山东蒙阴金伯利岩中石榴石特征及种类划分

The characteristics and classification of garnets from kimberlite in Mengyin,Shandong Province

石榴石是山东蒙阴金伯利岩型金刚石矿区的重要指示矿物之一,为了确定山东蒙阴金刚石矿区颜色复杂的石榴石种类,判断矿区石榴石特征与含矿性关系.项目组对矿区不同含矿程度金伯利岩中的石榴石进行了系统采样,测定了50件单晶石榴石微区化学成分和128个石榴石单晶的晶胞参数,测试结果显示：山东蒙阴金伯利岩中石榴石族矿物化学式A3B2[SiO4]3中的A组阳离子由Mg^2＋、Ca^2＋和Fe^2＋组成,二价阳离子主要为Mg^2＋,Mg^2＋占据十二面体空腔中心位置达53％ ～82％左右,Ca^2＋占据十二面体空腔中心位置小于7％ ～17％、Fe^2＋占据十二面体空腔中心位置5％ ～31％左右.B组阳离子主要由Al3＋、Cr3＋和Fe3＋组成,三价阳离子主要为Al^3＋,[AlO6]八面体占位达62％ ～92％左右,[FeO6]和[CrO6]八面体占位小于38％;按50个石榴石不同阳离子占据十二面体空腔和八面体空腔的位置多少,山东蒙阴石榴石可划分为10个亚种.根据山东蒙阴金伯利岩中石榴石成分,计算出石榴石形成压力为6.0～9.0 GPa,推测山东蒙阴矿区金伯利岩中有来自深源的石榴石.128件石榴石晶胞参数统计结果显示：山东蒙阴无矿、贫矿、中等含矿、富矿金伯利岩岩体中紫色石榴石晶胞参数分别为大于1.162nm、1.159～1.160nm、1.156～1.160nm和1.155～1.157nm之间,从无矿岩体→贫矿岩体→中等含矿岩体→富矿岩体,紫色石榴石晶胞参数值有明显变小的趋势.

Garnet is one of the main indicator minerals of kimberlite type diamond deposits in Mengyin,Shandong Province.In order to determine the types of complex color garnets in diamond deposits of Mengyin and the relations between characteristics of garnet and ore potential,the research group systematically sampled the garnets in kimberlite of different mineralization degrees in ore deposits.The MgO,SiO2,FeO,TiCO2,Al2O3,Cr2O3,CaO,MnO values of 50 single crystal garnets in the kimberlite were measured by electron microprobe spectrometer.128 garnet single crystal lattice parameters were measured by X-ray single crystal diffraction.Some data were obtained：In the garnet,MgO values vary in the range of 14.44 % ～23.44 %,SiO2 40.34 % ～43.64%,FeO 5.87% ～ 16.97%,TiO2 0.00% ～ 1.45%,Al2O3 14.25% ～ 22.30%,Cr2O3 0.00% ～11.88%,CaO 2.88% ～6.68%,CaO/MgO ratios are in the range of 0.12 ～ 0.44,FeO/MgO 0.25～1.15,Cr2O3/Al2O3 0.00 ～ 0.61,and TiO2/Al2O3 0.00～0.07.According to the above data,the garnets in kimberlite of Mengyin are probably of the magnesium,iron and calcium-aluminum chromium garnet series.In order to divide types of the garnets in kimberlite,the authors calculated the formula of the 50 garnets according to the chemical composition,and the results show that the chemical formula of garnet group minerals in kimberlite is A3B2 [SiO4]3.Group A cations are composed of Mg^2 ＋,Ca^2 ＋ and Fe^2 ＋.Divalent cation is mainly Mg^2 ＋.Mg^2 ＋occupies about 53 % ～82 % of the dodecahedron cavity center.Ca2 ＋ occupies less than 7 % ～17 % of the dodecahedral cavity center.Fe^2＋ occupies around 5%～31% of the dodecahedron cavity center.Group B cations are composed of Al^3＋,Cr^3 ＋ and Fe^3＋.Trivalent cation is mainly Al3＋,with [AlO6] octahedra occupying up to some 62 % ～ 92 %,and [FeO6] and [CrO6] octahedra occupying less than 38 %.According to different cations that occupy dodecahedral cavities and octahedral cavities,garnets in Mengyin can be divided into 10 subspecies.According to Dawson-Stephens garnet classification methods,garnets in kimberlite of Mengyin are mainly of G9 and G10.The content of Cr2O3 in G9 garnets is low,the content of CaO is high,and iron chromium garnet endmember is characterized by a high percentage of molecules,with the color being mainly of purple series.G10 garnets are mainly in the coarse-grained kimberlite diamond,with a large number of diamond inclusion garnets belonging to this class,which reflects the genetic relation of coarse-grained garnet with diamond.G10 garnet may indicate the existence of diamond-bearing kimberlite body.According to the composition of garnets in kimberlite of Mengyin,the garnet of kimberlite in diamond of this area had formation pressure of 6.0～9.0 GPa.It is inferred that garnet in kimberlite of Mengyin should belong to asthenosphere.Lattice parameter statistics of 128 garnets show that lattice parameters of garnets from Mengyin are distributed in the range of 1.151～1.161nm,with the frequency distribution in the range of 1.154～ 1.160 nm.Purple garnet lattice parameters indicating no ore,lean ore,medium ore-bearing rock and rich ore rock are 1.162 nm,1.159 ～ 1.160 nm,1.156～1.160 nm and 1.155～ 1.157 nm respectively.From barren rock through lean ore rock,medium ore-bearing rock,to rich ore rock,purple garnet lattice parameter values tend to decrease.Purple garnet cell parameters thus have obvious prospecting implications for kimberlite in Mengyin of Shandong Province.