米粒状白云石及其出溶成因

Rice-shaped Dolomite and Its Exsolution Origin

米粒状白云石产于奥陶系近岸碳酸盐岩序列的云斑(生屑)泥晶灰岩、砾屑灰岩和叠层石灰岩中,在虫孔和砾屑粒间最为富集,外形为伸长菱面体,薄片中呈米粒状.晶体无环带或雾心,但内部含大量呈规则分布的细小方解石“包体”,结晶C轴垂直于晶体延长方向.与背景方解石泥晶相比,Fe、Mn、Na和Ba含量较高,Sr含量较低.C轴的固定取向排除了石膏假晶的可能,也不是普通白云石化或交代钡白云石的结果,推测是沉淀镁方解石重结晶后在压力增加的条件下经出溶作用形成.

Rice-shaped dolomite crystals exist in dolomite-patched micritic, biomicritic, intraclastic and strornatolitic limestones in an Ordovician nearshore, carbonate succession, reaching their highest concentrations promixal to worm burrows and pore space between intraclasts. The crystals are elongate rhombohedra and appear to be elongate hexagons, parallelograms or trapezoids and look like rice grains in thin sections. No internal zoning or foggy cores are present but numerous minute calcite 'enclusions' distrib- ute parallel to the boundary of the crystals. The C-axis is perpendicular to the longer boundary of the crystal. It contains higher Fe, Mn, Na and Ba than surrounding calcite micrite which contains a greater proportion of Sr, especially its Fe and Ba are tens of times higher than those of calcite. C-axis orientation negates the dolomite representing replacement of gypsum.It can not be created by an ordinary dolomitization or the replacement of norsethite and is suggested to form in a process that the precipitated magnesian calcile is recrystallized into its inherent crystallographic shape, then followed by exsolution accompanied by exchange of calcium for magnesium ions under conditions of increased pressure. The agent triggering the recrystallization of magnesian calcite is probably in penecontamporaneous of early diagenetic interstitial wan er with high Mg / Ca ratio, in which there may be some substance that can release chemical energy not only to dehydrate Mg^(2+) but also to order the entering of the ions into lattice. As the water solution permeates the sediments downward and upper layers are dolomitized, the substance (or its energe) is consumed gradu ally, the concentration of which could become as less as can only carry out dehydration of local ions but can not make the activity of the oins ordered when the solution reaches the lower layers, As a result, parts of magnesian calcite in the lower sediments are favoured to grow up transforming to their inberent cryslallographic shape. Some Ba compound seems to be a candidate for the substance.