桐柏矿的晶体结构测定  

CRYSTAL STRUCTURE DETERMINATION OF TONGBAITE

作　　者：代明泉[1] 施倪承[1] 马喆生[1] 熊明[1] 白文吉[2] 方青松[2] 颜秉刚[2] 杨经绥[2] 

机构地区：[1]中国地质大学,北京100083 [2]中国地质科学院地质研究所,北京100037

出　　处：《矿物学报》2004年第1期1-6,共6页Acta Mineralogica Sinica

基　　金：国家自然科学基金资助项目(批准号:49972073;40242013)

摘　　要：桐柏矿(Cr3C2)是我国1983年发现的新矿物,因当时实验条件所限仅进行了晶体学参数测定而未进行晶体结构测定。最近在西藏罗布莎蛇绿岩块铬铁矿矿床中又一次发现该矿物,并获得了适宜进行X射线单晶衍射分析的单晶颗粒。采用SMARTAPEX单晶衍射仪电子耦合探测器技术,对该矿物进行了晶体结构的精确测定。测得该矿物属斜方晶系,空间群为Pnam,晶胞参数为a=0.5525(2)nm,b=1.1468(4)nm,c=0.2827(1)nm,晶胞体积V=0.1791(1)nm3,得到了精确的Cr和C原子坐标及其它晶体学参数,单位晶胞内分子个数Z=4{Cr3C2},计算密度D=6.677g/cm3,结构测定精度R(I>2σ(I))=0.0361。测定表明,该矿物属过渡金属碳化物结构,金属原子Cr与C原子呈非等大球的密堆积,C原子与最近邻的6个Cr原子配位,呈三方柱配位多面体,三方柱之间以共棱和共面方式连接。两种[CCr6]三方柱中,Cr—C平均键长分别为0.2047nm及0.2083nm。本文对该矿物与其它类似的过渡金属元素碳化物的晶体结构进行了对比研究。Tongbaite (Cr_(3)C_(2))was found as a new mineral in Chinain 1983. The mineral has been found again recently in the Luobusha ophiolite chromite deposit of Tibet, and a single crystal grain suitable for single crystal X-ray diffractionanalysis was finally selected. The CCD system on the SMART APEX single crystal diffractometer wasused in this experiment to determine the structure of the sample accurately. The mineral belongs to the orthorhombic system, its space group is Pnam, and its unit cell parameters are as follows: a=0.5525(2) nm, b=1.1468(4) nm, c=0.2827(1) nm, α=β=γ=90°,V=0.1791(1) nm^(3). The accurate atomic coordinates of Cr and C and other crystallographic parameters are also obtained. The number of molecules in a unit cell Z=4, the calculated density D=6.677g/cm^(3), and final R[I>2σ(I)]=0.0361. The determination shows that the crystal structure of this mineral belongs to that of the transition metal carbide. Cr atoms and C atomsform the closed packing mode of unequal-sized ball. The triangular prism polyhedra are coordinated by a C atom and six most adjacent Cr atoms, and each of the triangular prisms is connected by sharing edges or planes. The Cr-C bond distances including the two kinds of [CCr_(6)] triangular prism coordination polyhedra are 0.2047 nm and 0.2083 nm, respectively. The crystal structure of this mineral is compared with that of transition metal carbides in this paper.

关 键 词：西藏罗布莎 金属碳化物 桐柏矿 晶体结构 

分 类 号：P578.1[天文地球—矿物学]