富铁硅钛铈矿的晶体化学

Crystal Chemistry of Fe-Rich Chevkinite-(Ce)

通过电子探针分析和晶体结构测定 ,证明四川冕宁碱性花岗岩中的天然非变生铁硅钛铈矿 ,以富含铁、稀土和铌为特征 ,FeO(全铁 )质量分数为 10 .97%～ 11.5 9% ,Ce2 O3变化范围为 2 3 %～ 2 5 % ,Nb2 O5为 1.5 3 %～ 2 .10 % ;化学式中存在A型和 (B +C)型阳离子缺失 ;具有C2 /m空间群 (R =2 .41% ) ;晶胞参数为 :a =1.3 45 6( 3 )nm ,b =0 .5 72 80 ( 10 )nm ,c =1.10 83 ( 2 )nm ,β =10 0 .60 ( 3 )° ,V =0 .83 97( 3 )nm3,Z =2。富铁硅钛铈矿的晶体结构由 2种层组成 ,一是由Si2 O7基团和B -八面体构成的层 ,一是由C -八面体层构成的层 ;2种层均平行ab平面 ,沿c轴交替排列构成三维格架。稀土离子位于双硅酸根和八面体层之间 ,形成 ( 8+3 )配位的A( 1)多面体和 10配位的A( 2 )多面体。Fe和Ti呈部分有序分布于B ,C( 1) ,C( 2A)和C( 2B)位置中。

Chemical composition and crystal structure of natural non-metamict Fe-rich chevkinite-(Ce) from the aegirine-alkali granite, Mianning, Sichuan Province, China, were investigated. Microprobe analyses indicate that FeO (total iron) content in Fe-rich chevkinite (Ce) ranges from 10.97% to 11.59% and Ce_2O_3 from 23% to 25%. There are deficient A- and (B+C)-type cations in the formula. The crystal structure of Fe-rich chevkinite-(Ce) was refined in the space-group type C2/m (R=2.41%). Cell parameters are a=1.3456(3) nm, b=0.5728(1) nm, c=1.1083(2) nm, β=100.60(3)°, V=0.8397(3) nm^3, Z=2. The structure of Fe-rich chevkinite-(Ce) is made up of two kinds of sheets, one is a sheet of Si_2O_7 groups and B-octahedra, and the other is a sheet of C-octahedra, both parallel to the ab plane. The two kinds of sheets alternate along c axis and form a three dimensional framework. The rare earth ions lie between the disilicate groups and the sheets of octahedra to form (8+3)-coordinated A(1)-polyhedra and 10-coordinated A(2)-polyhedra. The Fe and Ti are partially ordered among the octahedral sites of B, C(1), C(2A) and C(2B).