冀东麻粒岩系变质峰期水活度,氧逸度及其化学位变化——以太平寨—娄子山地区

脱水反应的热力学计算确定本区太古宙麻粒岩系变质峰期水活度很低,太平寨地区富铝片麻岩和紫苏黑云片麻岩－麻粒岩中ａＨ２Ｏ值一般均在０．１０～０．２０之间,娄子山地区相对稍高,一般为０．１５～０．２５。全区峰期氧逸度ｌｇｆｏ２在－８～－１４之间。（эμＨ２Ｏ／эＸＭｇ＾Ｂｔ）和（эμＯ２／ＸＭｇ＾Ｂｔ）系数平均值太平寨地区分别为－０．２９３和－１．６０,娄子山地区分别为－０．３６４和－１．４２。全区αＨ２Ｏ很低,轴一地区上述各种数值相当均匀,不同地区则有一定差异,这些特征与“碳质变质”机理相符,说明峰期水活度可能受深源ＣＯ２流渗透的控制,属于外部缓冲性质。     

冀东太平寨-娄子山太古宙麻粒岩相变质峰期流体研究

冀东太平寨-娄子山地区太古宙麻粒岩系变质峰期流体包裹体首次见于夕线石中,按形 态 和产状可分三类,均形成于峰期至其稍后阶段。它们成分相似。特征是:①富碳质,XC　O2+C　H4=0.64～0.93; ②含少量H2O,XH 2O=0～0.22,且与峰期矿物 组合平衡热力学计算的水活度(aH2O=0.04～0.31)和含量( XH2O=0.03～0.22)完全吻合; ③有时含一定量H2S和SO2; ④盐水溶液 中阴离子总量极低(0.12～0.61mol/L)。石英和石榴石中也有特征相同的这期包裹体。峰 期包裹体中C　O 2均一温度多数 为-28℃～-22℃,密度为0.96～1.06g/cm3,包裹体整体密度应稍高,相应的压力为0.60 ～0.70G Pa。流体的氧逸度lgfO2在-17～-15之间。峰期流体上述各种特征在空间上较 均匀,相邻的不同岩石类型中aH2O值相似,结合地质和原岩特 征分析,认为流体中C　O2可能为深部地幔来源,其成因与基性岩浆底侵及其结晶作 用有关。     

集宁群和迁西群麻粒岩相变质流体的特征和成因

矿物中流体包裹体研究和脱水反应热力学计算结果表明，集宁群和迁西群麻粒岩相变质峰期流体均富碳质,x(CO2 CH4)一般为70%-80%，x(H2O)≈10%-20%，密度一般为1.0-1.1g/cm^3。集宁群中变质流体的成分和H2O活度在空间上分布不均匀，明显受岩性控制。富铝片麻岩、黑云紫苏片麻岩和基性麻粒岩中αH2O值分别为0.10-0.20、0.20-0.40和0.40-0.60，表明其非外源成因，未曾在岩石中均匀渗透，而可能与进变质过程原岩中普遍存在的有机质氧化和各种脱挥发分反应有关，其富碳质还与H2O相对易于逸散有关。迁西群变质峰期流体成分在空间上分布均匀，不受岩性控制，在玉平寨等地区上述3类岩石中αH2O值主要都在0.13-0.25之间，XH2O为0.1-0.2，反映流体似为外源成因，并确曾在岩石中均匀渗透。本区存在幔源基性岩浆底侵的地质条件，这种富碳质流体可能与它们结晶时析出的CO2流上升有关。     

冀东地区新太古代麻粒岩相变质作用及其大地构造意义

冀东地区的早前寒武纪基底保留有太古宙克拉通普遍发育的"穹窿-龙骨构造",如卢龙-双山子表壳岩系呈近南北向带状分布于以TTG片麻岩为主构成的太平寨卵形构造域、迁安片麻岩穹窿和安子岭片麻岩穹窿之间,洒河桥线性构造带呈北东向切割太古宙构造线分布。在太平寨卵形域和洒河桥线性带中常见有零散分布的麻粒岩相表壳岩块体和古元古代变质基性岩墙。表壳岩块体包括基性和泥砂质麻粒岩和BIF型铁矿等,变质基性岩墙也发育高压麻粒岩相组合,变质锆石年龄为～1.81Ga。本文总结了近年来对该区麻粒岩的研究进展。太平寨卵形域中的基性麻粒岩以中粒二辉麻粒岩为主,有些样品的角闪石周围出现叠加变质形成的微粒矿物组合,个别样品见辉石周围发育石榴石冠状体,形成‘红眼圈’结构;利用稀土元素温度计确定二辉麻粒岩的峰期达到了～1000℃的超高温条件,麻粒岩中的锆石仅记录新太古代末期变质年龄(～2.50Ga),与周围TTG质岩石的结晶时间近于相同。洒河桥线性带中的基性麻粒岩以细粒高压麻粒岩组合为主,偶见中粒二辉麻粒岩残留,其峰期P-T条件分别为800～860℃/1.0～1.2GPa和950～1070℃/1.0GPa;麻粒岩中锆石主体记录新太古代末期变质年龄,但出现少量古元古代变质锆石(1.97～1.83Ga),石榴石-全岩Lu-Hf等时线年龄为1.77～1.78Ga。由此推测太平寨和洒河桥地区都经历了新太古代末期超高温麻粒岩相变质作用,又在古元古代晚期遭受了高压麻粒岩相差异性叠加,太平寨地区受叠加较弱,仍然保留太古宙的卵形构造,而洒河桥一带受叠加较强,形成了线性变形带。太平寨卵形域的泥砂质麻粒岩可识别出4阶段的变质矿物组合:包括包体组合、峰期组合、固相线(或最后)组合以及叠加组合。相平衡模拟表明固相线组合的P-T条件为870～890℃/～0.7GPa,峰期组合可达到1000℃/1.1GPa,石榴石中富钙斜长石包体指示麻粒岩早期经历了低压高温变质阶段。由此构建麻粒岩P-T轨迹为逆时针型,包括3个阶段:低压加热至超高温(AG-I),近等温升压至压力峰期(～1.1GPa)(AG-II),和峰后降压降温至固相线(AG-IIIa)以及在亚固相线下的降温降压过程(AG-IIIb)。锆石定年表明泥砂质麻粒岩原岩沉积年龄稍早于2.50Ga,变质年龄为2.48～2.50Ga。泥砂质麻粒岩的峰期变质条件和时代均与二辉麻粒岩一致,叠加组合的P-T条件与高压麻粒岩相似,反映泥砂质麻粒岩也受到了古元古代晚期变质作用影响。依据太平寨麻粒岩的逆时针型P-T轨迹,推测麻粒岩相变质作用包括如下构造过程:(i)AG-I指示表壳岩层受到后续地幔极高温岩浆喷发被埋深加热,或者受到下部TTG质岩浆海的加热过程;(ii)AG-II指示被加热的岩石(总伴有BIF铁矿层)被破碎并在在密度驱动下沉入岩浆海深部,达到下地壳深度;(iii)AG-IIIa/b对应沉入岩浆海深部的岩石伴随穹窿上升发生减压冷却的过程。很多地质观测和数值模拟研究表明太古宙克拉通的形成受太古宙特有的垂直构造体制控制,与太古代之后线性造山带的构造体制完全不同。     

西昆仑塔什库尔干布伦阔勒群的岩石地球化学特征及变质P-T轨迹

西昆仑布伦阔勒群变质岩是西昆仑造山带的重要组成部分,但其成因一直存在争议.在塔什库尔干县的马尔洋地区,布伦阔勒群主要由石榴斜长角闪片麻岩和孔兹岩组成.根据地球化学特征,石榴斜长角闪片麻岩稀土元素配分曲线可以分为两种类型:一种稀土总量较高(∑REE=190.2×10-6 ～ 359.1×10-6),从轻稀土到重稀土逐渐亏损((La/Yb)N =4.28～5.79),与E-MORB类似;另一种稀土总量较低(∑REE=89.28×10-6～113.0×10-6),轻稀土亏损((La/Yb)N=0.59 ～0.84),重稀土曲线平坦((Gd/Yb)N =0.99 ～ 1.07),与N-MORB类似.微量元素蛛网图中石榴斜长角闪片麻岩具有Ba正异常,Sm、Cr、Zr、Hf和Ti的负异常,轻微的Nb、Ta的负异常,显示为岛弧拉斑玄武岩的特征.孔兹岩的原岩判别图解显示其原岩可能为岛弧环境沉积的页岩和硬砂岩.因此,推测塔什库尔干布伦阔勒群的石榴斜长角闪片麻岩和孔兹岩的原岩形成于岛弧环境.根据岩相学观察、矿物化学分析和温压计算,石榴斜长角闪片麻岩经历了三个变质阶段:M1为高压变质阶段,矿物组合为Grt+ Hbl(1)+ Pl1+ Qtz,变质温压条件为850～ 870℃/12.9 ～13.3kb;M2和M3为两期角闪岩相退变质阶段,矿物组合分别为Hbl2+ Pl2+Qtz和Hbl3+ Pl3+ Kfs+ Bt+ Qtz,变质温压条件分别为730 ～ 770℃/7.3～7.8kb和680 ～ 740℃/4.7 ～5.7kb.孔兹岩也经历了三个变质阶段,推测其早期M1阶段变质温压条件可能与石榴斜长角闪片麻岩的峰期变质阶段相同(850～870℃/12.9 ～ 13.3kb);峰期M2和峰期后M3阶段变质矿物组合分别为Grt2+ Pl2+ Bt2+ Sil+ Qtz和Grt3+ Pl3+ Bt3+Sil+ Mus+ Qtz,温压计算结果分别为800～830℃/7.9～9.2kb和670～700℃/5.1～5.6kb.孔兹岩的M1、M2和M3变质阶段对应于石榴斜长角闪片麻岩的M1、M2和M3变质阶段.上述温压计算结果形成顺时针的P-T轨迹,表现为峰期高压变质作用后叠加了由高角闪岩相-中压麻粒岩相到低角闪岩相的退变质作用,反映了西昆仑与碰撞相关的大地构造背景,这可能与海西期古特提斯洋的闭合有关,之后叠加了印支期构造抬升过程中的剪切作用.     

冀东地区英云闪长质片麻岩的变质结构、P-T-t和水活度

通过英云闪长质片麻岩变质结构的研究,把变质作用分为四个演化阶段,用三组分五个矿物相ACF体系P-T成因格子表示了这个演化过程。四个变质阶段矿物地质温度和压力计算结果构成了逆时针P-T-t轨迹。角闪石生成二辉石的平衡反应计算表明,主期变质水活度明显高于峰期变质水活度。水活度越高角闪石向二辉石转化温度越高,因此水活度是麻粒岩相变质作用的独立因素之一。     

威海地区超高压变质花岗片麻岩锆石U-Pb定年和氧同位素研究

在苏鲁超高压变质带东北端山东威海地区皂埠镇发现锆石δ18O值低至-7.8‰左右的花岗片麻岩, 与前人在苏鲁超高压变质带西南端江苏东海青龙山地区发现的锆石δ18O值为-7‰~-9‰左右的花岗片麻岩一致.对这些低δ18O值花岗片麻岩进行了锆石SHRIMP法UPb定年和系统的激光氟化法矿物氧同位素分析, 结果对低δ18O值锆石成因和花岗片麻岩的原岩性质提供了制约.研究得到: (1) 这些低δ18O值锆石以新元古代岩浆锆石为主, 但部分岩浆锆石在印支期超高压变质作用过程中发生了不同程度的重结晶作用.δ18O值为-7.08‰的岩浆核锆石UPb定年得到的花岗片麻岩原岩谐和年龄和不一致线上交点年龄分别为(760±49) Ma和(751±27) Ma, 变质谐和年龄和不一致线下交点年龄分别为(232±4) Ma和(241±33) Ma, 指示其原岩为新元古代花岗岩并经历了印支期变质作用; (2) 锆石δ18O值在局部范围内变化于-7.76‰~5.40‰之间, 低δ18O值岩浆锆石表明它们是从新元古代低δ18O值岩浆中直接结晶形成, 锆石δ18O值的局部变化表明其原岩岩浆的氧同位素组成具有不均一性, 指示低δ18O值岩浆源区物质曾经在地表与极度亏损18O的大气降水发生过不同程度的高温水岩反应; (3) 低δ18O值花岗片麻岩在印支期板块俯冲和折返过程中基本没有与外部发生显著的氧同位素交换, 在退变质作用过程中花岗片麻岩内部缓冲流体对原岩岩浆锆石的δ18O值影响不大.威海皂埠镇地区和东海青龙山地区的花岗片麻岩在原岩时代、变质时代和氧同位素组成等方面基本相同, 指示它们应具有相同的原岩性质, 并经历了相同的变质作用和水岩相互作用过程.因此, 极度亏损18O的新元古代双峰式基性-酸性岩浆岩可能分布于整个大别-苏鲁造山带.      

西伯利亚克拉通南缘奥里洪地块麻粒相变质作用及构造意义

早古生代西伯利亚克拉通南缘发生了大规模的增生-碰撞造山运动,本文研究的地区--奥里洪地块记录了巴尔古津微板块与西伯利亚克拉通碰撞造山的事件.对奥里洪地块出露的两种典型的高级变质岩--石榴辉石岩和石榴黑云片麻岩的矿物成分分析和变质温压计算,表明它们都经历了麻粒岩相的峰期变质作用,峰期变质温度达到770～800℃,而压力曾达到1.0GPa左右:峰后的退变质作用仍具有较高的温度,但压力明显降低(700～730℃,0.065GPa和710～766℃,0.50GPa),显示了一个近等温降压(ITD)的顺时针P-T轨迹特征.石榴黑云片麻岩中变质锆石的原位LA-ICP-MS U-Pb定年表明,麻粒岩相峰期变质年龄为479±2Ma,而峰前变质可能在500Ma就已经开始.峰后的退变质作用很可能发生在475～460Ma之后.整个造山作用持续了至少35Ma.对比蒙古-图瓦地块及中国东北佳木斯-额尔古纳地块已厘定出的变质作用及岩浆活动年龄可以发现,西伯利亚克拉通南缘不同地区增生-碰撞造山作用发生的时间是不同的,奥里洪地区造山作用相对年轻.     

江西庐山星子群片麻岩的变质年龄和变质机制:独居石和锆石SIMS年代学

江西庐山地区星子群片麻岩的构造属性对认识江西甚至华南的大地构造具有重要意义。对星子群片麻岩穹窿不同构造部位的4个片麻岩中独居石进行高精度SIMS U-Pb年龄测定,结果显示它们的²⁰⁶Pb/²³⁸U加权平均年龄分别为141.0±1.0 Ma、143.0±1.0 Ma、143.5±1.0 Ma和144.2±1.6 Ma,因此将星子群的峰期变质时代限定于141.0～144.2 Ma。同时本文首次报道了庐山观音尖辉长岩的锆石SIMSU-Pb定年,确定其侵位年龄为141.1±1.5Ma,这与星子群片麻岩独居石获得的峰期变质年龄在误差范围内一致。结合星子群的变质温度和变质分带特征,推测庐山片麻岩穹窿可能是中生代岩浆热穹窿,其角闪岩相的变质作用可能是岩浆热动力变质的产物。本研究不仅为庐山地区构造变形属性的厘定提供了详实的定年证据,还能为中国东部中生代板内强烈变质变形的研究提供重要参考。     

河南洛宁太华变质杂岩区早元古代变质作用P-T-t轨迹及其大地构造意义

太华变质杂岩广泛出露于华北克拉通南缘,总体呈SW-NE向展布.在河南洛宁地区,太华变质杂岩以TTG片麻岩、斜长角闪片麻岩和变泥质片麻岩为主.斜长角闪片麻岩中可识别出三个阶段的变质矿物组合:进变质阶段矿物组合(M1)为石榴子石变斑晶内部的包裹体矿物组合( Amp1+ Pl1+Qtz),变质高峰期矿物组合(M2)为石榴子石变斑晶边部和基质矿物组合( Grt2+ Amp2+ Pl2+ Qtz),退变质阶段矿物组合(M3)为“白眼圈”状后成合晶组合(Amp3+ Pl3+ Qtz).运用矿物温度计与压力计估算三个阶段的P-T条件分别为:进变质阶段约600 ～ 680℃/7.0～ 7.6kbar,变质高峰期为680 ～ 790℃/9.5 ～10.7kbar,退变质阶段为580～720℃/6.5 ～7.6kbar.变泥质片麻岩中保留了进变质阶段(M1)包裹体矿物组合(Bt1+Pl1+Qtz)和峰期变质阶段(M2)矿物组合(Grt2 +Bt2+Pl2 +Qtz)两个阶段.其中未发现后期退变质反应结构,石榴子石中也未发现成分环带.P-T条件估算结果分别为:M1阶段620 ～ 710℃/4.9 ～5.6kbar,M2阶段710～760℃/7.3～8.3kbar.洛宁地区太华变质杂岩记录了顺时针的近等温降压型的P-T轨迹,可能经历了与华北中部造山带其它杂岩类似的变质演化过程,推测其形成于华北克拉通东部陆块和西部陆块沿中部造山带的拼合过程中.SIMS与ICP-MS锆石U-Pb定年表明,斜长角闪片麻岩记录了1938～1967Ma的变质事件,比华北中部造山带其它变质杂岩区所广泛记录的～1850Ma变质事件早了约100Ma,暗示中部造山带的拼合可能是一个长期的、复杂的过程.     

岩石钾质变化及钾交代与成矿关系

钾质高低及变化可反映岩石演化程度和钾交代作用。发生钾交代的岩石为成矿有利岩石和找矿标志。岩石钾交代与钾质升高归根于富碱的地幔流体活动。上升的地幔流体交代上地幔和地壳岩石形成富碱岩石和碱交代岩,并活化迁移不同围岩中成矿元素形成各种矿床。大地构造单元边缘与陆内张裂活动带是地幔流体交代活动场所和大型矿床形成地。许多矿床包括金矿床和钾交代关系密切。     

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.     

298K时三元体系K^＋／CO3^2—,B4O7^2——H2O和Li^＋／CO3^2—,B4O7^2——H2O相关系

研究了二个三元体系Ｌｉ＋／ＣＯ２－３,Ｂ４Ｏ２－７Ｈ２Ｏ（１）和Ｋ＋／ＣＯ２－３,Ｂ４Ｏ２－７Ｈ２０（２）２９８Ｋ时的相平衡关系和平衡液相的物化性质（密度、折光密、粘度、电导率、ｐＨ值）。研究表明：这二个三元体系均属简单共饱型,无复盐或固溶体形成。体系（１）的两段溶解度曲线对应于无水Ｌｉ２ＣＯ３和Ｌｉ２Ｂ４Ｏ７·３Ｈ２Ｏ结晶区,体系（２）的两段溶解度曲线对应于Ｋ２ＣＯ３·３／２Ｈ２Ｏ和Ｋ２Ｂ４Ｏ７·４Ｈ２Ｏ结晶区。     

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     

P--T Stabilities of Laumontite, Wairakite, Lawsonite, and Related Minerals in the System CaAl2Si2O8-SiO2-H2O

Hydrothermal investigation of the bulk composition CaO.Al₂O₃.4SiO₂+excessH₂O has been conducted using conventional techniques over thetemperature ranges 200–450 °C and 500–6000 barsP_fluid. A number of reactions have been studied by employingmineral mixtures consisting of reactants and products in about9: 1 and 1: 9 ratios. The phase relations were deduced fromrelatively long experiments by observing which seeded assemblagedisappeared or decreased markedly in one of the paired run charges. Laumontite was synthesized in the laboratory, probably for thefirst time. Laumontite was grown from seeded wairakite to over99 per cent using a weak NaCl solution. The refractive indicesof the synthetic material are about = 1.504 and = 1.514. Theaverage unit cell dimensions are a₀ = 14.761±0.005 Å;b₀ = 13.077±0.005 Å; c₀ = 7.561±0.003 Å;and ß = 112.02°±0.04°. Within the errorof measurement, the optical properties and cell parameters arein good agreement with those of natural laumontite. The equilibriumdehydration of laumontite involves two reactions: (1) laumontite= wairakite+2H₂O, passing through about 230 °C at 0.5 kb,255±5 °C at 1 kb, 282±5 °C at 2 kb, 297±5°C at 3 kb and 325±5 °C at 6 kb; and (2) laumontite= lawsonite+2 quartz+2H₂O, taking place at about 210 °Cat 3 kb and 275 °C at 3.2 kb. Above 300 °C, the equilibriumcurve for the solid-solid reaction (3) lawsonite+2 quartz =wairakite passes through 305 °C, 3.4 kb and 390 °C,4.4 kb. Equilibrium has been demonstrated unambiguously forthe above three reactions. The hydrothermal decomposition ofnatural laumontite above its own stability limit appears tobe a very slow process. Combined with previously published equilibria determined hydrothermallyfor wairakite, the phase relations are further investigatedby chemographic analysis interrelating the phases, laumontite,wairakite, lawsonite, anorthite, prehnite+kaolinite, and 2 pumpellyite+kaolinitein the system CaAl₂Si₂O₈-SiO₂-H₂O. This synthesis allowed theconstruction of a semiquantitative petrogenetic grid applicableto natural parageneses and the delineation of the physical conditionsfor the various low-grade metamorphic facies in low µCO₂environments. The similar stratigraphic zonations, consistentlyfound in a variety of environments, are recognized to be a functionof burial depth, geothermal gradient, and mineralogical andchemical composition of the parental rocks. Departures fromthe normal sequences are believed to be due to the combinationsof mineralogical variations, availability of H₂O, differencesin the ratio µCO₂/µH₂O, and the rate of reaction.The possible P-T boundaries for diagenesis, the zeolite facies,the lawsonite-albite facies, the prehnite-pumpellyite facies,and the adjacent metamorphic facies are illustrated diagrammatically.     

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.     

High-pressure,high-temperature stability of surinamite in the system MgO-BeO-Al2O3-SiO2-H2O

The MgAl surinamite end member, (Mg₃Al₃)^[6]O[AlBeSi₃O₁₅], was synthesized in the requisite system with and without water. The new phase is monoclinic, space group P2/n, with a=9.881(1)Å; b=11.311(1) Å; c=9.593(1) Å; =109.52(2)°. Refractive indices are n _x=1.7015(20); n _y=1.7035(20); n _z=1.7055(20). The infrared spectrum shows characteristic differences against the structurally related and optically extremely similar phase sapphirine.Using the seeding technique, the preliminary stability field for MgAl surinamite was found to lie at high temperatures (650 °C) and high pressures (4 kbar). At lower temperatures breakdown takes place to hydrous assemblages of chlorite, talc, and chrysoberyl with kyanite or yoderite; at lower pressures chrysoberyl forms parageneses with sapphirine and cordierite. In crystal chemical terms the underlying principle for the stability of surinamite versus that of the low-pressure assemblages is the higher proportion of octahedrally coordinated Al in surinamite (75%). Following the same principle surinamite itself decomposes at still higher pressures to a paragenesis, in which all Al enters octahedral coordination (pyrope+a chrysoberyl-type phase and some unidentified X-ray peaks).The stability field of synthetic MgAl surinamite is in good agreement with P, T-estimates of some 8–12 kbar, 800°–950° C as taken from the literature for the few occurrences of natural, Fe-bearing surinamite in granulite and upper amphibolite facies environments. The incorporation of iron in surinamite must be limited, because this mineral is known to coexist with its more iron-rich breakdown assemblage almandine-rich garnet+chrysoberyl. As the minimum melting curve of granite under hydrous conditions lies outside the surinamite field up to a water pressure of about 20 kbar, the absence of surinamite in normal granitic pegmatites can already be explained by physical constraints. However, there are probably also chemical constraints in the generally high Fe/Mg bulk chemistry of the pegmatite environments.Now at Institut für Kristallographie, Technische Hochschule, Templergraben 55, D-5100 Aachen, FRG     

Synthesis and stability relations of wairakite,CaAl2 Si4 O12·2H2O

Hydrothermal investigation of the bulk composition CaO·Al₂O₃·4SiO₂ + excess H₂O has been conducted using conventional techniques over the temperature range 200–500° C and 500–5,000 bars P _fluid. The fully ordered wairakite was synthesized unequivocally in the laboratory, probably for the first time.The gradual, sluggish and continuous transformation from disordered to ordered wairakite evidently accounts for failure by previous investigators to synthesize ordered wairakite in runs of week-long duration. The dehydration of metastable disordered wairakite to metastable hexagonal anorthite, quartz and H₂O has been determined; this reaction takes place at temperatures exceeding 400° C, even at fluid pressures of 500 bars or less. The upper P _fluid-T boundary of the disordered phase is equivalent to the maximum temperature curve of synthetic wairakite presented by previous investigators. The hydrothermal breakdown of natural wairakite above its stability limit appears to be a very slow process.The equilibrium dehydration of wairakite to anorthite, quartz and H₂O occurs at 330±5° C at 500 bars, 348±5° C at 1,000 bars, 372±5° C at 2,000 bars and 385±5° C at 3,000 bars. Where fluid pressure equals total pressure, the thermal stability range of wairakite is about 100° C wide. At lower temperatures wairakite reacts with H₂O to form laumontite. Reconnaissance experiments dealing with the effect of CO₂ on stabilities of calcium zeolites suggest that wairakite or laumontite may be replaced by the assemblage calcite + montmorillonite in the presence of a CO₂-bearing fluid phase.The determined P _fluid -T field of wairakite is compatible with field observations in some metamorphic terrains where it is related to the shallow emplacement of granitic magma and with direct pressure-temperature measurements in certain active geothermal areas. Under inferred conditions of higher _CO2/_H2O ratios, essentially unmetamorphosed rocks grade directly into those characteristic of the greenschist facies; moderately high values of _CO2 in carbonate-bearing rocks result in the downgrade extension of the greenschist facies at the expense of zeolite-bearing assemblages.