我国羟硅铈矿的发现与矿物学特征

Mineralogy of tornebohmite-(Ce)discovered for the first time in China

本文研究的羟硅铈矿是国内首次发现的该种矿物,产于河南太平镇稀土矿的破碎带蚀变岩中,与铈褐帘石、gatelite-(Ce)、富氟硅铈石、氟碳铈矿、直氟碳钙铈矿、氟铈矿、氟镧矿等稀土矿物紧密共生,多呈铈褐帘石假象细粒状集合体或与gatelite-(Ce)平行交生沿外围交代铈褐帘石产出,粒径一般小于0.1 mm,为自形-半自形,粒状-板状。该矿物呈浅绿色至橄榄绿色,透明,玻璃光泽,不规则断口,摩氏硬度约为4.5,计算密度5.08 g/cm3。光学性质:二轴晶,正光性,多色性明显,单偏光下呈绿色,正交偏光下具有鲜艳的高级干涉色。电子探针分析显示,矿物成分w(Ce_2O_3) 32.05%～34.26%,w(La_2O_3) 13.67%～17.66%,w(Pr_2O_3) 2.92%～3.68%,w(Nd_2O_3) 7.52%～10.34%,w(Al_2O_3) 8.59%～10.93%,w(SiO_2) 21.35%～24.97%,按照Si+P=2(apfu)计算单位分子式中阳离子数,OH+F=1(apfu)计算单位分子式中羟基的离子数,得出该矿物的经验化学式为(Ce1.02La0.49Nd0.27Pr0.10 Sm0.02Gd0.01Th0.01Mg0.06 Ca0.01)Σ2.00 (Al0.99Fe0.03)Σ1.02(SiO4)Σ2.00(OH0.81F0.19)Σ1.00。X射线单晶衍射分析表明,该矿物属于单斜晶系,晶胞参数:a=7.4382(3)?, b=5.6730(2)?, c=16.9819(8)?,β=118.84(0)°, Z=4, V=655.13(5)?3,空间群为P21/c。矿物的激光拉曼光谱特征峰主要为241、362,862、895、954和3722cm-1等,上述特征均与国外已发表的羟硅铈矿数据非常相似,是对羟硅铈矿分子振动光谱特征的首次探索。本文还对羟硅铈矿的矿物族归属以及矿物成因进行了初步的探讨。

The t?rnebohmite-(Ce),which was discovered in China for the first time,occurred in altered rocks in fracture zones of the Taipingzhen REE deposit,Henan Province.It is closely associated with other REE minerals including allanite-(Ce),gatelite-(Ce),F-rich cerite-(Ce),bastn?esite-(Ce),synchysite-(Ce),fluocerite-(La),and fluocerite-(Ce)in forms of intergrowth.It mostly occurred as fine-grained aggregates in form of allanite-(Ce)pseudomorph,or as the t?rnebohmite-gatelite-(Ce)intergrowth which replaced marginal parts of allanite-(Ce)crystals.It has euhedral-subhedral,granular-tabular crystals with size of generally less than 0.1 mm.It is transparent.It has light green to olive green color,glass luster,irregular fracture,Mohs’hardness of about 4.5,with calculated density of 5.08 g/cm3.It has optical properties of biaxial crystal,optically positive,strong polychromatism,green color under single polarized light,with bright high-level interference color under crossed nicols.Its chemical compositions analyzed using EPMA(with 10 analytical points in 2 grains)are of 32.05%-34.26%Ce2O3,13.67%-17.66%La2O3,2.92%-3.68%Pr2 O3,7.52%-10.34%Nd2O3,8.59%-10.93%Al2O3,and 21.35%-24.97%SiO2.According to the equation of Si+P=2 to calculate the number of cations,and the equation of OH+F=1 to calculate the number of anions,its empirical formula is(Ce1.02 La0.49 Nd0.27 Pr0.10 Sm0.02 Gd0.01 Th0.01 Mg0.06 Ca0.01)Σ2.00(Al0.99 Fe0.03)Σ1.02(SiO4)Σ2.00(OH0.81 F0.19)Σ1.00.On the basis of the analysis of X-ray single crystal diffraction,the t?rnebohmite-(Ce)belongs to monoclinic system,with the unit cell parameters of a=7.4382(3)?,b=5.6730(2)?,c=16.9819(8)?;β=118.84(0)°,Z=4,V=655.13(5)?3,and the space group of P21/c,respectively.Furthermore,characteristic peaks of the laser Raman spectral were observed at 241,362,862,895,954,3722 cm-1,respectively.All those features of t?rnebohmite-(Ce)from the Taipingzhen are very similar to those of the reported t?rnebohmite-(Ce)in abroad.This is the first exploration for the molecular vibrational characteristics of t?rnebohmite-(Ce)from the Taipingzhen REE deposit.The genesis and affiliation of t?rnebohmite-(Ce)have also been basically discussed in this paper.