沅江矿─—一种金和锡的新矿物

Yuanjiangite──A New Auriferous and Stanniferous Mineral

沅江矿是迄今在自然界中发现的唯一的一种Ａｕ和Ｓｎ的新矿物，已于１９９３年８月被国际矿物学协会新矿物与矿物命名专业委员会批准。它发现于湖南中西部沉江中游沅陵县境内。产于沅江Ⅲ阶地中更新世冰碛冰水沉积的砂砾层中（２．５－２．６Ｍａ）。呈细小（＜５μ）粒状集合体；外观似瘤状－假葡萄状；银白色，易氧化为灰白色至黑色；微具延展性；不透明；金属光泽；条痕黑色；Ｄ＿计＝１１．７８，Ｄ＿测＝１１．７－１１．９；ＶＨＮ＿（２５）＝１７２－２７４。光片中可见到沅江矿与砂金紧密连生，二者界限不规则，晶粒间充填自然铅等矿物，呈砂岩状结构。反射色为白色略带黄。非均质性清楚。易溶于王水，在盐酸及硝酸中缓慢溶解。电子探针分析：含Ａｕ６０．５２９％－６４．５８０％，Ｓｎ３５．３０５％－３８．８０７％，此外还有少量的杂质Ａｇ和Ｐｂ，其化学式为ＡｕＳｎ。Ｘ射线粉晶衍射数据中强线为：３．７２６（３４），３．０８７（３８），２．２１８（１００），２．１５９（５７），１．５４６（３１），１．２５７（２５），１．１６２（１８）；为红砷镍矿型结构，空间群Ｐ６＿３／ｍｍｃ，Ｚ＝２，修正后的晶胞参数为：伴生矿物有砂金、锡石、黄铁矿、辰砂、自然铅及自然锡等４?

Yuanjiangite is the only auriferous and stanniferous new mineral ever discovered in nature and was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association in August 1993.It occurs in the sandy gravel(rich in placer gold)of Middle Pleistocene(2.6-2.5 Ma)moraine glaciofluvial deposition. Yuanjiangite occurs together with placer gold in the form of interlocking bodies,exhibiting irregular boun-daries between them.Yuanjiangite occurs as crystalline granular aggregates and appear in the nodular-pseudobotryoidal forms;its crystal grains are smaller than 5μm,with somewhat larger hexagonal prisms and related combinations merely observed in a few drusy cavities of aggregates.Silver white in color;opaque;with metallic luster;streak blacK;slightly ductile;with obvious anisotropism.It is a gold-tin mutually-combined substance whose electron microprobe analytical result is Au 60.529%-64.580%,Sn 35.303%-38.807%and small amounts of Ag and Pb impurities. Its chemical formula is AuSn ,with the atomic ratio close to 1:1.X～ray powder diffraction analysis shows that yuanjiangite has the structure of nickeline and the following unit cell parameters:c/a=1.27, space group P6_3/mmc and Z=2.Based on the electrochemical principle combined with the mode of occur-rence of yuanjiangite,it is considered that this mineral was formed by in-situ replacement of placer gold under the supergene condition.