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.     

陕西洛南县蓟县系洛南群层控玉化硅质岩特征、成因及意义*

华北板块南缘陕西洛南县北部中元古界蓟县系洛南群巡检司组顶部层控玉化硅质岩中,发育似层状硅质玉髓及透镜状燧石,这些燧石及硅质玉髓已经玉化,质细色艳,达到高品级石英岩质玉石级别,已申报为国家宝玉石矿种,并在宝玉石市场崭露头角,显示出巨大的经济开发潜力。作者以岩石学及地球化学分析为手段,研究该层控玉化硅质岩特征、成因及意义。研究表明: (1)层控玉化硅质岩赋存于中元古界蓟县系洛南群巡检司组顶部的紫红色、灰绿色薄层状砂泥质板岩层中,层厚一般0.5~4 m;似层状玉髓多呈紫红色、灰绿色、烟灰色和无色等,透镜状燧石多呈紫红色及灰绿色似玛瑙纹圈层状。(2)硅质玉髓主要成分为石英,隐晶、微晶结构,块状构造,质地细腻、坚硬,玻璃光泽,半透明—微透明;其Al/(Al+Fe+Mn)均值为0.62,Zr/Hf均值为34.83,Sr/Ba均值除1个异常点外为0.50,V/Ni均值为0.37,ΣREE均值为5.11,δCe均值为1.02,δEu均值为0.86,La_N/Ce_N均值为0.85,总体显示淡水沉积环境,硅质来源主要为陆源来源,也有后期热液作用来源的部分加入。(3)似层状硅质玉髓层是由于快速海退引起的淡水淋滤硅化作用形成的海底硬底构造或硅质壳;似玛瑙纹透镜状燧石及彩色似层状硅质玉髓层是由于沉积期及沉积期后成岩作用氧化还原条件变化,及后期热液侵入烘烤引起的紫色与绿色致色矿物或致色元素转变形成;角砾状燧石是由于后期秦岭强烈构造作用,引起似层状硅质玉髓上下岩层因软硬差异而产生的变形破裂作用形成的构造角砾岩。     

陕西洛南县蓟县系洛南群层控玉化硅质岩特征、成因及意义

华北板块南缘陕西洛南县北部中元古界蓟县系洛南群巡检司组顶部层控玉化硅质岩中,发育似层状硅质玉髓及透镜状燧石,这些燧石及硅质玉髓已经玉化,质细色艳,达到高品级石英岩质玉石级别,已申报为国家宝玉石矿种,并在宝玉石市场崭露头角,显示出巨大的经济开发潜力。作者以岩石学及地球化学分析为手段,研究该层控玉化硅质岩特征、成因及意义。研究表明:(1)层控玉化硅质岩赋存于中元古界蓟县系洛南群巡检司组顶部的紫红色、灰绿色薄层状砂泥质板岩层中,层厚一般0.5～4 m;似层状玉髓多呈紫红色、灰绿色、烟灰色和无色等,透镜状燧石多呈紫红色及灰绿色似玛瑙纹圈层状。(2)硅质玉髓主要成分为石英,隐晶、微晶结构,块状构造,质地细腻、坚硬,玻璃光泽,半透明—微透明;其Al/(Al+Fe+Mn)均值为0.62,Zr/Hf均值为34.83,Sr/Ba均值除1个异常点外为0.50,V/Ni均值为0.37,ΣREE均值为5.11,δCe均值为1.02,δEu均值为0.86,La_N/Ce_N均值为0.85,总体显示淡水沉积环境,硅质来源主要为陆源来源,也有后期热液作用来源的部分加入。(3)似层状硅质玉髓层是由于快速海退引起的淡水淋滤硅化作用形成的海底硬底构造或硅质壳;似玛瑙纹透镜状燧石及彩色似层状硅质玉髓层是由于沉积期及沉积期后成岩作用氧化还原条件变化,及后期热液侵入烘烤引起的紫色与绿色致色矿物或致色元素转变形成;角砾状燧石是由于后期秦岭强烈构造作用,引起似层状硅质玉髓上下岩层因软硬差异而产生的变形破裂作用形成的构造角砾岩。     

华南地区二叠纪栖霞组燧石结核成因研究及其地质意义

燧石结核是华南地区二叠纪栖霞组的重要识别特征之一，其成因具重要的古地理、古海洋意义。通过对湖北黄石、江苏南京和广西来宾三地栖霞组燧石结核的岩石矿物学研究，确定了栖霞组燧石结核的矿物组成和成岩作用序列。研究区燧石结核主要由微石英、负延性玉髓、粗晶石英组成，并含少量白云石、方解石及生物碎屑。其中，微石英、负延性玉髓、正延性玉髓、白云石形成于早期成岩作用，方解石晶粒形成于晚期成岩作用，粗晶石英的形成则具有多期性。结合栖霞组菊花状天青石和海泡石成因研究结果，本文认为组成栖霞组燧石结核的硅质来源与当时全球硅质生物的繁盛有关。燧石结核内玉髓和白云石形成环境条件及形成时间的确定，为建立更加合理的燧石结核成因模式和白云岩化模式提供了重要资料，同时也对深入探讨本区二叠纪层状硅质岩的成因具启发意义。     

地球化学自组织现象梗概

包括化学反应在内的自组织现象发生在沉积岩、火成岩和变质岩的所有岩石类型中。由于地球化学的自组织作用表现的一些例证通常不能由实验直接提供,所以常常难以确定它们就是自组织作用的真实情况,更不用说来鉴定形成机制。这些例子(具一定的可靠性)有:(1)氧化铁在石灰岩的燧石结核中形成富集环带,页岩中形成有规则间隔的、雪茄烟形的团块。(2)玛瑙(微晶石英纤维集合体)可表现出下列所有现象:(a)看上去与李泽冈环带极相似的条带;(b)由非扭曲与扭曲石英纤维交替组成的相邻条带;(c)正交偏光镜下明显可见玛瑙有特色的人字形干涉图。(3)前寒武纪铁建造在不同的尺度范围内表现了相互交替条带,且常常具有很大的横向延伸。(4)密西西比河型矿石走向上经常表现了由萤石、白云石、方铅矿、闪锌矿等任意一两种矿物交替出现的条带(～1cm);也可见到粒级条带。(5)多组间距均匀的夹层和缝合线(间距达10厘米多)常见于石灰岩和许多其它类型的沉积岩中。(6)大区域变质岩中有明显而大致规则的明暗矿物相间的条带(片理、解理或页理)。(7)某些接触变质岩(矽卡岩)所显示的规则条带。(8)基性杂岩中见有横向上大规模的英寸级平面状条带,如蒙大拿的Stillwater和格陵兰的Skacrgaard岩体。(9)许多花岗质岩石含有由同心状的,两种矿物交替环带(0.1～1cm)组成的“球体”(规则的球状结晶矿物集合体,直径几十厘米)。(10)中性火山岩中含有具屡变成分环带的斜长石晶体。     

松辽盆地南部HX井上白垩统青山口组黑色泥岩的矿物组成与自生微晶石英成因

泥质岩的矿物组成和成岩作用是制约其作为CO₂地质封存的盖层封闭能力和作为页岩气储层具易压裂性质的重要原因。通过地球化学分析、X-射线衍射分析和扫描电镜观测及能谱测定,确定了松辽盆地南部HX井上白垩统青山口组泥岩的矿物组成,识别出了蒙皂石转变伊利石反应过程中形成的亚微米级自生微晶石英。研究表明：HX井上白垩统青山口组泥岩的母岩岩石类型和风化效应一致,泥岩主要由黏土矿物、钠长石和石英组成,钾长石少量;黏土矿物以伊利石/蒙皂石混层为主,其次为绿泥石;脆性矿物（钠长石、石英和钾长石）的平均质量分数为44.43%,黏土矿物平均质量分数为54.26%。蒙皂石转变伊利石反应过程中形成的亚微米级微晶石英单晶呈自形－半自形粒状,集合体呈巢状,分布于伊利石/蒙皂石混层黏土矿物的微孔隙中。以伊利石/蒙皂石混层矿物为主、且存在一定量的自生微晶石英的泥岩盖层（例如HX井青山口组泥岩）将有助于注入CO₂的长期封存。HX井青山口组泥岩中的脆性矿物含量符合工业价值页岩气的储层标准,而黏土矿物含量则相差较多。     

深层碎屑岩自生矿物成因机理及其对储层物性的影响——以渤海歧南断阶带侏罗系为例

自生矿物特征和成因机理对深层碎屑岩储层物性具有重要影响。以渤海海域歧南断阶带侏罗系为例,通过岩心、薄片、扫描电镜、电子探针、同位素、包裹体、X衍射分析等技术手段,对研究区侏罗系深层碎屑岩储层自生矿物的类型、特征、成因机理及对优质储层发育的控制作用进行研究。结果表明：研究区侏罗系碎屑岩属于中孔-低渗储层,非均质性强;主要自生矿物类型为硅质矿物、碳酸盐矿物、黏土矿物3类;早期形成的硅质石英衬垫和碳酸盐胶结物控制了孔隙的发育和演化,沿颗粒表面分布的早期硅质石英衬垫有效抑制了后期石英加大生长,并增强了岩石的抗压能力,有利于原生孔隙保存;早期碳酸盐胶结物增强了储层抗压实能力,并为后期储层遭受溶蚀形成溶蚀孔提供了物质基础,有利于高孔隙储层形成;黏土矿物控制了储层渗透率差异,储层渗透率与自生高岭石体积分数呈正相关性,较高渗透率储层分布于侏罗系中段高岭石富集带内。研究区侏罗系碎屑岩发育原生孔隙体积分数高、微晶石英衬垫发育的Ⅰ类有利储层和粒间溶蚀孔隙较发育、具显著表生成岩作用特点的Ⅱ类有利储层,二者孔隙演化存在明显差异。Ⅰ类有利储层主要受早—中成岩期微晶石英衬垫抗压实作用控制,浅层和深层均可发育高孔隙储层;Ⅱ类有利储层主要受表生期风化淋滤作用控制,可在风化壳附近形成优质储层,整体上Ⅰ类储层物性优于Ⅱ类。     

胶北荆山群富铝岩系石榴石扩散环带特征及其成因指示意义

通过详细的微区成分测定，发现胶北荆山群富铝岩系中石榴石普遍发育扩散环带，但扩散环带的发育程度及样式很不均匀，明显受与其相邻矿物的控制。与黑云母接触时，石榴石晶体边部的镁含量最低，环带最为发育，与堇青石接触时次之，与长英质矿物接触时则环带发育较弱或不发育。这种特征的石榴石扩散环带样式与传统认识有很大差异，反映降温过程中石榴石与黑云母等镁铁矿物之间的Fe-Mg交换作用主要是通过彼此接触的界面来实现，粒间流体对组分的传输作用有限。但是当岩石中黑云母大量存在而石榴石含量又较低时，由于体系水活度增高，粒间流体也会传输一定的Fe、Mg组分，导致与长英质矿物相邻的石榴石晶体边部发育微弱的扩散环带。通过分析，确定粒径大于1500斗m的石榴石晶体核部可以保存变质峰期的平衡成分，基质中远离石榴石等镁铁矿物处于长英质矿物之间的大颗粒黑云母颗粒核部也基本可以保存变质峰期的平衡成分。     

斜长石环带非线性自组织生长过程的元胞自动模拟与分析

主要针对斜长石环带的成因动力学机制模型进行可视模拟的探讨.前人研究认为:斜长石韵律环带的形成仅仅与岩浆后期侵入的周期线性相关,因此,本模拟计算引入非线性元胞自动机分析方法并借助Matlab平台编写计算机软件来模拟斜长石环带生长机制.根据Fick扩散定律等,通过元胞自动分析机,模拟在不同开放程度的岩浆系统背景下的形成过程,结果表明韵律环带不仅仅与有岩浆后期侵入的环境相关,还与晶体附近的岩浆的An浓度的重新趋近饱和有关.     

玛瑙的宝石学分类及其鉴别特征

玉髓是地球上产量最大的玉石品种之一,玛瑙又是玉髓中最色彩缤纷、特色多样、美观与奇特兼具而广受人们喜爱的品种。但近些年来,国内外各地新发现的玉髓或玛瑙品种越来越多,受到市场不同程度热捧,不同地域各自命名,名称多种多样,价格混乱,让消费者眼花缭乱,更是让玉髓和玛瑙之间的关系及各自的概念混淆不清。玛瑙定义和分类的混乱已严重影响玛瑙市场的规范有序的良性发展,也不能满足现今玛瑙市场日新月异的变化需求。为解决这一问题,本研究在多年来多个相关科研、标准项目研究成果和大量实物样品测试验证基础上,根据现有市场上常见的传统玛瑙品种和非传统的新品种的特点特性,尝试完善玉髓和玛瑙的宝石学定义,并以玉髓和玛瑙产品最终呈现出来的最主要特征为分类依据,将玉髓划分为单色玉髓、玛瑙、碧石、硅化玉四大类。本次研究重点阐述玛瑙的宝石学定义及分类特征和鉴定特征,将玛瑙划分为多色玛瑙、纹带玛瑙、图案玛瑙、形态玛瑙、胆玛瑙、特殊光学效应玛瑙六类十六个特色品种,给出了科学合理并具有市场可行性的宝石学分类方案。     

Chert spheroids of the Monterey Formation,California (USA): early‐diagenetic structures of bedded siliceous deposits

Chert spheroids are distinctive, early‐diagenetic features that occur in bedded siliceous deposits spanning the Phanerozoic. These features are distinct in structure and genesis from similar, concentrically banded ‘wood‐grain’ or ‘onion‐skin’ chert nodules from carbonate successions. In the Miocene Monterey Formation of California (USA), chert spheroids are irregular, concentrically banded nodules, which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite. During shortening, there is brittle fracture of diatomite around, and horizontally away from, the convex surface of strain‐resistant chert nodules. Unlike most older siliceous deposits, the Monterey Formation still preserves all stages of silica diagenesis, thus retaining textural, mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures. Chert spheroids found in opal‐A diatomite form individual nodules composed of alternating bands of impure opal‐CT chert and pure opal‐CT or chalcedony. With increased burial diagenesis, surrounding diatomite transforms to bedded porcelanite or chert, and spheroids no longer form discrete nodules, yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony. Petrographic observations show that the purer silica bands are composed of void‐filling cement that precipitated in curved dilational fractures, and do not reflect geochemical growth‐banding in the manner of Liesegang phenomena invoked to explain concentrically banded chert nodules in limestone. Chertification of bedded siliceous sediment can occur more shallowly (< 100 m) and rapidly (< 1 Myr) than the bulk silica phase transitions forming porcelanite or siliceous shale in the Monterey Formation and other hemipelagic/pelagic siliceous deposits. Early diagenesis is indicated by physical properties, deformational style and oxygen‐isotopic composition of chert spheroids. Early‐formed cherts formed by pore‐filling impregnation of the purest primary diatomaceous beds, along permeable fractures and in calcareous–siliceous strata.     

The nature of crystalline silica from the TAG submarine hydrothermal mound, 26°N Mid Atlantic Ridge

Silica occurs in abundance in a variety of hydrothermal samples from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound, 26°N Mid-Atlantic Ridge. The water content, trace element chemistry, and mineralogy of crystalline silica from 15 different samples have been examined by vibrational spectroscopy and probe microanalysis. The samples are from: shallow subsurface ferric iron oxyhydroxide silica deposits (n=4), a fragment of an active white smoker chimney (n=1), anhydrite bearing hydrothermal breccias (n=2), pyrite silica breccias (n=3), and silicified wall rock breccias (n=5). Length-fast chalcedony occurs in association with variable quantities of ferric iron oxyhydroxides in hydrothermal breccias from the mound flanks, within shallower subsurface chert samples, and within white smoker chimney walls. Samples from the anhydrite zone contain textures which are suggestive of an origin involving replacement of anhydrite. Samples taken from TAG 1 and 5 from below the anhydrite zone contain no chalcedony. Instead they contain subhedral quartz crystals which show oscillatory zoning in aluminium. Two types of crystalline silica namely, type A and type B quartz, are defined on the basis of the infrared spectra in the OH region from 3200 cm⁻¹ to 3600 cm⁻¹. The type A quartz occurs beneath the anhydrite zone at TAG 1 and 5. We propose a model that relates specific varieties of crystalline silica to different thermal and chemical environments within the mound interior. Length-fast chalcedony occurs in an outer low temperature envelope across the top and sides of the mound. The common association between length-fast chalcedony and ferric iron oxyhydroxides suggests that chalcedony crystallization is favoured where catalysis by ferric iron can occur. The apparent suppression of fibrous silica at the expense of single quartz crystals with increasing depth is attributed to differing growth rates and degrees of supersaturation of silica-bearing solutions with increasing temperature within the mound. The transition from type A to type B single crystal growth is interpreted to occur at temperatures approaching ˜360 °C due to decreasing solubility of aluminium in quartz, so that aluminium is rendered unavailable for type A valence compensation. Received: 10 September 1998 / Accepted: 6 July 1999     

Occurrence and distribution of “moganite” in agate/chalcedony: a combined micro-Raman, Rietveld, and cathodoluminescence study

Agate/chalcedony samples of different origin were investigated by performing Raman, X-ray diffraction (using Rietveld refinement), and cathodoluminescence measurements. These analyses were performed to measure the content and spatial distribution of the silica polymorph moganite, which is considered to represent periodic Brazil-law twinning of α-quartz at the unit-cell scale in agate/chalcedonies. Homogeneous standard samples including the nearly α-quartz free moganite type material from Gran Canaria were analysed in order to compare results of the X-ray diffractometry and Raman spectroscopy techniques and to provide a calibration curve for the Raman results. However, due to the different length scales analysed by the two techniques, the “moganite content” in microcrystalline SiO₂ samples measured by Raman spectroscopy (short-range order) was found to be considerably higher than the “moganite content” measured by X-ray diffractometry (long-range order). The difference is explained by the presence of moganite nanocrystals, nano-range moganite lamellae, and single Brazil-law twin-planes that are detected by vibrational spectroscopy but that are not large enough (in the sense of coherently scattering lattice domains) to be detected by X-ray diffractometry. High resolution Raman analysis provides a measure of the moganite content and its spatial variation in microcrystalline silica samples with a lateral resolution in the μm-range. Variations in the moganite-to-quartz ratio are revealed by varying intensity ratios of the main symmetric stretching-bending vibrations (A₁ modes) of α-quartz (465 cm⁻¹) and moganite (502 cm⁻¹), respectively. Traces of Raman microprobe analyses perpendicular to the rhythmic zoning of agates revealed that the moganite-to-quartz ratio is often not uniform but shows a cyclic pattern that correlates with the observed cathodoluminescence pattern (colour and intensity). Data obtained from an agate sample from a fluorite deposit near Okorusu, Namibia and from a volcanic agate from Los Indios, Cuba were selected for detailed presentation. Variations of cathodoluminescence and Raman data between single bands in agates suggest alternating formation of fine-grained, highly defective chalcedony intergrown with moganite, and coarse-grained low-defect quartz. Multiple zones indicate dynamic internal growth during a self-organizational crystallization process from silica-rich fluids. Received: 4 December 1997 / Accepted 19 June 1998     

Structural defects in microcrystalline silica

The structure of the microcrystalline silica varieties chalcedony, flint, moganite, opal-C and -CT is characterized by X-ray powder diffractometry and transmission electron microscopy (TEM). The role of impurities is investigated by infrared spectroscopy and chemical analysis. Microcrystalline opal, chalcedony and flint have a disordered intergrowth structure composed of cristobalite and tridymite domains in opal, and quartz and moganite domains in chalcedony and flint. Each constituent phase has different cell dimensions and symmetry. The main impurity is water which is enriched at the intergrowth interfaces. Density and refractive indices of microcrystalline silica depend on the water content.     

Solubility of silica polymorphs in electrolyte solutions, II. Activity of aqueous silica and solid silica polymorphs in deep solutions from the sedimentary Paris Basin

Activity coefficient for aqueous silica in saline waters and brines from the Paris Basin was calculated using Pitzer's specific interaction model. Quartz and chalcedony are the only reported authigenic silica minerals in the Dogger aquifer of the Paris Basin (France). However, the measured silica concentrations fall between those of these two phases. The silica concentrations measured in Dogger fluids seem to be controlled by a microcrystalline quartz phase with a grain size computed to be about 20 nm. Studies have shown that pressure can preserve small grain size for a long time at the geological scale. The effective mechanism of pressure action is probably linked to the fact that pressure simultaneously favours dissolution at the grain-contact inducing a quartz supersaturation and prohibits the increase in size of reprecipitated microcrystalline quartz grains. This hypothesis is supported by other studies reported in the literature. The proposed model, which incorporates silica mineralogy and a precise calculation of aqueous silica activity, allows us to explain measured silica concentrations in the deep sedimentary solutions of the Dogger aquifers. In the Keuper brines, silica solubility can in most cases be explained by an equilibrium with either chalcedony or quartz. Another application of the present work is shown by an example, where we examined the importance of precisely evaluating the activity coefficient in basin characterisation, as the goal of reservoir characterisation is to describe the spatial distribution of petrophysical parameters such as porosity, permeability, and saturations.     

Opal and chalcedony speleothems on quartz sandstones in the Sydney region,southeastern Australia

Speleothems of silica are far rarer than those of calcite but occur in a range of types including stalactites, stalagmites and flowstones. This study has found a wider range and far greater number of silica speleothems on the quartz sandstones of the Sydney region than the small number of previous accounts had suggested. Speleothems on the Sydney region sandstones are composed of multiple layers of amorphous opal‐A and cryptocrystalline chalcedony. Silica slowly dissolved from detrital and diagenetic quartz and kaolinite clays of the host arenites is redeposited as opal‐A at the sandstone surface when groundwater evaporates. This amorphous silica converts over time by Ostwald‐type paragenesis to the cryptocrystalline form, but the expected intermediate opal‐CT phase has not been detected. The crystallisation of chalcedony at earth‐surface temperatures is generally believed to take an extremely long time and its presence makes these speleothems very significant, especially as it is reported in only a small number of silica speleothems elsewhere. Furthermore, a similar paragenetic silica‐‘ripening’ mechanism may also be involved in the low‐temperature earth‐surface formation of other crystalline silica deposits such as silcrete duricrusts and pedogenic quartz. Additional closely coupled laboratory and field investigations into the processes that control silica paragenesis under earth‐surface conditions are sorely needed.     

Extent of diagenetic transformations in severely altered biogenic silica deposits from Tunisia: new insights from mineralogy and geochemistry

Sedimentary biogenic silica from Redeyef in Gafsa basin (southern Tunisia) was analysed for its ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra and complemented by X-ray diffraction and SEM observations. The ²⁹Si MAS NMR spectrum is characterized by the abundance of hydroxylated silicon, displayed in resonance intensities and reflects a clear tendency towards dissolution of diatomaceous amorphous silica and the occurrence of the hydrated silica, which is the main component that ensures the diagenetic transition via the mechanism of dissolution–precipitation to other more crystalline silica phases, after the lost of its hydroxyls groups (water) by heating (burial). ²⁷Al MAS NMR reveals two coordinations of Al; the octahedrally coordinated Al suggests the presence of clay relics trapped during crystal growth or a microcrystalline zeolite (clinoptilolite detected by SEM observations), while the tetrahedrally coordinated Al suggests the presence of minor quantities of minerals with tetrahedral Al, such as an Al-rich fluid and/or minerals such as feldspars.     

A proposed mechanism for the growth of chalcedony

The structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica. Although the paragenesis of chalcedony has provoked marked disagreement among researchers, a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures (<100° C). These conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short-chain linear polymers via bridging silica monomers. This assembly occurs through a spiral growth mechanism activated by a screw dislocation withb=n/2 [110], wheren is an integer. The proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale, such as: (1) the direction of fiber elongation along [110] rather than [001]; (2) the periodic twisting of chalcedony fibers about [110]; (3) the high density of Brazil twin composition planes; (4) the common intergrowth of moganite within chalcedony.     

Hydrogeochemical factors effecting the scaling problem in Balçova geothermal field,İzmir, Turkey

Reservoir fluid compositions have been assessed from analytical data on water samples collected from thermal and cold waters in Balçova geothermal field. The results of mineral equilibrium modelling indicate that the waters, with some exceptions, are systematically supersaturated with respect to calcite, aragonite, dolomite, chalcedony and quartz, but undersaturated with respect to amorphous silica, celestite, anhydrite and gypsum and undersaturated or supersaturated with respect to barite, low-albite, K-feldspar, gibbsite and Fe(OH)₃(a). Calculation of mineral saturation states and geochemical analyses of scale and field observations show that carbonate minerals (calcite, aragonite and dolomite) are most likely to be precipitated as a scale type. Besides carbonates, scale formation risk of amorphous silica, Fe(OH)₃(a), anhydrite, barite and celestite minerals should be taken into account in some wells and surface equipment. Most of the waters, with some exceptions, have carbonate scaling risk at all temperatures, whereas the other scaling risks only exist over a limited temperature range. While silica, Fe(OH)₃(a) and barite show a scaling tendency at low temperatures, anhydrite and celestite scaling occurs at higher temperatures.     

Microzoning of agates as a reflection of their formation dynamics

Agates are natural hierarchicalal structural assemblies composed largely of silica minerals: chalcedony, quartz, and quartzine. Microstructural heterogeneities, most typical of those of agate-forming minerals, are characterized by variations in periods of helicoidal twisting of chalcedony and by micrometric rhythmic alternation of zones with a low refractive index within the matrix of normal chalcedony, quartz, and quartzine. A study of the degree of crystallization of chalcedony using the Barsanova-Yakovleva method showed that neither helicoidal twisting nor refractive index zoning brought about any significant disturbance in the chalcedony structure. Helicoidal twisting is a result of the stresses caused by substitution of Si⁴⁺ for Al³⁺ asymmetric with respect to chalcedony crystal. It is suggested that layering by the refractive index developed due to accumulation of small portions of gel consisting of high silica polymers at the front of crystal growth.