴辉岩中蓝晶石及绿辉石的分解——一种可能的新成因解释

在麻粒岩相条件下，荣成含蓝晶石榴辉岩发生退变质，其中的绿辉石退变质成透辉石＋中长石组成的后成合晶，蓝晶石大部分被冠状体取代，基于详细探针分析结果的成分迁移估算显示，冠状体的形成需要从绿辉石的分解中获取CaO,MgO,FeO，绿辉石的分解包含两个反应：一个为绿辉石中的硬玉分子与石英结合形成钠长石；另一个发生在绿辉石内部各端员组分之间，后者不但形成后成合晶，而且为蓝晶石周围冠状体的形成提供CaO,MgO,FeO，蓝晶石的分解未能给后成合晶的形成提供SiO2，绿辉石与蓝晶石在分解时并冰形成一个封闭体系，它们均从周围环境中获取SiO2。     

Mechanical twinning and slip in experimentally deformed plagioclases

Mechanical albite and pericline twins were induced in seven feldspars, (Na, Ca) (Al, Si)₄O₈, in triaxial compression tests at 800° C, 8 to 10 kb confining pressure. Two were initially in a high temperature structural state (disordered) (An₁, An₅₉). Three (An₃₂, An₃₉, and An₅₃) were slightly disordered, and two (An₇₆ and An₉₅) had the transitional and primitive anorthite structure, respectively. No microscopic twinning was detected in comparable tests on ordered albite and oligoclase (An₁, An₂₀), feldspars in the peristerite range (An₁₁ An₁₄) or in a low structural state labradorite (An₅₁). Other deformational features include lamellae 60 to 80° from the c axis and (010) slip in the opposite sense for (010) twinning.Work performed under the auspices of the U. S. Atomic Energy Commission.     

Thermodynamics of plagioclases I: Theory of the I\bar 1 - P\bar 1 phase transition in anorthite and Ca-rich plagioclases

Landau theory of the \(P\bar 1 - I\bar 1\) phase transition in Ca-rich plagioclases reveals the sensitivity of the phase transition behaviour to a) Al, Si disorder, b) structural replacement of Ca by Na, and c) inhomogeneities of lattice strains. The following effects are predicted:

1. A tricritical phase transition exists in fully ordered anorthite. Al, Si disorder and Na, Ca exchange lead to second order phase transitions.
2. The transition temperatures depend sensitively on the degree of Al, Si disorder and the chemical composition of the Ca-rich plagioclases. Increasing Na-content decreases the transition temperatures.
3. The thermal evolution of c and d reflections depends on the homogeneity of the crystal and do not necessarily reflect the temperature evolution of the macroscopic lattice strain. A simple quadratic dependence of the X-ray scattering intensity on the order parameter exists only for fully ordered, homogeneous anorthite.

The role of inhomogeneous Al, Si distributions and lattice relaxations are discussed including possible structural modulations.     

Variation of viscosity with temperature and composition in the plagioclase system

The viscosities of supercooled melts of 13 different plagioclase compositions have been measured in the range 10¹³–10⁸ dPas with a micro-penetration viscometer. Application of the most common empirical and theoretical viscosity-temperature models to the present data and plagioclase viscosity data available from the literature showed that the configurational entropy theory represents the best approach. Using this theory together with an entropy of mixing term evaluated from a modified two-lattice mixing model, the viscosities of intermediate plagioclase compositions have been calculated as a function of temperature and anorthite content solely from the appropriate data of the end members albite and anorthite. The agreement between experimental and calculated viscosity data is excellent.Dedicated to Professor Wolf von Engelhardt on the occasion of his 75th birthday     

Crustal thickening of the lower crust of the Kohistan arc (N. Pakistan) deduced from Al zoning in clinopyroxene and plagioclase

The lower-crustal rocks of the Kohistan complex (northern Pakistan) are mostly composed of metabasic rocks such as pyroxene granulites, garnet granulites and amphibolites. We have investigated P–T trajectories of the relic two-pyroxene granulites, which are the protolith of the amphibolites within the Kamila amphibolite belt. Aluminous pyroxene retains igneous textures such as exsolution lamellae developed in the core. The significant amount of Al in clinopyroxene is buffered by breakdown reactions of plagioclase accompanied by film-like quartz as a product at grain boundaries between plagioclase and clinopyroxene. Distinct Al zoning profiles are preserved in pyroxene with exsolution lamellae in the core and in plagioclase adjacent to clinopyroxene in pyroxene granulites. In the northern part of the Kamila amphibolite belt, Al in clinopyroxene increases towards the rim and abruptly decreases at the outer rim, and anorthite in plagioclase decreases towards the rim and abruptly increases near the grain boundary between plagioclase and clinopyroxene. In the southern part of the Kamila amphibolite belt, Al in clinopyroxene and anorthite in plagioclase simply increase towards the margins of the grains. The anorthite zoning in plagioclase is in agreement with the zoning profiles of Ca-Tschermaks and jadeite components inferred from variations of Al, Na, Ti and Fe³⁺ in clinopyroxene. Assuming that the growth surface between them was in equilibrium, geothermobarometry based on Al zoning in clinopyroxene coexisting with plagioclase indicates that metamorphic pressures significantly increased with increasing temperature under granulite facies metamorphism. The peak of granulite facies metamorphism occurred at conditions of about 800 °C and 800–1100 MPa. These prograde P–T paths represent a crustal thickening process of the Kohistan arc during the Early to Middle Cretaceous. The crustal thickening of the Kohistan arc was caused by accretion of basaltic magma at mid-crustal depths.     

Late Quaternary uplift and subsidence of the west coast of Tanna,south Vanuatu,southwest Pacific: U–Th ages of raised coral reefs in the Median Sedimentary Basin

An X‐ray determinative curve, based on the parameter (Smith &; Gay, 1958) has been derived from twelve analysd plagioclases in the range An₆₂‐₈₇. The line differs from previous calibrations of against anorthite content, and it is suggested that this is due to the differing thermal histories of the materials used.

A break in slope in the determinative line at about An₇₅ may correspond to the change in lattice type from low intermediate plagioclase, to “body centred” anorthite. Heating of the plagioclases results in an increase in and the disappearance of the discontinuity.     

Margarite in kyanite- and corundum-bearing anorthosite,amphibolite, and hornblendite from central Fiordland,New Zealand

Margarite is both abundant and widespread throughout a sequence of interstratified amphibolite, hornblendite, and metamorphosed anorthosite from the upper Lyvia River, central Fiordland. These rock types comprise part of a metamorphosed layered intrusion. Assemblages recorded from these rocks are the product of two distinct phases of metamorphism. First generation assemblages typically comprise plagioclase (An_84–96), hornblende, kyanite, and minor corundum. Clinozoisite and chlorite occur as late stage breakdown products of plagioclase and hornblende. Margarite developed during the second phase of metamorphism.Within the corundum-bearing rocks replacement of corundum or plagioclase by margarite can be observed directly. On the basis of these observations the following reaction is evident: 1 corundum+1 anorthite+1H₂O=1 margarite.In other assemblages the formation of margarite can be attributed to the breakdown of kyanite and clinozoisite according to the reaction: 2 kyanite+2 clinozoisite=1 margarite+3 anorthite.Margarite is found, however, to contain appreciable amounts of paragonite solid-solution (up to 28 mol%) and plagioclase produced (second generation) is not pure anorthite but of intermediate compositions (An_46–62). The reaction therefore involves the introduction of both soda and silica. Margarite also crystallized independently of clinozoisite according to a reaction of the general form: 5 pargasite+17 kyanite+19 H₂O =8 margarite+4 chlorite+7 plagioclase.Application of available experimental data suggests that the margarite formed between 550 and 720 ° C up to a maximum pressure of 9.5 kb. Whereas the involvement of albite component (second generation plagioclase) will tend to lower the temperatures and pressures necessary for the occurrence of margarite, this effect is partially offset by the significant amounts of paragonite end-member held within the margarite. An independent estimate of the metamorphic conditions in metapelites suggests that the introduction of albite lowers equilibration temperatures by about 2 ° C for every 1% albite.     

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.     

Kationenaustausch an Plagioklasen

A series of K-plagioclases have been produced metastably by ion-exchanging the plagioclases in KCl-melt. Chemical analyses and re-exchange experiments (with NaCl-melt) have proved that the K-plagioclases are truly isomorphous with plagioclases. Because of their isomorphous nature it was possible to mix them in different proportions and homogenise them, thereby producing ternary feldspars of different compositions in the system Or-Ab-An. A comparative study of the lattice constants of plagioclases, their K-equivalents and the different ternary feldspars calculated using powder data leads to the following conclusions:

1. It is possible to dertermine by X-ray methods the anorthite content of a plagioclase to an accuracy of ±1% An.
2. The influence of the ionic size of the bigger cations Na, Ca, and K and that of order/disorder are different on different lattice constants and it is possible to distinguish these two effects.
3. The b-parameter indicates a definite structural change in plagioclases at about 25–30% An.
4. It is possible to predict the average Al/Si-distributions in the four different positions T_1(o), T_1(m), T_2(o) and T_(m) of all plagioclases of all stable structural states, using the c-parameter of their K-equivalents.

     

Enthalpies in (Na,Ca)- and (K,Ca)-feldspar binaries: a high-temperature solution calorimetric study

A series of high structural state plagioclases (Ab-An) was crystallized from glasses. By exchanging Na for K in KCl melts, metastable K-plagioclases (Or-An) were prepared which possess the same structural state as the starting plagioclases. Both series were investigated at 980 K by lead borate solution calorimetry. Continuing the ideas of Carpenter and McConnell (1984) and Carpenter (1992a), the results can be interpreted as follows. In the high plagioclase series, the enthalpies of solution, jH_sol, reflect the schemes of Al,Si ordering: (1) analbite-like (C2/m) ordering in the An-poor region 0hX_AnА.2, (2) high albite-like (C1¥) ordering in intermediate plagioclases, and (3) anorthite-like (I1¥) ordering in the An-rich region 0.7AnБ. In regions 1 and 2, jH_sol decreases as a function of X_An, but increases in region 3 as a consequence of the C1¥MI1¥ ordering reaction. Therefore, it is not a mixing effect but a compositionally restricted ordering effect which causes the excess enthalpies, jH^ex, to be positive in the plagioclase binary as a whole. Neglecting the existence of phase transitions at X_An=0.2 and X_An=0.7, jH^ex can be approximated by a two-parameter Margules model yielding W^H_AnAb=14Dž kJ/mol and W^H_AbAn=40Dž kJ/mol. jH_sol values of I1¥ plagioclases (X_An>0.7) can be "corrected" for the C1¥MI1¥ ordering effect (Carpenter 1992a). When combining the corrected values with the jH_sol data which were actually measured on the C1¥ plagioclases (X_An<0.7), negative excess enthalpies are generated in the plagioclase binary. This may be expected when C1¥ ordering occurs relative to topochemically monoclinic reference states of analbite and hypothetical anorthite devoid of I1¥ order. The solution experiments on the K-plagioclases resulted in similar characteristics as those found for the plagioclases. However, in addition to the ordering effects observed in the plagioclase binary, volume mismatch effects contribute to jH^ex in the K-plagioclase series. jH^ex can be represented by a Margules model with W^H_AnOr=60ᆞ kJ/mol and W^H_OrAn=91ᆢ kJ/mol when the phase transitions at X_An=0.2 and X_An=0.7 are again neglected. The contribution of the volume mismatch effect to jH^ex is considerable, as appears from the large difference between the K-plagioclase and the plagioclase Margules parameters. Their difference corresponds to a practically symmetrical dependence of jH^ex_volmism on composition, with W^H_volmism=48ᆡ kJ/mol.     

An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H₂O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H₂O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H₂O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H₂O content can notably increase the An content of plagioclase, up to 10 mol % for H₂O-undersaturated melts, and 20 mol % for H₂O-saturated melts. Exceptionally calcic plagioclases (up to An₁₀₀) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H₂O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na₂O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An₉₄) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na₂O = 12–15, or from melts with exceptionally high Al₂O₃(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H₂O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An₉₅ in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na₂O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994     

南极乔治王岛第三纪火山岩中斜长石成分特征及成因意义

乔治王岛第三纪火山岩含大量斜长石斑晶。斜长石的主元素、稀土和微量元素分析表明,它们主要为中长石－钙长石。斜长石种类与寄主岩之间有较好对应关系,即玄武安山岩和安山岩中以中长石和拉长石为主,而高铝玄武岩中为倍长石和钙长石;它们的ΣＲＥＥ、Ｌａ／Ｙｂ和δＥｕ等之间也有明显对应关系。但不同种类的斜长石之过渡金属元素特征相近,反映了过渡金属元素在斜长石中的含量和分异趋势主要受制于斜长石的晶体结构。综合研究表,斜长石成分取决于其母岩浆成分,并反映了区域岩浆演化的趋势．     

Heat capacity and entropy of low structural state plagioclases

The heat capacity of 11 different compositions of the low structural state plagioclase binary was measured between 5 and 773 K. The results are compared to those obtained from synthetic high structural state samples investigated in a previous study and obtained from a heat-treated natural sample of this study. The heat capacity of anorthite samples with slightly different order parameters shows large differences at ~500 K, where the Ibar1-Pbar1 phase transition occurs, which affects petrological calculations. At T = 298.15 K, the vibrational entropy versus composition behaviour of the low structural state plagioclases is almost ideal, in contrast to the high structural state plagioclases with positive excess vibrational entropies. At higher temperatures, the low structural state plagioclases show a negative deviation from ideal vibrational entropy composition behaviour in the Na-rich region.     

On the mechanisms of cation diffusion processes in ternary feldspars

Cation diffusion processes have been studied in single crystals of intermediate plagioclase and albite composition by tracer-diffusion experiments and optical absorption spectroscopy. Tracer-diffusivities were determined by the residual activity method, using the radioactive isotopes ²²Na, ⁴⁵Ca and ⁵⁹Fe. In most cases, diffusion experiments were performed at 1 bar, at controlled oxygen activity and at temperatures between 750 and 1300°C. The obtained Na-diffusivities for plagioclases were much smaller then previously determined for albite. This indicates a strong composition dependence of Na-diffusion. In contrast, Ca-diffusivity in albite does not differ very much from that in intermediate plagioclases. The relative diffusivities determined for plagioclase of composition An62 at 1200° C (CO/CO₂ =50∶50) were D _infNa ^sup* ∶D _infFe ^sup* ∶D _infCa ^sup* =5000∶10∶1. Despite the an isotropic structure of feldspars, no difference was found for Na-and Ca-diffusion normal to (001) and normal to (010). Water pressure of 2 kbar has no influence on the Na-diffusivity. In contrast to the Ca-diffusion, a dependence on oxyggen activity was found for Na-and Fe-diffusion. Fe-diffusivity increases with decreasing oxygen activity. This can be correlated to changes in oxidation state of iron dissolved in the plagioclases. Optical absorption spectroscopy shows that iron is oxidized in the plagioclases by annealing in air. This effect can be reversed by annealing at reducing conditions. A model is proposed to explain the oxidation of iron by a chemical diffusion process in which A-vacancies are formed by out-diffusion of Na⁺. Preannealing of samples in air gives a temperature independent decrease of Na-diffusivity by a factor of about 2.5. This effect is explained with help of a simple disorder model for A-cations in ternary feldspars. It is concluded that Na⁺ diffuses via interstitials and that the A-vacancy concentration in the plagioclases is controlled extrinsically, probably by dissolved SiO₂.     

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.     

Intergrowth of andesine and labradorite in marbles of the Central Alps

Crystallographic intergrowth of An 34 andesine and An 66 labradorite which can be seen with a petrographic microscope in sections cut perpendicular to [001] has been observed in amphibolite facies calcsilicate rocks of the Central Alps. The texture indicates that both plagioclases grew simultaneously in equilibrium, andesine mainly nucleating on faces (1¯10), (110), (100) and (130) of labradorite. The chemical composition measured on the U-stage and confirmed by microprobe establishes the endmembers of the andesine-labradorite miscibility gap. As shown by X-ray and electron diffraction both endmembers have the same intermediate plagioclase structure with periodic antiphase boundaries parallel to 10¯3 and spaced 25–30 Å which is characteristic of the structure observed in andesine from igneous rocks. Two different types of domains have been observed; one is in contrast with e-reflections and the other with strong a-reflections.