The crystal and magnetic structure of Li-aegirine LiFe3+Si2O6: a temperature-dependent study

Single crystals of Li-aegirine LiFe³⁺Si₂O₆ were synthesized at 1573?K and 3?GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223?K. LiFe³⁺Si₂O₆ is monoclinic, space group C2/c, a=9.6641(2)?Å, b= 8.6612(3)?Å, c=5.2924(2)?Å, β=110.12(1)^° at 300?K as refined from powder neutron data. At 229?K Li-aegirine undergoes a phase transition from C2/c to P2₁ /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622?Å and 〈SiB–O〉=1.624?Å at 100?K are present. The bridging angles of the SiO₄ tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100?K in the two independent tetrahedral chains in space group P2₁ /c, whereas it is 180.83(9)° at 300?K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100?K four Li–O bond lengths lie within 2.072(4)–2.172(3)?Å, the fifth Li–O bond length is 2.356(4)?Å, whereas the Li–O3?A bond lengths amount to 2.796(4)?Å. From ⁵⁷Fe Mössbauer spectroscopic measurements between 80 and 500?K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5?K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe³⁺Si₂O₆. Distinct preordering phenomena can be observed up to 35?K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T _N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.     

汉诺坝幔源单斜辉石巨晶氢同位素组成的离子探针分析：微尺度不均一性

运用离子探针技术测定了河北汉诺坝新生代玄武岩中3个单斜辉石巨晶的氢同位素组成，结果显示同一颗粒内部表现出微尺度的不均一性，2mm范围内δD的变化达到60‰。δD和氢含量之间不存在同步的变化，巨晶内部的化学成分均一，因此我们认为巨晶的氢同位素不均一性继续自母岩浆。母岩浆的氢同位素变化可能是去气过程中气相与熔体之间的分馏引起的。单斜辉石巨晶形成后很短的时间内即被寄主岩浆带至地表并经历了快速淬火。     

湖北铜绿山铜铁矿床中煌斑岩的矿物学特征及岩石成因初探

湖北铜绿山矽卡岩型铜铁矿床处于长江中下游铁铜成矿带西部的鄂东南矿集区。最近我们在该矿区工作中采集到两条侵入于含矿石英二长闪长玢岩中的煌斑岩脉。本文对这些煌斑岩脉进行了详细的镜下岩相学观察,并对其中造岩矿物黑云母、辉石和长石进行了电子探针成分分析。根据这些数据本文论述了煌斑岩的物源、形成环境以及岩浆体系的物理化学条件。研究表明,煌斑岩中云母为富镁黑云母,其形成的岩浆体系处于中低氧逸度(近Ni-Ni O);黑云母与全岩都具有高钛特征,表明煌斑岩的物源深度可能来自较深部地幔(如软流圈)。煌斑岩中辉石为单斜辉石,环带较为发育,有韵律环带和正环带,其韵律环带的形成可能是由于岩浆温压环境存在振荡性变化而形成的。铜绿山矿床中煌斑岩属于云煌岩;其岩浆来源具幔源性,受到地壳的混染作用不强。     

东疆二叠纪镁铁-超镁铁岩体中辉石的成分特征及其对成岩和Ni-Cu成矿的指示

新疆东部早二叠世镁铁-超镁铁岩体主要分布在天山东段觉罗塔格构造带、中天山地块和北山地区,与这类岩体相关的Ni-Cu矿床主要产出在觉罗塔格带和中天山地区,北山目前并未发现典型Ni-Cu矿床。这些岩体中的斜方辉石主要为古铜辉石,少量为紫苏辉石;单斜辉石属透辉石、次透辉石或普通辉石。矿物成分系统变化显示橄榄石Fo值北山最高、觉罗塔格带到中天山逐次降低的特点,橄榄石Fo值在78~86和Ni含量低于1800×10-6范围内更有利于Cu-Ni成矿;斜方辉石En端元和单斜辉石Wo端元极差值也显示北山最高、觉罗塔格带和中天山较低的规律,且相对小的极差值更有利于成矿。单斜辉石AlⅣ/AlⅥ比值结果显示北山岩体在矿物结晶过程中压力较低,硫溶解度较大,可能是其矿化程度弱于觉罗塔格带和中天山的一个重要因素。觉罗塔格带、中天山和北山镁铁-超镁铁质岩体的母岩浆为非碱性系列、大陆拉斑玄武质岩浆。根据北山和觉罗塔格带岩体单斜辉石微量元素成分估算的母岩浆具有LREE富集以及Nb、Ta、Zr、Hf负异常的特征,指示出两者源区为受俯冲板片交代的地幔,且坡北源区俯冲交代程度更低。觉罗塔格带和中天山单斜辉石Alz vs.Ti显示裂谷堆晶趋势向弧堆晶趋势演化,而北山单斜辉石Alz vs.Ti则显示明显的裂谷堆晶趋势,这说明俯冲交代作用在觉罗塔格带和中天山地区表现的更为强烈。北山镁铁-超镁铁岩体中发现单斜辉石Ti的高异常及矿物温压计算结果显示,表明北山地区具有比觉罗塔格带和中天山更高的岩浆温度,这可能与塔里木二叠纪地幔柱的活动有关。     

High-pressure X-ray and Raman study of a ferrian magnesian spodumene

The high-pressure behaviour of a synthetic P2₁/c ferrian magnesian spodumene, ^M2 (Li_0.85Mg_0.09Fe²⁺ _0.06)^M1(Fe³⁺ _0.85Mg_0.15)Si₂O₆, has been investigated using in situ single-crystal X-ray diffraction and Raman spectroscopy. No phase transition has been observed within the pressure range investigated. The isothermal equation of state up to 7 GPa was determined. V₀, K_T0 and K, simultaneously refined with a Murnaghan equation of state, are: V₀= 415.66(7) ų, K_T0=83(1) GPa and K=9.6(6). The magnitudes of the principal unit-strain coefficients were calculated and their ratios 1:2:3=1.00:1.85:2.81 at P=6.83 GPa indicate a very strong anisotropy. Monitoring of the intensity of b-type reflections (h+k= 2n+1) confirms that from room conditions up to 7 GPa the primitive lattice is maintained. Raman spectra have been collected up to 7.4 GPa. No change in the number of observed vibrational modes occurs in the pressure range investigated. At high frequency, the Raman doublet relative to the Si–O–Si vibrations of the two distinct tetrahedral chains is a broad band at room pressure, however, the frequency difference between the two modes increases with increasing pressure.Operating system: Windows NT     

腾冲火山岩斜长石斑晶的过剩氩兼论火山活动的分期

在腾冲黑空山、打鹰山和马鞍山火山岩的斜长石斑晶里首次发现存在过剩氩,并从基质中得到比以往的全岩年龄更年轻的KAr年龄,论证了以往所发表这一时间段的KAr 年龄结果可能因过剩氩而偏老,结合马鞍山已经取得的10 ka左右的U系年龄结果认为,腾冲最近一期火山活动的终止时间应为全新世。     

南秦岭早古生代玄武岩的岩浆源区及演化过程

南秦岭地区早古生代玄武岩中发育的大量单斜辉石斑晶为研究火山岩的深部演化过程及源区属性提供了重要的载体。本文通过对早古生代玄武岩及其中的单斜辉石斑晶进行矿物学、岩石学及地球化学分析,讨论火山岩的演化历程及源区属性。电子探针分析结果表明玄武岩中单斜辉石斑晶属于透辉石,其成分与全岩成分并不平衡,暗示岩石经历了单斜辉石的堆晶作用。通过质量平衡计算得到了与单斜辉石斑晶平衡的熔体并计算了单斜辉石结晶的温压条件,结果显示单斜辉石斑晶结晶压力为7.6～14.0kbar,温度为1201～1268℃。高压下的分离结晶作用导致了单斜辉石成为主要的结晶相。重建后的玄武岩具有高镁、高钙、富钛,富集高场强元素Nb、Ta,亏损Rb、K、Sr和P,Dy/Yb比值高的地球化学特征,指示其源区为含单斜辉石、磷灰石及石榴石的交代岩石圈地幔。同地区发现的玄武质角砾也具高镁及高钙的特征,其富集Ba、Nb、Ta、Ti及低Dy/Yb比值表明源区为含单斜辉石、角闪石及尖晶石的交代岩石圈地幔。     

河北矾山杂岩体中单斜辉石的研究

河北矾山杂岩体属于二氧化硅不饱和的超钾质碱性-过碱性岩浆系列,由单斜辉石岩、辉石正长岩和碱长正长岩等不同类型岩石组成。各类型岩石主要组成矿物为单斜辉石、黑云母、石榴石和正长石。本文利用电子探针对单斜辉石进行了详细研究,发现所有单斜辉石属于高钙透辉石,随着岩浆的演化,主要表现为Fe²⁺对Mg²⁺的替代关系,结晶趋势为透辉石→钙铁辉石,这揭示矾山杂岩体岩浆体系的特点是高温、中等大小的氧逸度、贫硅、富碱（尤其是钾）。辉石中的Al^IV含量取决于岩浆的硅饱和度,硅越不饱和,Al^IV含量越高;Al^VI含量则与体系中的Al含量呈正相关关系。由Al对Si的替代引起的电荷不平衡主要由八面体位置的Fe³⁺来补偿,其次为少量的Ti⁴⁺和更少量的Al³⁺。体系中的Ti含量与体系的温度呈正相关关系,而Na含量则与辉石中的Ti和Fe³⁺含量成正相关关系。不同类型岩石中单斜辉石从核部到边部随着Mg^#的降低,Al、Ti含量呈现不同的演化趋势,这是因为它们于岩浆演化的不同阶段开始结晶,经历了不同的岩浆演化史。碱性岩中高钙辉石的出现和成分环带的普遍发育是由岩浆的贫硅富钾特征决定的。单斜辉石的成分不仅受控于结晶时的温度、压力条件,也受控于岩浆的总成分及其变异。     

松辽盆地东南隆起区营城组珍珠岩斑晶矿物研究

松辽盆地早白垩世营城组火山岩系含有丰富的珍珠岩,为松辽盆地重要的油气勘探目的层位。在松辽盆地边缘东南隆起九台地区出露有比较新鲜的珍珠岩,内部斑晶保存完好,裂隙发育;斑晶矿物主要有斜长石、黑云母、角闪石,还有镁铁质的单斜辉石和斜方辉石。对含斑晶珍珠岩和剔除斑晶后的珍珠岩玻璃质分别进行地球化学分析,斑晶矿物进行单矿物电子探针成分研究。其结果表明：该珍珠岩属于酸性高钾钙-碱性系列,微量和稀土元素特征显示与上部地壳相似;斑晶矿物单斜辉石为斜紫苏辉石和次透辉石,斜方辉石为紫苏辉石,均显示变质成因,属于捕虏晶;与珍珠岩相邻层位玄武岩的斑晶斜长石为拉长石,单斜辉石为铁次透辉石,属于岩浆成因。这些特征表明,珍珠岩原岩可能来自于上部地壳,其辉石等捕虏晶为岩浆上侵运移过程与围岩发生混染作用的结果。     

The effect of Ca-Tschermaks component on trace element partitioning between clinopyroxene and silicate melt

We have studied the influence of Ca-Tschermaks (Calcium Tschermaks or CaTs) content of clinopyroxene on the partitioning of trace elements between this phase and silicate melt at fixed temperature and pressure. Ion probe analyses of experiments carried out in the system Na₂O–CaO–MgO–Al₂O₃–SiO₂, at 0.1 MPa and 1218°C, produced crystal-melt partition coefficients (D) of 36 trace elements (Li, Cl, Sc, Ti, V, Cr, Fe, Co, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and W), for clinopyroxene compositions between 10 and 32 mol% CaTs. Partition coefficients for 2+ to 5+ cations show, for each charge, a near parabolic dependence of log D on ionic radius of the substituting cation, for partitioning into both the M1 and M2 sites of clinopyroxene. Fitting the results to the elastic strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452–454] we obtain results for the strain-free partition coefficients of theoretical cations (D₀), with site radius r₀, and for the site's Young's Modulus (E).

In agreement with earlier data our results show that increasing ^ivAl concentration in cpx is matched by increasing D, E_M1, E_M2 and D₀ for tri-, tetra- and pentavalent cations. The degree of fractionation between chemically similar elements (i.e. Ta/Nb, Zr/Hf) also increases. In contrast, D values for mono-, di- and hexavalent cations decrease with increasing ^ivAl in the cpx. The large suite of trace elements used has allowed us to study the effects of cation charge on D₀, r₀ and E. We have found that D₀ and r₀ decrease with increasing cation charge, e.g. r₀=0.66 Å for 4+ cations and 0.59 Å for 5+ cations substituting into M1. Values of E_M1 and E_M2 increase with cation charge as well as with increasing ^ivAl content. The increase in E_M2 is linear and close to the trend set by Hazen and Finger [Hazen, R.M., Finger, L.W., 1979. Bulk modulus-volume relationship for cation–anion polyhedra. J. Geophys. Res. 84 (10) 6723–6728] for oxides. E_M1 values are much higher and do not fit the trend predicted by the Hazen and Finger relationship.