赣西湘东菊花状天青石的特征及其成因

Characteristics and Origin of Chrysanthemum-shaped Celestite from Western Jiangxi-Eastern Hunan Area

赣西湘东地区的天青石,主要产于下二叠统由生物屑泥晶灰岩或泥灰岩构成的瘤状灰岩中,少数产于钙镁质页岩中。晶体常沿a轴或b轴延伸呈柱状,集合体在三度空间由内向外呈放射状排列,形似盛开的菊花,是一种比较少见的天青石结晶习性。本文通过沉积学、矿物学、地球化学的研究阐述了它的特征和成因,提出它是在成岩作用过程中碳酸盐灰泥中的SrSO_4围绕雏晶或胶态SiO_2核心生长而成。这种天青石的发现不仅具有矿物学和沉积学的意义,而且可作为我国华南地区沉积型海泡石的找矿标志。

Celestite in western Jiangxi--eastern Hunan area occurs mainly in LowerPermian nodular limestone composed of bioclastic micritic limestone or marl,with a small quantity seen in calc--magnesian shale. Its crystals commonlyelongate in prismatic form parallel to a or b axis, and the aggregates arespatially in radiate arrangement and hence look like chrysanthemum in fullbloom. Such a crystal habit is rather unique in celestite. The chrvsanthemum--shaped celestite is greyish white, commonly withvellowish or reddish tint; transparent to trenslucent; vitreous luster; fragile;hardness 3--3.5; specific gravity 4.0. Crystals elongate in prismatic formparallel to a or b axis, and aggregates assume radiate form. Under microscope,the mineral is colorless to pale blue, its shape changes from prismatic toparallelogramic or rhombic depending on its orientation in thin section, andits cleavages are well developed. Refractive indexes measured by the immersionmethod: Ng= 1. 630, Np= 1. 622. Its X--ray diffraction and infrared data areconsistent with the data of standard celestite. Compared with celestite abroad,celestite from this area is purer in compsition and contains lower barium andcalcium, which exist in the form of isomorphous substitution. Based on studies, the atrthors consider that the celestite may have beencrystallized around the crystallite or the colloidsl SiO_2 nucleus in favorablehorizons of the carbonate mud during diagenesis. Its crystallization process issimilar to the formation of spherulitic siderite: under the redtiction conditionof diagenesis, free Sr^(2+) combines with SO_4^(2-)to form dispersed SrSO_4 molecules,which, with the intensification of diagenesis, transform into tiny colloidalspots and grow around the nucleus and along the a axis in the form ofradiate aggregates that look like chrysanthemum in full bloom.