【摘要】 丁道衡矿-(Ce):(Ce,La)4Fe2+(Ti,Fe2+,Mg,Fe3+)2Ti2Si4O22,理想的结构化学式Ce2Ce2Fe2+Ti2Ti2Si4O22,单斜晶系,a=1.34656(15)nm,b=0.57356(6)nm,c=1.10977(12)nm,β=100.636(2)°,V=0.84239(46)nm3,空间群P21/a(假C2/m),Z=2。作为硅钛铈矿(chevkinite)族矿物的新成员,它产于举世闻名的白云鄂博铁-稀土-铌矿床镁夕卡岩中,多数单晶体长0.2~1.0 cm,最大者长大于1.5 cm。共生矿物有透辉石、透闪石、钠透闪石、褐帘石-(Ce)、磁铁矿、尖晶石、烧绿石、氟金云母、氟磷灰石、石英和萤石等。矿物具有褐色条痕和半金属-金属光泽的黑色,其褐黑色碎片半透明。性脆、具有贝壳状断口、无解理和裂开。显微硬度(VHN25g)为606.0~717.4 kg/mm2(相当于摩氏硬度约5.9)。矿物实测密度4.83(7)g/cm3,计算密度值4.88(0)g/cm3。丁道衡矿-(Ce)的反射色为带灰色的浅黄色,多色性为不同色调的灰色。平均反射率(λ=589nm)为11.4%~12.5%。矿物为二轴晶负光性。作过结构精测的丁道衡矿-(Ce)晶体的电子探针测值:SiO2:19.29,TiO2:18.26,A l2O3:0.04,FeO:8.49,Fe2O3:1.67,ThO2:0.16,MgO:1.32,CaO:2.17,Nb2O5:0.47,Ta2O5:0.00,La2O3:19.53,Ce2O3:28.08,Y2O3:0.00,Na2O:0.01,总量99.46%;其中Fe3+/Fe2+的比值依穆斯堡尔谱换算。根据结构精测每个单位分子式中O=22和C(1)位上Ti优先占位,计算的矿物晶体化学式为(Ce2.13La1.49Ca0.48Th0.01)Σ4.11Fe2+(Ti0.88Fe02.+47Mg0.41Fe03.2+6A l0.01)Σ2.03(Ti1.96Nb0.04)Σ2.00(Si2O7)2O8。分别用P21/a和C2/m对丁道衡矿-(Ce)的晶体结构作了精测。结果表明,丁道衡矿-(Ce)的真空间群是P21/a。换言之,它才是真正珀硅钛铈矿[perrierite-(Ce)]的同质多象体。显而易见,硅钛铈矿族矿物既可以有P21/a空间群,又可以有C2/m空间群。更多还原

【Abstract】 Dingdaohengite-(Ce),(Ce,La)4Fe2+(Ti,Fe2+,Mg,Fe3+)2Ti2Si4O22,idealy Ce4Fe2+ Ti2Ti2(Si2O7)2O8,monoclinic,a=13.4656(15),b=5.7356(6),c=11.0977(12),β=100.636(2)°,V=842.39(46)3,space-group P21/a(pseudo-C2/m),Z=2,is a new member of the chevkinite group minerals from the world-known Bayan Obo REE-Nb-Fe mine in China.The mineral occurs as euhedral or subhedral,short columnar or thick tabular//(001) crystals in the magnesian skarn rocks,located mainly in the exocontact between granite and dolomitic marble.Most individual crystals are 0.2-1.0 cm long,the largest one is > 1.5 cm in size.Associated minerals are diopside,tremolite,richterite,allanite-(Ce),magnetite,ilmenite,spinel,titanite,pyrochlore,F-rich phlogopite,fluorapatite,quartz,fluorite,etc.The mineral is black with a brown streak and a submetallic-metallic luster,translucent in thin fragments being brownish-black in colour.It is brittle with conchoidal fractures.No cleavage or parting was observed.Its hardness is VHN25g 606.0-717.4 kg/mm2(Mohs nearly 5.9).And the measured density is 4.83(7) g/cm3,and the calculated density is 4.88(0) g/cm3.In the reflected light,dingdaohengite-(Ce) is yellowish with gray tone,it is pleochroic with different gray tones,reflectance values(in the case of λ=589nm) are within the range of 11.4%-12.5%.It is biaxial negative.Electron-microprobe analysis of the refined structural crystal gave SiO2 19.29,TiO2 18.26,Al2O3 0.04,FeO 8.49,Fe2O3 1.67,ThO20.16,MgO 1.32,CaO 2.17,Nb2O5 0.47,Ta2O5 0.00,La2O3 19.53,Ce2O3 28.08,Y2O3 0.00,Na2O 0.01,and 99.46 wt% in sum;the Fe3+/Fe2+ ratio was converted by Mossbauer spectroscopy.The corresponding chemical formula is(Ce2.13La1.49Ca0.48Th0.01)∑4.11 Fe2+(Ti0.88Fe2+0.47Mg0.41Fe3+0.2Al0.01)2.03(Ti1.96Nb0.04)∑2.00(Si2O7)2O8,based O=22 apfu(atoms per formula unit) and Ti-occupancy at the C(1) site of the structure from the structural refinement.Certainly,a natural member like dingdaohengite-(Ce) with Ti domination at the C(1) site of the structure in the chevkinite sub-group had not been found before.There exist essentially two different viewpoins on the true space-groups of the chevkinite group,viz.,P21/a and C2/m,but a natural material with P21/a space-group in the chevkinite sub-group had not been discovered before,either.For this reason,the crystal structure of dingdaohengite-(Ce) was refined with space-groups P21/a and C2/m,which showed that the true space-group of dingdaohengite-(Ce) is P21/a.In other words,the new mineral is truly a polymorph of perrierite-(Ce).Clearly,the chevkinite-group minerals might have both P21/a and C2/m space-groups.更多还原