核-幔物质晶体化学、矿物学及矿床学初探

A PRELIMINARY STUDY OF THE CRYSTAL CHEMISTRY,MINERALOGY, AND ORE DEPOSITS OF EARTH'S CORE-MANTLE MATERIAL

大量的高温、高压实验表明随着温度和压力的增加 ,地幔氧化物及硅酸盐的晶体结构会发生与其温度和压力相适应的多型转变 ,并导致元素金属性的增加 ,即氧离子半径的缩小以及硅和其他阳离子半径的增大。来自地核部分物质的晶体化学及晶体物理目前还很少涉及 ,根据其物质组成 ,可能具有金属及金属互化物的性质 ,即原子呈等大球最紧密堆积 ,具有高熔点、难溶性等特征。在地球形成时作为地核物质是最先凝聚的 ,并由于重力分异沉入地核 ,上升至浅部时不与其他元素化合 ,保持了零价状态 ;等大球最紧密堆积使其在地球各层圈中晶体结构保持恒定。地幔热柱使核、幔物质上升至地球浅部成为可能 ,并形成矿物及矿床。以西藏罗布莎铬铁矿床为例 ,已经发现了多达 5 0种的铂族元素矿物、铁族元素矿物及其金属互化物 ,包括成分复杂多变的Os Ir ,Os Ir Ru ,Pt Fe ,Ir Fe Ni,Fe Ni Cr及Fe Co等矿物 ,它们还与来自核幔边界的Si,Fe ,FexSiy,FeO和SiO2 矿物共生 ,其中铂属矿物及其金属互化物矿物应视为来自E层的地核矿物 ;探讨了该区矿物与陨石矿物在共生组合方面的相似性 。

A lot of high temperature and high pressure experiments indicate that along with the increment of temperature and pressure the polymorphic transformation of the crystal structure of mantle oxides and silicates will occur in conformity with the temperature and pressure conditions, which results in the increment of elemental metallic property, for example, the decrement of radius of oxygen atom and increment of radius of other cations. By now, the investigation on crystal chemistry and crystal physics of earth's core material has not much involved. According to its composition, the core material may possess the properties of metallic and intermetallic phase. The atomic structure of the material should be the closest packing of equal spheres structure, and it might possess high melting-point and refractory property. During the formation of the earth, the core material (metal and alloy) was the foremost to condense among the other planet substances and sank down into the earth's core by gravitative differentiation. When the material rose to the shallow part it did not combine with any other elements, therefore its chemical valence maintained a value of zero. It maintains the same crystal structure in all layers of the earth. The plumes provide the possibility for earth's core and mantle materials of rising to the crust and forming minerals and deposits. Taking the mineral group in Luobusha chromite deposit of Tibet as an example, it is indicated that the minerals are from earth's E and F layers. In this deposit, about 50～60 mineral species of platinum group and iron group elements, and of intermetallic compounds were found, including Os-Ir, Os-Ir-Ru, Pt-Fe, Ir-Fe-Ni, Fe-Ni-Cr and Fe-Co minerals with highly variable compositions. They were associated with Si, Fe, Fe xSi y, FeO and SiO 2 minerals coming from the earth's core-mantle boundary. A reasonable explanation is that these platinum group element minerals and their alloy minerals are from E layer of the earth's core. The similarities in the paragenesis of meteorites to that of the mineral group in this area are discussed. The aggregation of earth's core minerals may be considered as a new type of metal element ore deposit.