法国Beauvoir花岗岩中烧绿石族矿物的研究

Study on Pyrochlore-Group Minerals from the Beauvoir Granite,France

在法国Ｂｅａｕｖｏｉｒ花岗岩中，烧绿石－细晶石族矿物是重要的铌钽矿物之一，主要出现在岩体的上部。大部分晶体为自形，白色、谈黄色或谈绿色，粒径为微米至毫米级。３０多个电子探针分析结果表明，主要成分为Ｎａ、Ｃａ、Ｕ、Ｎｂ，Ｔａ、Ｆ．从岩体的下部到上部，烧绿石族矿物的Ｎｂ／（Ｎｂ＋Ｔａ）比值呈升高的趋势。这些矿物富含铀，Ｕｏ＿２含量最高达１５．０５％，部分烧绿石和细晶石为铀烧绿石和铀细晶石；另一方面，在岩体下部，细晶石中Ｕｏ＿２含量平均为６．３％，而在岩体上部，烧绿石和细晶石中的Ｕｏ＿２平均含量为９．０％

Miner als of the pyrochlore-microlite group belong to principal Nb,Ta accessory minerals in the Beauvoir granite, France.Most of their grains are euhedral with colors ranging from white,yellowish to greenish. Their sizes vary from microns to millimeters.Microlite is absent in the BⅢfacieS and rare in the B Ⅱ facies,but very abundant in the BI facies;pyrochlore appears only at the top of the Beauvoir granite.Over 3 0 electron-microprobe analyses are co-nsistent with the general crystalchemical formula:A_2-mB_2X_6Y_1-N·pH_2O, where principally A= Ca,Na,U;B=Nb,Ta,W,Sn,X=O;Y=F,OH. The hydra tion often appears at the rim or along micro-fracture in grains of pyrochlore-microlite, accompanied by leaching of Na,Ca,U from the A site and of F from the Y site,and by the introduction of K,Ba into the A site.The Nb/(Nb + Ta)ratio of the pyrochlore-group minerals has an increasing tendency upward in the granite body. These minerals are generally rich in uranium,the highest percentage found of UO_2 is 15. 05%.On the one hand,certain pyrochlore-micro-lites may be considered as uranpyrochlore-uranmicrolite, and on the other ha-nd,microlite of the lower part contains on average 6.3% UO_2 in contrast.microlite and pyrochlore of the upper part have the mean content of 9.o%.Pyrochlore-group minerals show zoning structure,due chemically to the osci-llatory variations in Nb and Ta. At the top of the granite,minerals of thisgroup may replace manganocolubite, leading to the form ation of a special re-placement structure related to the late stage subsoli dus interaction betweenmagma and meteoric fluid.