Ordering and exsolution processes in Or-rich alkali feldspar megacrysts from the Eldzhurtinskiy granite (Caucasus)

 The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Central Caucasus) is uniform with respect to modal composition, major and trace element chemistries of bulk rocks and mineral phases. In contrast, it reveals two types of alkali feldspar megacrysts differing in tetrahedral Al-content (2t₁) and exsolution microtextures: 1. Alkali feldspar megacrysts (Or₇₀An₂Ab₂₈) from the top of the body consist of ideally coherent intergrowths of fine-scale regular Or- and Ab-rich lamellae. The exsolved K-feldspar host is monoclinic (2t₁=0.7), the exsolved Na-rich phase consists of Albite- and/or Pericline-twinned albite. 2. Megacrysts from greater depths have the same bulk composition, but the exsolved Ab-rich phase occurs in the form of optically visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering (2t₁=0.8). The evolution of the distinct types of megacrysts reflects differences in the cooling history within the upper and lower part of the granite body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of albite patches in the megacrysts from the lower part are caused by fluid-feldspar interaction. The transition zone in the body between the two types of megacrysts is sharp (in a depth interval of 100–200 m) and not related to shear zones. Received: 12 June 1995 / Accepted: 29 January 1996     

An alkali feldspar series

A series of single-phase alkali feldspars in the system Na₂O-K₂O-Al₂O₃-SiO₂-H₂O were crystallized from dehydrated gels at 500 bars and 800° C, in the presence of an aqueous vapor phase, quartz, and corundum. Twenty of these bulk samples were analyzed for K₂O and Na₂O by atomic absorption spectrophotometry. The unit cell parameters for 19 of these samples and 8 others in the series were obtained by least squares refinement of x-ray powder diffraction data collected with a focusing camera. Polynomial expressions giving the variation in unit cell parameters as a function of composition, in mol fraction potassium feldspar, were obtained, and compared with similar polynomial expressions developed from the avilable data on the sanidine-high albite series.This series of alkali feldspars is distinct from the previously studied series, but is closely related to the sanidine-high albite series. The differences illustrated by this series are principally in the higher values of the b and c unit cell edges, for a given N_or, and consequently higher unit cell or molar volume for a given N_or.Although the source of these differences may be related to crystalline solution with a hydronium (or oxonium) feldspar molecule, H₃OAlSi₃O₈, a more probable source of these differences is that this feldspar series is less ordered in terms of Al/Si distribution than the sanidine-high albite series.     

The nature of potassium feldspar,exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars

Transmission electron microscope data on the morphology of exsolution lamellae, the nature of the potassium feldspar and the development of dislocations at lamellar interfaces in coherent cryptoperthites and fine microperthites are reviewed. Dislocations have been reported previously in only two crystals, and periodic dislocations noted in only one, an Or-rich microperthite. Periodic dislocations (spacing 100–150 nm) are here described from a ternary mesoperthite (Or₂₆ Ab₅₂ An₂₂). Small crystallites (<30 nm) of other phases have sometimes nucleated on the dislocations. The 020 lattice fringes of the feldspar phases have been imaged; the difference in 020 spacings can be almost entirely accommodated by the regular dislocations, so that the boundaries may be termed nearlyperfectly semicoherent.Dislocations have been found so far only in cryptoperthites with lens-shaped or straight lamellae, either in Or-rich feldspars or in Ab-rich ternary ones. In intermediate compositions with wavy or zig-zag albite lamellae, or lozengeshaped albite areas (braid microperthites) dislocations have not been observed. Strain reduction in intermediate compositions occurs by migration of lamellar interfaces from (¯601) to near (¯6¯61) as microcline forms in the diagonal association. In Ab-rich ternary feldspars the relatively high Ancontent blocks interface migration, and strain reduction occurs by nucleation of dislocations; the Or-rich feldspar phase is tweed orthoclase. In Or-rich bulk compositions the low volume of albite exerts insufficient stress to promote microcline formation, and tweed orthoclase develops. Interfaces do not migrate, and dislocations again develop. Fields in which different potassium feldspar polymorphs occur and in which the different exsolution textures are developed are summarized on a ternary diagram.     

Chemically induced fracturing in alkali feldspar

Fracturing in alkali feldspar during Na⁺–K⁺ cation exchange with a NaCl–KCl salt melt was studied experimentally. Due to a marked composition dependence of the lattice parameters of alkali feldspar, any composition gradient arising from cation exchange causes coherency stress. If this stress exceeds a critical level fracturing occurs. Experiments were performed on potassium-rich gem-quality alkali feldspars with polished (010) and (001) surfaces. When the feldspar was shifted toward more sodium-rich compositions over more than about 10 mole %, a system of parallel cracks with regular crack spacing formed. The cracks have a general (h0l) orientation and do not correspond to any of the feldspar cleavages. The cracks are rather oriented (sub)-perpendicular to the direction of maximum tensile stress. The critical stress needed to initiate fracturing is about 325 MPa. The critical stress intensity factor for the propagation of mode I cracks, K _Ic, is estimated as 2.30–2.72 MPa m^1/2 (73–86 MPa mm^1/2) from a systematic relation between characteristic crack spacing and coherency stress. An orientation mismatch of 18° between the crack normal and the direction of maximum tensile stress is ascribed to the anisotropy of the longitudinal elastic stiffness which has pronounced maxima in the crack plane and a minimum in the direction of the crack normal.     

Perthite microstructure in magmatic alkali feldspar with oscillatory zoning; Weinsberg Granite, Upper Austria

Feldspar megacrysts from the Weinsberg granite (Moldanubian Zone) show oscillatory zoning with respect to the albite- and orthoclase components. All growth zones show perthitic exsolutions which take the form of bleb- and lens shaped albite-rich precipitates in an orthoclase-rich host. The average sizes and shapes of the precipitates show systematic variation with the integrated bulk compositions of the respective growth zones. The precipitates are abundant and relatively small in growth zones with intermediate bulk composition (Or50Ab41An09 - Or80Ab18An02), and they are less abundant and larger in more orthoclase-rich zones (Or88Ab11An01). Small precipitates have a relatively high aspect ratio, whereas the large precipitates in the potassium-rich zones are more spherical. The relation between microstructure and integrated bulk composition suggests that exsolution and subsequent growth and coarsening occurred by different mechanisms in the respective growth zones. Numerical modeling shows that rapid growth of precipitates over extended periods of time and attainment of relatively large final size is favored, if only few nuclei are formed in an oversaturated host. In contrast, precipitates can grow rapidly only over limited time intervals and remain relatively small, if abundant nuclei are present. During cooling of the oscillatorily zoned alkali-feldspar, exsolution started at relatively high temperatures in growth zones of intermediate integrated bulk composition as compared to exsolution in the more orthoclase-rich growth zones. Irrespective of whether exsolution occurred by spinodal decomposition or by nucleation at relatively high temperatures in the growth zones of intermediate integrated bulk composition, it produced abundant nuclei and resulted in relatively small precipitates. In contrast, comparatively few nuclei were formed in the orthoclase-rich growth zones resulting in large precipitates. The Na/K partitioning between precipitates and the host is independent of the integrated bulk composition of the respective growth zone reflecting re-equilibration during cooling down to relatively low temperatures (<?400°C). The shape of the precipitates probably has evolved from an initially lamellar or spindle-like geometry with high aspect ratio to more isometric, spheroidal shapes during precipitate growth and coarsening. Host/precipitate interfaces served as fluid pathways during late stage deuteric alteration.     

Potassium and sodium self-diffusion in alkali feldspar

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

Nucleation on perthite-perthite boundaries and exsolution mechanisms in alkali feldspars

A transmission electron microscope study of intracrystalline boundaries between two perthites of markedly different composition in composite crystals, one a tenary mesoperthite (Or₂₆Ab₅₂An₂₂, initially a homogeneous potassian monalbite) the other a more potassic cryptoperthite (Or₆₁Ab₃₃An₆, initially a homogeneous sodian sanidine), shows that the two perthites are in nearly parallel intergrowth. Most boundaries examined were of (hkO) type; (010) boundaries are straight, whereas other (hkO) boundaries are curved or stepped. Exsolution occurred first in the potassian monalbite (mesoperthite) and was unaffected by the boundary. Subsequent exsolution in the sodian sanidine (cryptoperthite) was affected by the boundary, but for up to only a few micrometers. Exsolution occurred by heterogeneous nucleation and growth of oligoclase on and from the intracrystalline boundary. At almost the same time the rest of the volume of sanidine exsolved by spinodal decomposition. 1–2 μm from the boundary in the intervening K-rich matrix of the sodian sanidine, further exsolution occurred by homogeneous nucleation. Time — temperature — transition curves for continuous cooling have been devised to account for the unusual complexity of the exsolution texture. Except in such exceptional circumstances as the example studied, the initial exsolution in high-temperature alkali feldspars of intermediate composition, unlike other minerals, probably does not occur by nucleation, but only by spinodal decomposition.     

The red to near-infrared luminescence in alkali feldspar

Results are presented for the radio-, cathodo- and ionoluminescence (RL, CL and IL) of an ordered, exsolved alkali feldspar from South Greenland (BM2). BM2, a microperthite typical of many evolved rocks, has a broad red near-infrared luminescence, which, using lifetime-resolved CL, reveals four overlapping emissions, of which two are dominant. These derive from Fe³⁺-activated luminescence from the low albite and maximum microcline components of the microperthite in which the Fe is on the most ordered tetrahedral site. The temperature dependence of the emission shows a smooth change from longer wavelengths at 20 K to shorter values at room temperature. There is a broad relationship between Fe-O bond distance and the energy of the luminescence. Comparisons of RL, CL and IL allow features in the luminescence associated with surface states to be identified. The dependence of red RL intensity versus temperature is divided into four regions for which the activation energies and temperature ranges are estimated. Variations in RL intensity between 67 and 225 K are interpreted as a phase transition in microcline. RL intensity indicates a multi-stage transformation, proceeding via a first step between 67 and ~100 K in which luminescence intensity increases, followed by a region between 100 and ~225 K in which intensity falls. A discontinuity in intensity at ~225 K marks the end of the second region. These variations may result from changes in the bond angles of bridging oxygens. IL dose dependence studies have been performed. As implantation progresses, the red/IR emission profile skews towards short wavelengths, reflecting amorphisation of the structure and local variations in Fe-O bond distances. If the sample recovers for 24 h, the luminescence profile partly returns to its original state, but remains skewed, reflecting partial resumption of short-range order. Such profiles are also observed from natural shocked feldspars, and it may be that ion beam damage is akin to structural modifications associated with impact events. Ion implantation also causes the formation or enhancement of UV, blue and yellow emissions. Recovery of the sample over 24 h causes the UV and blue emissions to be greatly reduced, whereas the yellow emission may be enhanced. Tentative interpretations of this behaviour are presented.     

Melting kinetics of a plagioclase feldspar

Partial melting experiments on plagioclase feldspar have been carried out to investigate textures and kinetics of the melting. A labradorite single crystal was heated at one atmosphere pressure and temperatures within its melting interval as a function of time. So called honeycomb, fingerprint, or sieve textures were produced except for the runs just below the liquidus. The melting was initiated by heterogeneous nucleation of melt at the surface and/or interior (cracks and possively dislocations) of the crystal. The pattern of the melt is dendritic with a few μm arm spacing. After the melt develops throughout the crystal, the volumes of melt and residual crystal become larger and smaller, respectively, without changing the arm spacings. The melt is homogeneous and has the approximate temperature dependent liquidus composition irrespective of the time. There are compositional gradients in the residual crystal after short periods of melting. The An content of the crystals increases with increasing time until it finally reaches equilibrium with the melt after several thousands minutes of heating. It is concluded that the enlargement of the melt, the main process of the melting, is controlled by diffusion in the crystal. The fact that partial melts have the composition of the equilibrium liquidus even from the first several minutes strongly suggests that the local equilibrium at the crystal-liquid interface is satisfied during the melting. Some of the honeycomb, fingerprint, and sieve textures found in xenoliths and phenocrysts of sodic plagioclase in volcanic rocks would be caused by heating events (such as magma mixing) during which temperatures of magmas were temporarily higher than the solidus of some of the minerals.     

Fracture induced emission of alkali atoms from feldspar

Emission of neutral atoms (K and Na) and molecules (H₂O and KOH) observed during fracture of K-feldspar have been accounted for by two independent mechanisms. H₂O and KOH emissions are attributed to the venting of fluid-filled inclusions, while emission of atomic K is due to surface effects accompanying cleavage of crystalline feldspar. The intensity of emitted potassium, at least 6 × 10¹⁴ atoms/cm² of surface area, is sufficient to affect K activities in solution during microbrecciation in the presence of rock-dominated fluids.     

Microstructural evolution during experimental albitization of K-rich alkali feldspar

Crystals of K-feldspar (C2/m), in contact with highly concentrated aqueous NaCl solutions at 500°C and 200 MPa, are pseudomorphically replaced by high albite (C[`1]) (C\bar{1}) as a result of an interface-coupled dissolution/reprecipitation process. The reaction occurs at an extremely sharp reaction front (<10 nm) and involves the complete breakdown of the initial framework structure. This results in the release of tetrahedrally incorporated elements such as Fe³⁺ and Ti⁴⁺ and a significant increase in Si/Al disorder across the reaction interface. The evolving microstructure is controlled by crystallographic relations between the phases. This leads to highly anisotropic, sawtooth-shaped intergrowths of albite and initial K-feldspar, resulting in the least structural misfit between the two framework structures. As a result, the newly formed interfaces appear to be semicoherent, and cracks across the reaction fronts even indicate elastic strain. The reaction produces 2 distinctive albite types (albite-1 and albite-2). Both are polycrystalline, with albite-2 showing significantly larger subgrain sizes. This indicates a secondary coarsening step driven by the reduction in interfacial energy within the polycrystalline replacement product. The reaction also produces a highly porous rim. However, the porosity is not evenly distributed resulting in a porous albite-1 and a non-porous albite-2 that mostly surrounds large, euhedral pores. Despite the substantial volume fraction of porosity in albite-1, no significant 3D interconnectivity could be detected, making the presence of a pervasive porosity unlikely. However, the result of coarsening is the continuous modification of the 3D porosity distribution. This could potentially provide a mechanism for fluid transport through the replacement rim until textural and chemical equilibration is achieved.     

The coarsening kinetics associated with exsolution in an iron-free clinopyroxene

The coarsening kinetics of exsolution lamellae have been determined for an initial composition of 0.541 CaMgSi₂O₆–0.459 Mg₂Si₂O₆ (Di_54.1) at 1300 °, 1200 ° and 1100 ° C. Portions of this material were annealed for varying lengths of time, and the average wavelength () was obtained from measurements made by utilizing transmission electron microscopy (TEM). All experiments were conducted within the (001) coherent spinodal for this system (McCallister and Yund, 1977), and the lamellae are coherent up to and including the largest wavelength observed, 1054 Å.At all three temperatures mentioned previously the data are consistent with the following relationship: The activation energy for the process, as determined from the respective kinetic constants (k), is 99±2 Kcal/mol. This value is similar to that found for Ca self diffusion in pseudo-wollastonite and wollastonite, and suggests the possibility that the diffusion of Ca may be controlling the rate of coarsening.A comparison of exsolution microstructures in the synthetic samples with those observed in clinopyroxenes found in kimberlites and with similar Ca/Ca + Mg indicates that the latter cooled rapidly to the effective quench temperature.     

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.     

石英脉型钨矿床中云英岩析离体及岩浆液态分异成矿研究——以湖南瑶岗仙钨矿床为例

云英岩析离体(包体)常见于南岭地区石英脉型(黑)钨矿床的成矿花岗岩体顶部附近,这些析离体多呈不规则椭球状,与寄主岩石花岗岩围岩界线明显,无与外面连通的脉体或构造,分布无规律。在湖南瑶岗仙钨矿床,云英岩析离体赋存于瑶岗仙岩体主体的灰色斑状碱长花岗岩Ⅰ中,大部分有分带,外带为云英岩化碱长花岗岩,富含辉钼矿,最高达5%以上,钠长石An平均为0.64;内核部分为云英岩,主要矿物组合为白云母、石英、萤石、黄玉、黑钨矿等,与黑钨矿石英脉相同,其黑钨矿含量最高达20%以上,属热液沉淀形成。云英岩析离体整体呈现出浆液过渡态流体作用的特征,是富Li-F碱长花岗岩浆液态分异作用的产物。与碱长花岗岩围岩相比,析离体中富集金属元素W、Mo、Bi及Li、Rb等元素,相对贫Co、Ni、Sr、Ba等元素,Cu、Pb、Zn的含量变化不明显。这种元素富集规律与区域岩浆演化趋势基本一致,显示岩浆晚期液态分异形成浆液过渡态流体是W等成矿元素富集的重要机制,但这一机制并不会导致Pb、Zn的富集,这一点不同于矽卡岩型钨矿床。石英脉型(黑)钨矿床的成矿地质体是灰色斑状碱长花岗岩Ⅰ,云英岩析离体是该岩体岩浆液态分异的结果,可以作为识别碱长花岗岩含矿性的重要标志(寻找石英脉型钨矿矿床)。     

Kalgan volcanic complex: optical properties of its alkali feldspar phenocrysts

The Mesozoic Kalgan volcanic complex is distributed around the city of Kalgan in an area of about 1000–1200 km² and contains a great variety of rocks, among which are basalts, augite clacites, quartz-trachytes, quartz porphyry, rhyolite and various kinds of glassy rocks, tuffs and other volcanic ejecta. These diverse forms of volcanic rock impart the splendid geomorphological features. Six extinct craters were found, all of the central type; positions of eruptive centers were shifted from the southeast of the area in the early period to the northwest later. Two vertical sections of volcanic bodies of quartz porphyry and rhyolite were chosen to study precisely the optical angles of feldspars from each specimen. Results showed that there are different variation features of 2V between the basal, intermediate and top portions of volcanic bodies. Values of 2V are smaller, and of 2Vd, greater in the basal and top portions than those in the intermediate portions. Two main varieties of alkali feldspar occur in the Kalgan volcanic rocks. One is high temperature variety-low sanidine (–)2V=26°?40°, distributed mainly in the rhyolite and glassy rocks; another is the intermediate temperature variety-high orthoclase (–)2V=40°–62° , found mostly in the quartz porphyry. But their compositions are all in the intermediate range of K-Na feldspar series. Variations of the optical angles of alkali feldspar in the Kalgan complex proved that 2V is a sensitive optical constant of petrogenesis. Its variation is not only shown in one and the same extrusive or intrusive body, but also in one and the same specimen of rocks or crystal, with some definite regulations. – Author.     

A model of water allocation in alkali feldspar,derived from infrared-spectroscopic investigations

Polarizedinfrared (IR) spectra of sanidine crystals from Volkesfeld, Eifel show the existence of two broad pleochroic absorption bands at 3,400 and 3,050 cm^?1. Because overtones near 5,150 cm^?1 were observed, the former bands are assigned to OH stretching frequencies of H₂O molecules. On the basis of the pleochroic scheme of the bands it is proposed that H₂O molecules occur as structural constituents entering theM site of the sanidine structure; the plane of the H₂O molecules lies parallel to the symmetry plane.     

广西栗木钨锡稀有金属矿床碱长花岗岩的厘定

广西栗木钨锡稀有金属矿床内的花岗岩传统定名为中粒斑状铁白云母花岗岩和钠长石花岗岩.电子探针分析表明,栗木两类花岗岩中的钠长石An值均小于5,95％以上的样品An＜2,均属于碱性长石.显微镜下工作说明,矿物结晶顺序为钠长石→石英+钾长石→黄玉→云母,钠长石是岩浆演化过程中最早结晶的产物,并非钠长石化的结果.在QAP投影中,两类花岗岩落入QA线上的碱长花岗岩区域.故两类花岗岩的确切定名应为中粗粒铁锂云母碱长花岗岩(Ⅰ)和细粒碱长花岗岩(Ⅱ).     

Myrmekite associated with alkali feldspar megacrysts in felsic rocks from New South Wales

Comparison of myrmekites in undeformed and deformed ‘porphyritic’ felsic rocks shows that they have different morphologies and spatial distributions within their megacrysts. In particular, plagioclase inclusions in the undeformed megacrysts carry myrmekite rims which may be explained by solid-state exsolution, as may the quartz blebs in associated perthitic plagioclase. Myrmekite at the rims of the megacrysts may be due to an interaction between exsolution and metasomatic processes. Stress concentrated at the margins of pre-existing megacrysts in the deformed rocks and the occurrence here of random non-coherent plagioclase nuclei will explain the preponderance of myrmekite at this location.     

南岭锡钨多金属矿区碱长花岗岩的厘定及其意义

南岭地区锡钨多金属矿成矿作用主要与燕山早期花岗岩岩浆活动有关,这些花岗岩一直被认为是黑云母花岗岩、二长花岗岩、细粒二长花岗岩等,少量矿区岩体顶部出现钠长石花岗岩。通过对湖南瑶岗仙钨矿岩体的深入解剖,以及对杮竹园、黄沙坪、锡田、邓阜仙、栗木、梅子窝等矿区花岗岩全面的岩矿鉴定和电子探针分析,确定钨矿区致矿花岗岩的长石主要为碱性长石,其中绝大部分样品中钠长石An<5,因此确定这些花岗岩均属于碱长花岗岩。与成矿有关的由浆液过渡态流体形成的云英岩包体中的钠长石更加富Na,An<3。碱长花岗岩的成分以及钠长石成分在岩体顶部约1 000 m深度范围内无明显垂向变化。致矿花岗岩体中部分早期花岗岩包体、晚期花岗斑岩,部分花岗岩基以及印支期花岗岩、加里东期花岗闪长岩等,钠(斜)长石An值明显高,很多属更长石、中长石甚至基性长石。这种包含有两种碱性长石的碱长花岗岩由富挥发分的岩浆形成,在岩体顶部附近广泛发生液态不混溶作用,是导致锡钨多金属矿富集成矿的主要分异方式。钨锡矿区碱长花岗岩钠长石An值明显低于区域大花岗岩基或不致矿花岗岩,可作为花岗岩的钨锡成矿评价标志之一。