超临界流体的快速泄出:来自霓辉石英脉的证据

河南外方山地区熊耳群火山岩中新发现了一种霓辉石英脉群,脉表现出独特的矿物组合及晶体特征,如霓辉石英脉具有复杂的矿物组成,表现出强烈的不平衡特点;霓辉石英脉具明显的对称分带特征,各分带中矿物组成及特征显著不同;存在具熔蚀边结构的钾长石-霓辉石集合体团块;霓辉石具在明显核幔结构,内核化学成分与幔边部存在较大差异;钾长石晶体元素分布存在似"文象"结构,表现出Fe元素的熔体分离特征;脉中矿物普遍发育环带结构;霓辉石英脉交代致密火山岩围岩形成蚀变带,新生大量榍石、霓辉石等矿物且含量随着与脉壁距离的增加急剧减少,显示出强烈的流体/气体扩散交代作用.霓辉石英脉独特的矿物组合及晶体特征反映了其成因的复杂性以及生长环境的变化.根据霓辉石英脉矿物的特点,推测其源于流体、熔体和晶体的混合物,并经历了快速侵位、快速冷却的过程,因而能够保存流体演化各阶段的结晶信息,保留不同成因的晶体群.外方山霓辉石英脉矿物可划分为捕虏晶亚群、熔体晶体群、超临界流体亚群、热液晶体亚群和凝聚晶体亚群等.外方山霓辉石英脉矿物晶体群的深入研究可能有助于揭示岩浆熔体向流体转换的机制及侵位机制,进而可揭示岩浆-流体成矿作用.     

Orthopyroxene oikocrysts in the MG1 chromitite layer of the Bushveld Complex: implications for cumulate formation and recrystallisation

Two typical mineral textures of the MG 1 chromitite of the Bushveld Complex, South Africa, were observed; one characterised by abundant orthopyroxene oikocrysts, and the other by coarse-grained granular chromitite with only minor amounts of interstitial material. Oikocrysts form elongate clusters of several crystals aligned parallel to the layering, and typically have subhedral, almost chromite-free, core zones containing remnants of olivine. The core zones are surrounded by poikilitic aureoles overgrowing euhedral to subhedral chromite chadacrysts. Chromite grains show no preferred crystal orientation, whereas orthopyroxene grains forming clusters commonly share the same crystallographic orientation. Oikocryst core zones have lower Mg# and higher concentrations of incompatible trace elements compared to their poikilitic aureoles. Core zones are relatively enriched in REE compared to a postulated parental magma (B1) and did not crystallise in equilibrium with the surrounding minerals, whereas the composition of the poikilitic orthopyroxene is consistent with growth from the B1 magma. These observations cannot be explained by the classic cumulus and post-cumulus models of oikocryst formation. Instead, we suggest that the oikocryst core zones in the MG1 chromitite layer formed by peritectic replacement of olivine primocrysts by reaction with an upwards-percolating melt enriched in incompatible trace elements. Poikilitic overgrowth on oikocryst core zones occurred in equilibrium with a basaltic melt of B1 composition near the magma-crystal mush interface. Finally, adcumulus crystallisation followed by grain growth resulted in the surrounding granular chromitite.     

Natural staurolite breakdown reactions at moderate to high pressures

Existing staurolite breakdown reactions are briefly discussed and examples of each of the main types suggested from textural evidence from different well studied areas are analyzed. From the staurolite textural data, it is shown that: the pseudomorphic replacement is generally equal area; the replacement is usually by a single mineral or, where a number occur, one volumetrically greatly predominates; and successive reaction coronas exist around a single reactant grain. The analogy between the textural coronas and “monomineralic zones” at the contact of dissimilar rock types is drawn with the subsequent inference that reaction coronas in some, if not most, cases are out of equilibrium with the rest of the assemblage as well as the reactant mineral. In staurolite reaction textures, different grains in a single thin section normally have: different amounts (ratios) of product minerals, different product minerals, some grains which have reacted while others have not and different grains which have reacted to different degrees. Traverses of essentially total chemical analyses across staurolite grains in two test sections showed that considerable intertextural chemical variation in single thin sections exist which is much greater than the chemical variations within textures. Variations of, from 0.59 to 1.86 weight percent ZnO in different homogeneous staurolite grains occur in a single thin section (Z-5670911) and measurable variations of Si, Al, Fe, Mg and Mn are recorded. In all the textures analyzed both Zn and Al was deficient on the product side of balanced reactions, and particularly Zn can be shown to have been lost from the area of the section. The chemical and textural conclusions show that the limits of equilibrium systems, from which metamorphic reactions can be inferred, must be confined to the boundaries of the pseudomorphic texture, which means that ionic (complex bearing ?) species are involved and these systems must be treated as being open.     

Constraints on diamond genesis from the study of the dependence of diamond properties on the composition of kimberlites and lamproites

It was shown that the crystal morphology, content, and size of diamonds depend on the concentrations of silica, magnesium, calcium, and carbon dioxide in the host kimberlites and lamproites. The character of this dependence suggests that the viscosity of the initial melts of these rocks was the main control of the morphology and properties of diamond crystals and indicates a magmatic genesis for this mineral. Two genetic varieties of diamond crystals were distinguished: larger residual grains coeval with the formation of the sources of kimberlite and lamproite magmas during the slow high-pressure fractionation of the near-bottom peridotite layer of the global magma ocean and smaller early magmatic grains, which crystallized during the decompression-friction transformation of kimberlite and lamproite protoliths into magmas.     

“偏高岭石—碱—水”体系中4A沸石晶体生长规律及其机理探讨

煅烧高岭土与氢氧化钠的水热反应体系中，偏高岭石不仅为４Ａ沸石的形成提供了物质基础，而且为晶核的形成提供了有利界面。４Ａ沸石的晶核总是优先形成于发生了溶解的偏高岭石表面，而晶体生长的方式则受偏高岭石颗粒大小的影响。随着晶体的大量生长，偏高岭石逐渐消失，与此同时，产生了二次成核作用，晶体在先成的４Ａ沸石晶体表面生长，形成双晶。在整个反应过程中，液相中的［Ａｌ２Ｏ３］及［ＳｉＯ２］浓度的变化与４Ａ沸石晶体生成具有一定对应关系。这表明，偏高岭石不断溶解，固相的Ａｌ２Ｏ３·２ＳｉＯ２不断转化为液相的Ａｌ２Ｏ３和ＳｉＯ２。沸石晶体的生成是以液相各组分浓度的过饱和为动力的，为液相生成机理     

In situ characterization of dolomite crystals: Evaluation of dolomitization process and its effect on zoning

Dolomitization is a dissolution–re‐precipitation process which proceeds via micrometre‐scale interface reaction horizons. However, the effect of this fundamental local medium of replacement process on precipitated dolomite crystals is rarely investigated. For this purpose, dolomite samples of Ordovician Boat Harbour Formation carbonates (Newfoundland, Canada) from Main Brook and Daniel's Harbour (about 150 km apart) were studied. The investigation used high‐resolution approaches to carry out imaging and elemental analyses of individual dolomite crystals, including scanning electron microscopy, scanning electron microscopy‐based cathodoluminescence, secondary ion mass spectrometry and electron microprobe analysis. The purpose was to better understand geochemical variations across the crystal traverses and their association with dolomite replacement processes and conditions. The scanning electron microscopy‐based cathodoluminescence‐zoning character reveals three major crystal facies. Distinctly (planar boundaries) zoned core and indistinctly zoned core are zoned dolomite crystal facies in burial dolomite from Main Brook. Relatively unzoned crystal facies was identified in that of Daniel's Harbour. Compositionally zoned dolomite crystals are characterized by decreasing Sr, Na, Y and ∑REE but increasing Mn and Fe from core to rim sections. ‘Core to rim’ zoning was not found for major (Mg and Ca) elements. The zoning exhibited by the replacive dolomite crystals is interpreted to be, principally, a result of limited exchange between solutes of the bulk pore fluid and those of dissolving precursor marine carbonate. In tandem with petrographic features, the systematic compositional zoning character demonstrates that the effectiveness of the dolomitization process grades from indistinctly zoned core to relatively unzoned crystal facies via interface reaction horizons. Homogenization of major elements of core and rim sections, seen in distinctly zoned core crystal facies and to a much greater extent in relatively unzoned crystal facies, via a secondary reaction interface, is the first recognized stage of the ‘maturation process’ followed by homogenization of the trace elements seen largely in relatively unzoned crystal facies.     

Schlieren formation in diatexite migmatite: examples from the St Malo migmatite terrane, France

Schlieren are trains of platy or blocky minerals, typically the ferromagnesian minerals and accessory phases, that occur in granites and melt‐rich migmatites, such as diatexites. They have been considered as: (1) unmelted residue from xenoliths or the source region; (2) mineral accumulations formed during magma flow; (3) compositional layering; and (4) sites of melt loss. In order to help identify schlieren‐forming processes in the diatexites at St Malo, differences in the size, shape, orientation, distribution and composition of the biotite from schlieren and from their hosts have been investigated. Small biotite grains are much less abundant in the schlieren than in their hosts. Schlieren biotite grains are generally larger, have greater aspect ratios and have, except in hosts with low (< 10%) biotite contents, a much stronger shape preferred orientation than host biotite. The compositional ranges of host and schlieren biotite are similar, but schlieren biotite defines tighter, sharper peaks on composition‐frequency plots. Hosts show magmatic textures such as imbricated (tiled), unstrained plagioclase. Some schlieren show only magmatic textures (tiled biotite, no crystal‐plastic strain features), but many have textures indicating submagmatic and subsolidus deformation (e.g. kinked grains) and these schlieren show the most extensive evidence for recrystallization. Magmas at St Malo initially contained a significant fraction of residual biotite and plagioclase crystals; smaller biotite grains were separated from the larger plagioclase crystals during magma flow. Since plagioclase was also the major, early crystallizing phase, the plagioclase‐rich domains developed rapidly and reached the rigid percolation threshold first, forcing further magma flow to be concentrated into narrowing melt‐rich zones where the biotite had accumulated, hence increasing shear strain and the degree of shape preferred orientation in these domains. Schlieren formed in these domains as a result of grain contacts and tiling in the grain inertia‐regime. Final amalgamation of the biotite aggregates into schlieren involved volume loss as melt trapped between grains was expelled after the rigid percolation threshold was reached in the biotite‐rich layers.     

A method for extending a general constitutive model to consider the electro-chemo-mechanical phenomena of mineral crystals in expansive soils

The mechanical behavior of expansive soils varies according to the chemical composition of the pore fluid. It is well known that electrochemical phenomena on the surface of clay mineral crystals considerably affect their macrostructural behavior. In particular, a change in the pore fluid composition causes osmotic consolidation or swelling. In this study, a model is constructed to describe the characteristic behavior of expansive soils by coupling the interlaminar behavior of clay mineral crystals and the soil skeleton behavior. The interlaminar behavior is derived from the electro-chemo-mechanical coupling equilibrium of mineral crystals, and the soil skeleton behavior is given by a general elastoplastic constitutive model for soils. This modeling approach extends a general model to consider the electro-chemo-mechanical phenomena of mineral crystals. Oedometer tests with the replacement of the cell fluid and the swelling pressure and deformation tests on expansive clays are simulated with the proposed model. The simulation results indicate that the proposed method can reasonably represent the typical behavior of expansive soils.     

危地马拉与缅甸含绿辉石翡翠的矿物学对比研究

以危地马拉蓝水料与缅甸油青种翡翠为研究对象,通过显微镜观察、岩石薄片观察、X射线粉末衍射分析、激光拉曼光谱分析、扫描电镜分析、电子探针分析等测试手段对两者进行了常规宝石学特征、结构特征、杂质矿物、化学成分等方面的对比研究。结果表明,危地马拉蓝水料发育等粒变晶结构,主要成分为硬玉,含有绿辉石,硬玉环带以2～3层为主,绿辉石分别以自形颗粒、交代残余、沿微裂隙充填3种形式存在,其中白色点状物为钠长石,绿色点状物为霓辉石和绿辉石的混杂物;缅甸油青种翡翠以硬玉为主颗粒相对粗大,呈柱粒状镶嵌结构,硬玉发育3～5层环带结构,绿辉石以脉状充填硬玉颗粒间隙或被硬玉颗粒交代呈孤岛状。危地马拉蓝水料硬玉颗粒与脉状绿辉石的CaO含量均高于缅甸油青种翡翠,另外由于霓辉石的存在,推测两产地翡翠结晶环境中的Ca、Fe含量可能存在差异。     

Oxygen isotope heterogeneity and disequilibria of olivine crystals in large volume Holocene basalts from Iceland: Evidence for magmatic digestion and erosion of Pleistocene hyaloclastites

This work considers petrogenesis of the largest Holocene basaltic fissure eruptions of Iceland, which are also the largest in the world: Laki (1783-84 AD, 15 km³), Eldgjá (934 AD, 18 km³), Veidivötn (900, 1480 AD, multiple eruptions, >2 km³), Núpahraun (ca. 4000 BP, >1 km³) and Thjórsárhraun (ca 8000 BP, >20 km³). We present oxygen isotope laser fluorination analyses of 55 individual and bulk olivine crystals, coexisting individual and bulk plagioclase phenocrysts, and their host basaltic glasses with average precision of better than 0.1‰ (1SD). We also report O isotope analyses of cores and rims of 61 olivine crystals by SIMS with average precision on single spots of 0.24‰ (1SD) in 13 samples coupled with electron microprobe data for major and trace elements in these olivines. Within each individual sample, we have found that basaltic glass is relatively homogeneous with respect to oxygen isotopes, plagioclase phenocrysts exhibit crystal to crystal variability, while individual olivines span from the values in equilibrium with the low-δ¹⁸O matrix glass to those being three permil higher in δ¹⁸O than the equilibrium. Olivine cores with maximum value of 5.2‰ are found in many of these basalts and suggest that the initial magma was equilibrated with normal-δ¹⁸O mantle. No olivines or their intracrystalline domains are found with bulk or spot value higher than those found in MORB olivines. The δ¹⁸O variability of 0.3-3‰ exists for olivine grains from different lavas, and variable core-to-rim oxygen isotopic zoning is present in selected olivine grains. Many olivines in the same sample are not zoned, while a few grains are zoned with respect to oxygen isotopes and exhibit small core-to-core variations in Fe-Mg, Ni, Mn, Ca. Grains that are zoned in both Mg# and δ¹⁸O exhibit positive correlation of these two parameters. Electron microprobe analysis shows that most olivines equilibrated with the transporting melt, and thin Fe-richer rim is present around many grains, regardless of the degree of olivine-melt oxygen isotope disequilibrium.The preservation of isotopic and compositional zoning in selected grains, and subtle to severe Δ¹⁸O (melt-olivine) and Δ¹⁸O (plagioclase-olivine) disequilibria suggests rather short crystal residence times of years to centuries. Synglacially-altered upper crustal, tufaceous hyaloclastites of Pleistocene age serve as a viable source for low-δ¹⁸O values in Holocene basalts through assimilation, mechanical and thermal erosion, and devolatilization of stoped blocks. Cumulates formed in response to cooling during assimilation, and xenocrysts derived from hyaloclastites, contribute to the diverse δ¹⁸O crystalline cargo. The magma plumbing systems under each fissure are likely to include a network of interconnected dikes and sills with high magma flow rates that contribute to the efficacy of magmatic erosion of large quantities (10-60% mass) of hyaloclastites required by isotopic mass balance.Olivine diversity and the pervasive lack of phenocryst-melt oxygen isotopic equilibrium suggest that a common approach of analyzing bulk olivine for oxygen isotopes, as a proxy for the basaltic melt or to infer mantle δ¹⁸O value, needs to proceed with caution. The best approach is to analyze olivine crystals individually and demonstrate their equilibrium with matrix.     

Auger Nanoprobe analysis of presolar ferromagnesian silicate grains from primitive CR chondrites QUE 99177 and MET 00426

We have investigated the presolar grain inventories of two CR chondrites, QUE 99177 and MET 00426, which are less altered than most members of this meteorite group. Both meteorites contain high abundances of O-anomalous presolar grains, with concentrations of 220 ± 40 and 160 ± 30 ppm for QUE 99177 and MET 00426, respectively. The presolar grain inventories are dominated by ferromagnesian silicates with group 1 oxygen isotopic compositions, indicative of origins in low mass red giant or asymptotic giant branch stars. Grains with pyroxene-like compositions are somewhat more common than those with olivine-like compositions, but most grains are non-stoichiometric with compositions intermediate between these two phases, consistent with recent work suggesting that amorphous interstellar silicates have stoichiometries between olivine and pyroxene type silicates. Although structural data are not available, one grain contains only Si and O, and has a stoichiometry consistent with SiO₂.Our presolar grains are much more Fe-rich than predicted by astronomical observations. Although secondary alteration may play a role in enhancing the Fe contents of presolar grains, it seems unlikely that the large and ubiquitous Fe enrichments observed in the grains from this study can be due only to secondary processing, particularly given the highly primitive nature of these two meteorites. Grain condensation in the stellar outflows where these grains formed likely proceeded under rapidly changing kinetic conditions that may have enhanced the incorporation of Fe into the grains over that expected based on equilibrium condensation theory.Both QUE 99177 and MET 00426 appear to contain unusually low abundances of oxide grains and have higher silicate/oxide ratios than other primitive meteorites analyzed to date. We explore various possibilities for this discrepancy, but note that most scenarios are not likely to result in the preferential destruction of oxides relative to silicates. Thus, the highest silicate/oxide ratios, such as those observed in the CR chondrites, should reflect the true initial proportions of presolar silicate and oxide grains in the parent molecular cloud from which the solar nebula evolved.     

储层孔隙系统“水—岩”反应结晶动力学研究进展

“水—岩”反应作为储层成岩作用研究的重要组成部分,其研究结果对解释储层非均质性成因机制、综合评价储层品质等方面都具有重要意义。近年来随着基础理论、分析测试手段、物理实验方法和模拟技术的进步,该领域研究取得了诸多进展。成岩物理模拟实验的进步实现了对流体—围岩/矿物体系“水—岩”作用过程的宏观尺度观测和研究,也明确了各物理化学参数对体系内矿物溶解—结晶过程的影响,但分辨率限制了其在微观（纳米）尺度解释许多现象成因机制和约束条件方面的应用。晶体生长理论的建立和发展为解释储层孔隙系统中“水—岩”作用过程的结晶动力学原理奠定了基础,特别是近年来伴随纳米科技而发展起来的各种在线和非在线测量技术大大提高了储层孔隙系统结构和内部晶体生长情况的观测精度,将“水—岩”作用研究分辨率提升至纳米级,这为从微观尺度了解孔隙系统中流体—矿物体系的溶解—结晶（沉淀）相平衡过程及其控制因素提供了方案。储层孔隙系统中流体的结晶动力学行为与孔隙介质和流体性质密切相关,是成核自由能、矿物表面电化学特征、传质速率等多因素综合作用的结果。对不同成岩环境和孔隙系统中孔隙流体结晶动力学行为的系统研究,有助于了解孔隙系统空间结构、底衬表面化学特性、表面能效应等对矿物晶体成核生长与溶解过程的影响,为进一步深入解释储层非均质性的结晶动力学原理奠定了理论基础。     

贵州碳酸盐岩风化壳物源判别的新证据:石英颗粒形态、表面结构特征

利用扫描电镜分析了黔北、黔东、黔中4个剖面土样中的石英颗粒形态和表面特征,发现土样中的石英有3种类型:碳酸盐岩中的原生碎屑石英,呈次圆状或圆状,表面有被长时间、长距离水搬运的痕迹,无风成特征;燧石团块石英,具有新鲜的表面,呈不规则的尖棱状、次棱角状,表面基本无搬运痕迹;晶形较完整的次生石英,呈棱角状,晶形较完整,晶面有微生长晶体,无搬运特征。这些信息指示,3种石英均具有原位特征,前两种石英是直接对基岩的继承,第三种石英是风化壳剖面的次生产物,该结果为碳酸盐岩风化壳的物源及原位特征的确认提供了新证据。     

The time scales of magma mixing and mingling involving primitive melts and melt–mush interaction at mid-ocean ridges

We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between mush disaggregation and eruption was ≤1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts.     

Experimental study of syntaxial vein growth during lateral fluid flow in transmitted light: first results

In-situ observations of transmitted-light experiments of syntaxial vein growth during lateral fluid flow in a simulated fracture show a decrease in growth rate towards the downstream end. As a consequence, the fracture is sealed at the inlet. Our observations show the rapid, non-linear growth competition of grains, which is a result of anisotropic growth kinetics and the complex fluid flow around the individual crystals.We compared our results with simulations based on a simplified numerical model, which builds on existing simulation techniques of this system. The model incorporates plug flow and a fluid-flow velocity dependent crystal growth rate law based on literature data. Simulations are in reasonable agreement with experimental results. A sensitivity analysis shows that a high fluid flow velocity and a low supersaturation increase the potential to seal a vein homogeneously, in agreement with previous work. Additionally, the effect of the initial crack shape is explored.     

Reaction mechanism for the replacement of calcite by dolomite and siderite: implications for geochemistry, microstructure and porosity evolution during hydrothermal mineralisation

Carbonate reactions are common in mineral deposits due to CO₂-rich mineralising fluids. This study presents the first in-depth, integrated analysis of microstructure and microchemistry of fluid-mediated carbonate reaction textures at hydrothermal conditions. In doing so, we describe the mechanisms by which carbonate phases replace one another, and the implications for the evolution of geochemistry, rock microstructures and porosity. The sample from the 1.95 Moz Junction gold deposit, Western Australia, contains calcite derived from carbonation of a metamorphic amphibole—plagioclase assemblage that has further altered to siderite and dolomite. The calcite is porous and contains iron-rich calcite blebs interpreted to have resulted from fluid-mediated replacement of compositionally heterogeneous amphiboles. The siderite is polycrystalline but nucleates topotactically on the calcite. As a result, the boundaries between adjacent grains are low-angle boundaries (<10°), which are geometrically similar to those formed by crystal–plastic deformation and recovery. Growth zoning within individual siderite grains shows that the low-angle boundaries are growth features and not due to deformation. Low-angle boundaries develop due to the propagation of defects at grain faces and zone boundaries and by impingement of grains that nucleated with small misorientations relative to each other during grain growth. The cores of siderite grains are aligned with the twin planes in the parent calcite crystal showing that the reactant Fe entered the crystal along the twin boundaries. Dolomite grains, many of which appear to in-fill space generated by the siderite replacement, also show alignment of cores along the calcite twin planes, suggesting that they did not grow into space but replaced the calcite. Where dolomite is seen directly replacing calcite, it nucleates on the Fe-rich calcite due to the increased compatibility of the Fe-bearing calcite lattice relative to the pure calcite. Both reactions are interpreted as fluid-mediated replacement reactions which use the crystallography and elemental chemistry of the calcite. Experiments of fluid-mediated replacement reactions show that they proceed much faster than diffusion-based reactions. This is important when considering the rates of reactions relative to fluid flow in mineralising systems.     

GASEOUS-LIQUID INCLUSIONS IN CRYSTALS OF ICELAND SPAR

Study of inclusions in Iceland spar crystals might furnish important data on the composition of mother liquors, temperature of mineral formation, and, for industrial purposes, insight into the problem of optical defects in crystals. Two basic types of inclusion were distinguished according to time of formation and orientation, relative to the host crystal: primary inclusions (seven types were recognized), originated as solution particles captured and oriented during crystal formation; secondary inclusions (including four types) indicate solution entrance through fissures or adherence to twinning surfaces. Inclusions vary in size from near-submicroscopic to 30 millimeters, and in shape, from pronounced irregularity to crystalline. Temperatures of mineral formation, established at several sites, ranged from a minimum of 27° to as high as 200°C. Generally, crystals of superior optical quality were formed at relatively high temperatures. Analysis of the gaseous liquid contents shows from 1 to not over 8 percent of the volume is occupied by gas. Mineral-forming solutions are highly concentrated (386 grams per millimeter), weakly alkaline, and composed of chlorides and other salts. --D. D. Fisher.     

Ontogenetic analysis of individual olivine grains in ultramafic rocks

The morphology and internal structure of individual olivine grains from ultramafic rocks in the Guli and Gal’moenan dunite massifs differing in origin are considered. To restore the ontogeny of mineral aggregates, traces of elastic deformation retained in mineral grains have been used. Comparison of anatomy of olivine grains from these two massifs showed that the mechanism of accommodation of rocks to changing geological settings is expressed as the response of the mineral aggregate structure and variation in the anatomy of individual mineral grains. At the level of individual grains, this is annihilation of older defects and origination of younger dislocations; refinement of the crystal lattice; exsolution; formation and transformation of new mineral phases; and creep and migration of subboundaries within grains. At the aggregate level, this is rotation and migration creep of the internal boundaries of rock; formation of new boundaries of mineral intergrowths; reorientation of boundaries; and variation in their extent, density, and grain dimensions. The prehistory of massifs controls the manifestation and abundance of various elastic deformations and related types of recrystallization of olivine grain boundaries and subboundaries in aggregates. New conditions and accommodation of mineral aggregates to these conditions have instigated specific schemes of recrystallization, which bear information on the history of rocks and their massifs.     

Circumgranular bassanite in a gypsum crust from eastern Algeria – a potential palaeosurface indicator

Detrital sand grains are surrounded by thin bassanite coatings in the upper part of a coarse-crystalline gypsum crust from the Algerian Sahara. The bassanite developed by topotactic replacement of the surrounding gypsum in the absence of a liquid phase. Heating experiments using a gypsum crystal with sand inclusions produced similar patterns and textures. In one experiment, bassanite developed around quartz and carbonate grains but not along the sides of the heated gypsum crystal. This is the result of differences in heat capacity between gypsum, quartz and calcite. Bassanite formation in the crust from Algeria was not controlled by differences in thermal properties. Instead, the bassanite apparently formed under conditions of thermal equilibrium. The occurrence of bassanite as circumgranular coatings in the crust is interpreted as being related to the availability of submicroscopic space along the contact between the gypsum cement and the enclosed sand grains. The presence of coatings of this type, or derived relict features, is a potential criterion for the recognition of palaeosurfaces.     

Radiaxial fibrous calcite: a replacement after acicular carbonate

Radiaxial fibrous calcite, a common cavity fill in ancient limestones, is characterized by curved twin lamellae, optic axes that converge away from the substrate and subcrystals which diverge in this same direction. The optic axes radiate about three or four axes located in positions between adjacent crystals and orientated parallel with the crystal elongation. The crystals are commonly turbid with inclusions, which may be concentrated along twin lamellae and subcrystal and inter-crystalline boundaries, or form zones parallel to the substrate. Some inclusion patterns reveal the position of former crystal faces. From a consideration of the occurrences, gross morphological characters, the fabrics of the crystals and the inclusion patterns, radiaxial fibrous calcite is interpreted as a replacement of an early diagenetic acicular cement, composed of interfering bundles of radiating crystals. It is suggested that replacement takes place by a solution-precipitation process and the migration of a fluid film through the acicular host, with replacement occurring most rapidly between bundles of acicular carbonate. The optic axis pattern of the fibrous calcite is considered to be inherited from the c-axis orientations of the host acicular crystals. Fibrous calcite intercrystalline boundaries form as fractures after replacement. Some inclusion patterns record the characters of the replaced acicular cement; others, formed of impurities reorganized during the replacement, reveal the form of the replacement front.