西大别造山带红安高压榴辉岩的变质演化：岩相学与Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-O(Fe 2O3)体系中相平衡关系

西大别造山带红安高压榴辉岩主要矿物为石榴石、绿辉石、冻蓝闪石、石英和绿帘石,有时可见蓝闪石、多硅白云母和钠云母。石榴石具有生长环带且边缘成分变化大,可分为代表峰期的Ⅰ型边（X_Mg高、Grs低）和受退变质改造的Ⅱ型边（X_Mg低、Grs高）。石榴石内蓝闪石包体发育冻蓝闪石退变边,说明包体不能完全反映进变质条件。基质绿辉石比包体绿辉石Jd含量低,在一个晶体内成分有明显变化和沿解理缝发育冻蓝闪石,显示峰后绿辉石有成分变化和退变质改造。基质中冻蓝闪石晶体较大,核部见有蓝闪石残留,说明二者有成因联系。冻蓝闪石和绿辉石都发育后成合晶结构,石榴石有韭闪石的反应冠状体。在THERMOCALC程序计算的P-T视剖面图中,石榴石Ⅰ型边反映的峰期P-T条件为2.4~2.6GPa、570~585℃,和基质中多硅白云母Si含量等值线限定范围一致,对应硬柱石蓝闪石榴辉岩组合。石榴石Ⅱ型边P-T范围为1.9~2.4GPa、530~570℃,低于峰期条件。在可能的峰后降压过程中,岩石先后主要经历了硬柱石脱水生成绿帘石和蓝闪石、绿辉石退变为冻蓝闪石的反应阶段。绿辉石、冻蓝闪石发育的后成合晶说明晚期退变过程缺乏流体,石榴石的韭闪石冠状体也可能在该阶段产生,都受局部成分域控制。红安高压榴辉岩中各矿物与成分代表不同变质阶段,称其为冻蓝闪石榴辉岩只是对现有主要组成矿物的描述,不是基于共生关系的严格岩石学命名。     

西藏新达多地区榴辉岩的变质过程研究及其对古特提斯俯冲带演化的限定意义

位于拉萨地块中部松多榴辉岩带西端的新达多地区出露两种类型榴辉岩:含蓝闪石榴辉岩和双矿物榴辉岩。含蓝闪石榴辉岩主要矿物组合为石榴石、绿辉石、蓝闪石、多硅白云母及少量的绿帘石、角闪石、石英、金红石。石榴石不具有成分环带结构,蓝闪石存在于基质中,边部大多退变为冻蓝闪石并普遍发育有角闪石和石英的后成合晶。双矿物榴辉岩的主要矿物组合为石榴石、绿辉石、石英及少量的绿帘石、角闪石、金红石、钛铁矿、榍石。石榴石具有典型的进变质环带特征,从核部到边部镁铝榴石和钙铝榴石组分先升高后降低,铁铝榴石组分变化与之耦合,石榴石边部发育角闪石和钠长石的冠状体,推断石榴石记录了进变质的生长过程后又受到了退变质改造。结合传统温压计和变质相平衡模拟两种温压计算方法对榴辉岩的峰期变质条件进行限定,得到含蓝闪石榴辉岩的峰期温压条件为:615±5℃,33±0.5kbar;双矿物榴辉岩的峰期变质温度为630±10℃,压力不超过27kbar。变质相平衡模拟计算结果显示:(1)含蓝闪石榴辉岩经历了退变初期近等温减压过程,这一过程以硬柱石和少量滑石的脱水反应生成蓝闪石和绿帘石为主要特征;中晚期退变质阶段以大范围硬柱石消失后局部富余流体的消耗在蓝闪石边部形成冻蓝闪石以及蓝闪石边部发育后成合晶为特征,部分石榴石边部的韭闪石冠状体大致也发生在这一过程;(2)双矿物榴辉岩则记录了从进变质生长阶段到峰期变质阶段,最后再到退变质演化阶段的完整变质过程。结合前人对松多榴辉岩的工作,对新达多地区新近发现的两类榴辉岩的岩石学研究表明:拉萨地块内部的榴辉岩为典型的大洋俯冲带产物,代表了古特提斯洋盆的存在。俯冲折返过程中复杂的构造机制使得不同类型榴辉岩在同一地区出露共生。     

青藏高原拉萨地块松多蓝闪石榴辉岩的变质演化：相平衡及变质作用P-T轨迹

青藏高原拉萨地块松多蓝闪石榴辉岩主要矿物组合为石榴石、绿辉石、蓝闪石、绿帘石/斜黝帘石,及少量的金红石、石英、多硅白云母和普通角闪石。石榴石具有成分环带,从核到边Xpy升高,Xgr降低,部分石榴石外边缘受退变质改造影响,形成富Xgr的成分带。利用NCKMnFMASHTO体系中的P-T视剖面图,结合石榴石边部最大Xpy等值线和多硅白云母最大Si-含量值确定了松多榴辉岩的峰期变质条件为30±0.6kbar和610±6℃。石榴石核部到幔部成分环带记录的相对平缓的PT轨迹反应了岩石早期经历了以加热升温为主、轻微加压的缓慢俯冲过程,地温梯度为7~8℃/km。石榴石幔部到边部成分环带,结合多硅白云母最大Si含量等值线模拟了以缓慢升温、快速增压为特征的P-T轨迹,反应了岩石由早期的缓慢俯冲进入到后期的快速俯冲阶段,地温梯度由7~8℃/km减小到5~6℃/km。峰期之后的榴辉岩经历了早期近等温减压的变质过程,以硬柱石和少量滑石的脱水反应生成蓝闪石和绿帘石(约22~23kbar)为主要特征。其后的晚期退变质阶段以硬柱石消失后局部成分域内由富余流体的消耗形成冻蓝闪石(约16kbar)以及蓝闪石和绿辉石边部发育后成合晶为特征(11~12kbar),石榴石边部的韭闪石冠状体和金红石边部生成的榍石退变边也大致发生在该阶段。榴辉岩近等温减压的变质过程可能代表了早期的构造快速抬升过程。松多榴辉岩带可能代表了青藏高原拉萨地块内一条新的大洋型高压-超高压变质带,大约266Ma的榴辉岩相变质时代说明在拉萨地块内部可能存在过一个二叠纪的古特提斯洋盆。     

新疆西南天山哈布腾苏河高压-超高压变质带中榴辉岩和蓝片岩的岩石学及变质演化

对西南天山哈布腾苏河一带出露的典型榴辉岩和蓝片岩进行了详细的岩相学、矿物化学和温压条件综合研究。榴辉岩可分为蓝闪石榴辉岩、钠云母榴辉岩、绿帘石榴辉岩和蓝闪石榴角闪岩(退变榴辉岩)4类,蓝片岩可分为含蓝闪石石榴白云母钠长片岩、石榴白云母蓝闪片岩和石榴白云母蓝闪石英片岩3类。新鲜榴辉岩主要矿物组合为石榴石+绿辉石+钠云母+绿帘石,退变榴辉岩则为石榴石+蓝闪石+角闪石;蓝片岩主要矿物组合为石榴石+蓝闪石+多硅白云母+钠云母+钠长石+石英。榴辉岩和蓝片岩中石榴石变斑晶均保存进变质生长环带,从核部到边部XMn和XFe降低,XMg和XCa升高,指示了升温进变质的演化过程。根据榴辉岩矿物共生组合、石榴石内部包体组合分布特征及传统地质温压计估算结果,确定榴辉岩经历了4阶段的变质演化:早期硬柱石蓝片岩相进变质阶段、峰期榴辉岩相变质阶段(t=543～579℃,p=1.5～1.6 GPa)、峰后绿帘蓝片岩相退变质阶段(t=～450℃,p1.0GPa)和晚期蓝闪绿片岩相退变质阶段(t400℃,p0.5 GPa)。利用p-T视剖面图计算的榴辉岩、蓝片岩峰期变质温压条件与传统地质温压计估算结果十分相近,其中榴辉岩的峰期变质条件t=520～550℃,p=1.7～1.9 GPa;蓝片岩峰期变质条件t=520～620℃,p=1.7～2.3 GPa。本文估算的榴辉岩峰期变质压力条件与前人根据柯石英的发现而认为研究区部分榴辉岩及其围岩曾经历超高压变质作用的认识明显相悖,原因可能如下:①后期退变质作用引起研究区榴辉岩全岩成分、矿物化学成分的调整,在采用Grt-Cpx-Phe温压计和以全岩成分为基础的相平衡模拟方法估算峰期温压条件时受到影响,从而使估算峰期压力条件普遍偏低;②西南天山的榴辉岩可能并非全都经历了超高压变质作用,高压、超高压榴辉岩可能分别代表了不同变基性岩块在不同俯冲深度变质的产物。     

西大别造山带红安高压榴辉岩的变质相平衡研究

西大别造山带红安高压榴辉岩含有大量角闪石,被称为角闪石榴辉岩.其主要矿物组成为石榴子石、绿辉石、冻蓝闪石、石英和绿帘石,有时可见蓝闪石、多硅白云母和钠云母.该区高压榴辉岩引起很多学者的关注,但在峰期变质条件和矿物共生组合方面尚有争论.     

新疆西天山高压变质带的变质矿物与变质作用演化

新疆西天山高压变质带主要由石榴石，角闪石，绿辉石，多硅白云母，钠云母，绿帘石，绿泥石，钠长石，石英，榍石和金红石等组成，石榴石主要含铁铝榴石组份，角闪石有蓝闪石，亚铁蓝闪石，青铝闪石，冻蓝闪石等类型，变质矿物组合显示高压变质带经历了由硬柱石蓝片岩相，榴辉岩相，绿帘蓝片岩相至绿片岩相的变质作用演化进程。     

拉萨地块松多榴辉岩的变质演化过程:NCKMnFMASHTO体系中的相平衡关系

拉萨地块松多榴辉岩主要矿物组合为石榴子石、绿辉石、角闪石、多硅白云母、绿帘石、金红石。石榴子石环带不明显,核部成分均一,从核部到边部,镁铝榴石和钙铝榴石含量降低,可能分别记录了榴辉岩峰期及退变质过程信息。绿辉石显示微弱的成分环带,硬玉含量从核部到边部略有升高,部分绿辉石边部发育韭闪石退变质边,反映了在减压过程中外来流体进入体系的过程。多硅白云母具有高的Si含量(3.5~3.6),其中石榴石包体中的多硅白云母相对基质中的白云母有更高的Si值。本文利用Thermocalc变质相平衡模拟软件,结合详细岩相学观察,在NCKMn FMASHTO体系下,模拟松多含多硅白云母榴辉岩的变质演化过程。其中,榴辉岩峰期矿物组合为g+o+law+phn+ru,石榴石核部最大镁铝榴石值和石榴子石包体中多硅白云母最大Si值确定的榴辉岩峰期温压条件约为620℃,32×105Pa,榴辉岩经历了近等温降压的退变质过程。相平衡模拟结果表明拉萨地块松多榴辉岩经历了超高压变质作用过程,并经历了相对快速的折返过程到中部地壳层次。     

大别山西段含蓝闪石-蓝晶石榴辉岩的相平衡研究

目前对于大别山西段超高压榴辉岩仍存在一些不清楚的问题和模糊的认识,如蓝闪石和蓝晶石组合的稳定范围,峰期温压条件和矿物组合,以及早期退变质过程的矿物演化和流体作用。本文对取自大别山西段新县高压-超高压榴辉岩单元内不同地点的超高压榴辉岩样品进行了详细的岩石学和矿物学研究,在此基础上使用相平衡定量分析方法的 PT 视剖面图对它们进行了正演模拟计算,结果表明:含蓝闪石和蓝晶石榴辉岩处于相对低温或低压的蓝闪石榴辉岩和相对高温高压的蓝晶石榴辉岩的过渡区,其稳定的温压范围大致为温度590～700℃,压力1.7～3.3GPa,而且压力大于2.5GPa 时温度范围很窄,为600～640℃。由石榴石边缘成分和 PT 视剖面图确定的榴辉岩峰期温压条件为压力2.85～2.95GPa 和温度625～630℃,峰期矿物组合为石榴石 绿辉石 蓝闪石 蓝晶石 硬柱石 柯石英±多硅白云母。峰期之后,榴辉岩经历了快速近等温降压(ITD)的早期高压退变质作用,这是一个非平衡过程,所发生的主要变化如下:柯石英→石英,硬柱石→黝帘石 蓝晶石,在相对富镁岩石中出现滑石,当水含量较高时可以出现钠云母,蓝闪石在原来基础上有一定量的生长,并且绿辉石和多硅白云母很可能只部分地发生了成分变化,而石榴石几乎未发生改变。这样形成了目前观察到的矿物组成为石榴石 绿辉石 蓝闪石 蓝晶石 黝帘石/绿帘石 石英±多硅白云母±钠云母±滑石,它代表了 UHP 榴辉岩在早期高压退变质阶段结束时所具有的矿物组成,这一阶段结束时的温压条件大致为2.0～2.2GPa 和600～630℃;早期高压退变质阶段是脱水过程,流体是内部缓冲的。     

榴辉岩中传统地质温压计新解：来自PT视剖面图的证据

石榴石-单斜辉石(GC)温度计和石榴石-单斜辉石-多硅白云母(GCP)压力计是确定榴辉岩形成温压条件的最常用方法,二者主要依据石榴石、绿辉石和多硅白云母中相组分之间的交换和转换变质反应.依据MORB成分计算的PT视剖面图表明,在不同榴辉岩矿物组合中,控制3个矿物相成分变化的相组分之间的变质反应不同.在低温含绿泥石、滑石和蓝闪石榴辉岩组合中,石榴石和绿辉石的镁含量主要受到含水矿物脱水反应的控制,并都随温度升高而升高,二者之间的铁镁交换反应并不起主要作用.因此,在自然界含有蓝闪石等含水矿物的低温榴辉岩中,由于绿辉石相对富镁而常常导致GC温度计结果偏低.在含有硬柱石的高压-超高压榴辉岩中,石榴石中的钙含量受到硬柱石的控制,随着压力升高或温度降低,硬柱石含量增加,使石榴石中钙降低,此时石榴石-绿辉石-多硅白云母之间的转换反应对石榴石成分的影响会很微弱,由于石榴石相对贫钙而导致GCP压力计结果偏低.在含有蓝晶石的中温高压-超高压榴辉岩中,矿物成分的变化受到石榴石-绿辉石之间的铁镁交换反应和石榴石．绿辉石．多硅白云母-蓝晶石-石英/柯石英之间的一系列转换反应控制,因此,GC和GCP温压计都能给出相对合理的结果.在低压普通角闪石榴辉岩中,石榴石和绿辉石中的镁含量主要反应压力变化,有时并不指示变质作用温度.在含有蓝闪石等含水矿物的低温榴辉岩中,Thermocalc程序中的平均温压(avPT)方法可以给出比较合适的温度,但压力结果与GCP压力计一样也会偏低一些.在蓝闪石和绿帘石等含水矿物消失后的中温蓝晶石榴辉岩中,avPT方法难以给出合理的PT信息.相对来说,视剖面图方法能够给出最多的PT信息,是目前确定变质岩PT条件的最好方法.     

拉萨地块吉朗榴辉岩的岩石学研究及其对古特提斯洋壳俯冲折返过程的限定

拉萨地块东部松多(超)高压榴辉岩记录了古特提斯洋俯冲及折返过程。松多榴辉岩带已发现松多、新达多、白朗和吉朗4个榴辉岩出露区,它们的峰期温压条件及变质p-T轨迹的研究对揭示拉萨地块古特提斯时期的俯冲及折返过程有重要意义。松多榴辉岩带东段吉朗榴辉岩的主要矿物为石榴子石、绿辉石、多硅白云母、角闪石、金红石、绿帘石、石英以及退变形成的后成合晶结构(透辉石+角闪石+斜长石)和少量的黑云母。石榴子石具有含丰富矿物包裹体的"脏"核和极少包裹体的"净"边,具有典型的进变质成分环带特征,从核部到边部镁铝榴石组分升高,锰铝榴石和钙铝榴石组分降低。石榴子石边部发育窄的角闪石+斜长石(An=28)组成的冠状体,表明石榴子石边部发生了后期角闪岩相退变质作用。通过变质相平衡模拟计算得到石榴子石以及多硅白云母记录的峰期温压条件为563℃、2. 4 GPa。结合岩相学特征,确定吉朗榴辉岩经历了4期变质演化阶段:(1)进变质阶段以石榴子石核部及其包裹体为代表性矿物组合;(2)峰期变质阶段矿物组合为石榴子石边部、绿辉石、多硅白云母、蓝闪石、硬柱石、金红石和石英;(3)早期退变质阶段以硬柱石分解产生绿帘石为特征;(4)晚期退变质阶段以绿辉石发育后成合晶和石榴子石生长冠状体为特征。认为吉朗榴辉岩为典型的低温高压榴辉岩,经历了顺时针p-T演化轨迹,折返过程为近等温降压过程。与松多带内其他(超)高压岩石相比,吉朗榴辉岩峰期温压条件较低,其围岩为变石英岩,区别于区内其他(超)高压榴辉岩的石榴子石白云母片岩及蛇纹岩围岩。推测吉朗榴辉岩来自于俯冲带浅部,由俯冲隧道中低密度沉积物裹挟折返。     

Plumboselite, Pb3O2(SeO3), a new oxidation-zone mineral from Tsumeb, Namibia

Plumboselite, ideally Pb₃O₂(SeO₃), is a new selenite (IMA2010?C028) from the Tsumeb mine, Namibia. It occurs as fibres on clausthalite and is also associated with smithsonite, mimetite and vaterite. Plumboselite occurs in subparallel to divergent clusters of thin, flattened, colourless fibres up to 0.3?mm in length, but not exceeding 5???m in width and 2???m in thickness. The fibres are elongated parallel to [001] and flattened on {010}, with {010} the only form observed. The crystals have a dull to adamantine lustre and a white streak. The tenacity is brittle and the Mohs hardness is estimated to be between 2 and 3. Plumboselite crystals are optically biaxial with parallel extinction and are length fast in all orientations. The Gladstone-Dale relationship predicts n _av?=?2.115. The high indices of refraction and small crystal size prevented the determination of other optical properties. The calculated density is 7.814?g/cm³. The empirical formula (based on 5 O atoms) is Pb_2.92Ca_0.01Se_1.03O₅. Plumboselite is orthorhombic, space group Cmc2₁, a?=?10.5384(11), b?=?10.7452(13), c?=?5.7577(7) ?, V?=?651.98(12) ?³ and Z?=?4. The five strongest lines in the powder X-ray diffraction pattern are [d _obs in ?/(I)/hkl]: 3.155/(100)/221; 1.956/(26)/042,402; 2.886/(22)/311,002; 1.713/(21)/223; 2.691/(17)/040. The crystal structure was solved from single-crystal X-ray diffraction data and refined to R ₁?=?0.0371 on the basis of 200 unique reflections with F _o?>?4??F. The structure is based on double [O₂Pb₃] chains of edge-sharing oxo-centered [OPb₄] tetrahedra along c, between which are sited SeO₃ triangles. The two independent Pb²⁺ atoms and the Se⁴⁺ atom have sterochemically active lone electron pairs.     

Carbon in silicate liquids: the systems NaAlSi3O8-CO2, CaAl2Si2O8-CO2, and KAlSi3O8-CO2

To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO₂ vapor. The depression of the temperature of the solidus of albite by CO₂ decreases from 30° C at 10 kbar, to 10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO₂ in NaAlSi₃O₈ liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO₂ lowers the temperature of the solidus of anorthite by 30° C at 14 kbar, and by 70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si₃O₈ composition to pressures of 35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO₂ and CaO and depleted in Al₂O₃ relative to CaAl₂Si₂O₈.The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO₂ component in the liquid ( ). We predict that this increases the ratio of molecular CO₂/CO ₃ ^2– in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO₂ in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi₃O₈ liquids, which is in contrast to that in CaAl₂Si₂O₈ liquid. Also, reaction of CO₂ with NaAlSi₃O₈ liquid to form CO ₃ ^2– that is complexed with Na⁺ must be accompanied by a change in Al³⁺ from network-former to network-modifier, as Na⁺ is no longer abailable to charge-balance Al³⁺ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO ₃ ^2– with the excess Ca²⁺ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable.The depression of the temperature of the solidus of sanidine resulting from the addition of CO₂ increases from 50° C at 5 kbar to 170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO₂-rich liquid up to the singular point at 15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the with increasing pressure in the interval up to 15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant relative to those of albite and anorthite, and CO₂ produces a nearly constant freezing-point depression of about 170° C. Because of the low at pressures above the singular point, we infer that most of the carbon dissolves as CO ₃ ^2– , resulting in a low CO₂/ CO ₃ ^2– , but a high total carbon content.The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.Institute of Geophysics and Planetary Physics, Contribution No. 2744     

Production of Bauxite Certified Reference Material (BARC-B1201) for Nine Property Values (Al2O3, Fe2O3, SiO2, TiO2, V2O5, MnO,Cr2O3, MgO and LOI) Traceable to SI Units

The National Centre for Compositional Characterisation of Materials (NCCCM) / Bhabha Atomic Research Centre (BARC) and National Aluminium Company Limited (NALCO), India have produced an Indian origin bauxite certified reference material (CRM), referred to as BARC-B1201, certified for major (Al₂O₃, Fe₂O₃, SiO₂, TiO₂, loss on ignition - LOI) and trace contents (V₂O₅, MnO, Cr₂O₃, MgO). Characterisation was undertaken by strict adherence to ISO Guides. A method previously developed and validated in our laboratory, using single step bauxite dissolution and subsequent quantitation (of Al₂O₃, Fe₂O₃, SiO₂, TiO₂, V₂O_5, MnO, Cr₂O₃ and MgO) by ICP-AES (SSBD ICP-AES) was used for homogeneity studies and an inter-laboratory comparison exercise (ILCE) of the candidate CRM. LOI was determined by thermo-gravimetric analysis. Property values were assigned after an ILCE with participation from seventeen reputed government and private sector laboratories in India. The CRM was certified for nine property values: Al₂O₃, Fe₂O₃, SiO₂, TiO₂, V₂O_5, MnO, Cr₂O₃, MgO and LOI, which are traceable to SI units.     

A Single Step Acid Assisted Microwave Digestion Method for the Complete Dissolution of Bauxite and Quantitation of its Composition (Al2O3, Fe2O3, SiO2, TiO2, Cr2O3, MgO,MnO and V2O5) by ICP-AES

An acid assisted microwave-based method for the complete dissolution of bauxite using mixture of H₂SO₄, H₃PO₄ and HF acids in a single step was developed for the determination of various analytes (Al₂O₃, Fe₂O₃, SiO₂, TiO₂, Cr₂O₃, MgO, MnO and V₂O₅) using ICP-AES. The method was validated with respect to ruggedness, linearity, trueness, precision, limit of detection (LOD), limit of quantification (LOQ), working range and measurement uncertainties by analysing a bauxite reference material (Alcan BXT-12) and four certified reference materials (IPT-131, BXBA-4, NIST SRM 600, NIST SRM 697). The expanded uncertainties obtained for Al₂O₃ (40.0%), Fe₂O₃ (17.0%), SiO₂ (20.3%), TiO₂ (1.31%), Cr₂O₃ (0.024%), MgO (0.05), MnO (0.013), and V₂O₅ (0.60%), were 0.80, 0.40, 0.50, 0.033, 0.0008, 0.002, 0.0007 and 0.002 respectively, which are fit for the intended use to characterise bauxite. The developed method was also evaluated through participation in an interlaboratory comparison exercise organised by the Jawaharlal Nehru Aluminium Research Development and Design Centre (JNARDDC), Nagpur, India, using bauxite sample (BXT-JNA), with satisfactory z-scores achieved.     

Compressibility of P2O5-Al2O3-Na2SiO3 melts

The effect of composition and temperature on the relaxed adiabatic bulk modulus of melts in the P₂O₅-Al₂O ₃-Na₂SiO₃ system have been investigated in the temperature range of 1140 to 1450 °C using ultrasonic interferometric methods at frequencies of 3, 5 and 7 MHz. The density of these melts was determined using Pt-double-bob Archimedean densitometry techiques. P₂O₅ is known to dramatically affect the structure and the chemical and physical properties of granitic and pegmatitic melts as a function of the peralkalinity of the melt. The physical results of the structural changes occurring in Na₂O-Al₂O₃-SiO₂ melt upon the addition of P₂O₅ are observed by variations in the properties such as density and compressibility. For the present peralkaline melts, the bulk modulus and density decrease with addition of 15 mol% P₂O₅, and increase with the addition of 15 mol% Al₂O₃. The addition of P₂O₅ to the present melts results in a larger increase in melt compressibility than that observed with increasing polymerization between Na₂SiO₃ and Na₂Si₂O₅ melts. This would suggest that not only is the polymerization of the melt increasing with the addition of P₂O₅ (Mysen et al. 1981; Nelson and Tallant 1984; Gan and Hess 1992), but that the tetrahedrally co-ordinated phosphorus complexes are influencing the bond lengths and energies within the melt structure; resulting in the structure becoming more compressible than expected, although incompressible (Vaughan and Weidner 1987) tetrahedral P₂O₅ polyhedra (Mysen et al. 1981; Gan and Hess 1992; Toplis et al. 1994) are being added to the melt structure.     

Phase relations between Ca3Fe 2 3 +Si3O12-Fe 3 2 +Fe 2 3 +Si3O12 garnet and CaFeSi2O6-Fe2Si2O6 pyroxene solid solutions

Editorial responsibility: J. Hoefs     

Stability of the assemblage orthopyroxene-sillimanite-quartz in the system MgO-FeO-Fe2O3-Al2O3-SiO2-H2O

The stability of coexisting orthopyroxene, sillimanite and quartz and the composition of orthopyroxene in this assemblage has been determined in the system MgO-FeO-Fe₂O₃-Al₂O₃-SiO₂-H₂O as a function of pressure, mainly at 1,000° C, and at oxygen fugacities defined mostly by the hematite-magnetite buffer. The upper stability of the assemblage is terminated at 17 kbars, 1,000° C, by the reaction opx+Al-silicate gar+qz, proceeding toward lower pressures with increasing Fe/(Fe+Mg) ratio in the system. The lower stability is controlled by the reaction opx+sill+qz cord, which occurs at 11 kbars in the iron-free system but is lowered to 9 kbars with increasing Fe/(Fe+Mg). Spinel solid solutions are stabilized, besides quartz, up to 14 kbars in favour of garnet in the iron-rich part of the system (Fe/(Fe+Mg)0.30). Ferric-ferrous ratios in orthopyroxene are increasing with increasing ferro-magnesian ratio. At least part of the generally observed increase in Al content with Fe²⁺ in orthopyroxene is not due to an increased solubility of the MgAlAlSiO₆ component but rather of a MgFe³⁺AlSiO₆ component. The data permit an estimate of oxygen fugacity from the composition of orthopyroxene in coexistence with sillimanite and quartz.     

The System Na2O-Al2O3-Fc2O3-SiO2 at 1 Atmosphere, and the Petrogenesis of Alkaline Rocks

Equilibria involving acmite, albite, nepheline, quartz, anda liquid phase constitute the petrologically important partof the system Na₂O–Al₂O₃–Fe₂O₂–SiO₂, and theunivariant and invariant relations provide useful analogiesfor a wide variety of alkaline igneous rocks. These relationsare dominated by the incongruent melting behaviour of acmite,which does not appear on the liquidus of the join acmite-nepheline-silica;instead, a broad field of hematite is present and acmite crystallizesonly from liquids containing potential sodium silicate. Consequently,the oversaturated and undersaturated eutectics, correspondingto granitic and nepheline syenitic liquids, are rich in sodiumsilicate and distinct from those found in Petrogeny's Residuasystem: the temperatures of the eutectics are 7285C and 7155C, respectively. Survival of peralkaline granite in the aluminouscontinental crust can be explained by the strongly peralkalinecomposition of the oversaturated eutectic. Magma of this typemay be the primitive granite of the non-orogenic zones. Theubiquitous alkali metasomatism around alkaline complexes canalso be interpreted in terms of residual liquids enriched inalkali silicates. Transition from undersaturated to oversaturatedliquids is possible by fractionation of hematite and a new processfor achieving the reverse transition has been found. This dependson the substitution of Fe³ for Al³ in feldspar and suggestsa more important role for syenite in any scheme of petrogenesis. Each of the two eutectics is linked to a corresponding peritecticat which hematite reacts to give acmite. The liquid at the undersaturated,quaternary reaction point is of ijolitic type, providing thefirst intimation that ijolite may represent a low-melting fractionin nature. The system Na₂O–Al₂O₃–Fe₂O₃–SiO₂thus constitutes the peralkaline residua system and on thisbasis a coherent picture of stable continental magmatism canbe constructed. Ijolite is seen as the low-melting fractionfrom a range of peralkaline compositions and from rocks suchas melilite basalt, while the frequently associated carbonatiteis considered to be the volatile-rich, fugitive material fromthe mantle. Such a relationship is consistent with the dualassociation of carbonatite with either ijolite or kimberliteunder different tectonic conditions. The more common syenite,nepheline syenite, and alkaline granite of the non-orogenicregions are regarded as low-melting fractions from basalticmaterials in the deep crust. Most of this activity, involvingmagmas of residual type, could thus be explained in terms ofpartial melting in the deep crust and upper mantle. A possiblemechanism for this would be arching of the rigid continentalcrust, the consequent relief of lithostatic load giving riseto melting, and the concentration of fugitive constituents,in the underlying zones.