基于实验分析的乳房状石钟乳成因机理初步研究——以湖北阳新地心大峡谷洞穴沉积形态为例

目前，关于乳房状石钟乳成因的研究仍然停留在“科学猜想”阶段。通过对湖北阳新“地心大峡谷”洞穴内采集的一对孪生乳房状石钟乳样品进行矿物构成、晶体形态以及年代学分析，获得如下新认识:①乳房状石钟乳由致密—疏松互层递序生长的方解石构成。②致密层方解石呈菱面体晶型，无色透明，垂直向下生长，没有横向扩张；疏松层方解石呈细小颗粒状，乳白色不透明，呈“皮壳”状聚集，有一定程度的横向扩张，是乳房状石钟乳基部和主干部位膨胀的主要原因。③致密层和疏松层之间并不是连续生长的，存在显著的沉积间断。疏松层的232Th含量普遍比致密层高1~2个数量级，说明疏松层中不溶于水的杂质含量显著高于致密层。结合以上实验结果，以及洞内“皮壳”状沉积物可以在洞壁、洞顶分布的特征，推测:①“皮壳”沉积的溶液是从围岩或相关沉积物（石钟乳）基底以面状缓慢渗出的。②当溶液过饱和析出碳酸钙后，溶剂以水分蒸发的形式离开沉积界面。③“皮壳”沉积结构疏松的原因可能是由于地下河改道，洞穴密闭度变差，洞穴空气CO₂浓度降低，方解石结晶速率加快，从而造成方解石晶体缺陷增多，并以微晶集合体堆积。 

Forming mechanism of breast-like stalactites based on the experimental analysis of cave deposition from Dixin canyon,Yangxin, Hubei

The current study on the forming mechanism of breast-like stalactites is still at the stage of "scientific conjecture". In this study，samples of a pair of twin breast-like stalactites were collected in a cave of Dixin canyon at Yangxin county， Hubei Province. We carried out the mineral composition， crystal morphology and geochronology studies on the samples， and obtained new understanding. The breast-like stalactite is composed of calcite that formed the compact/porous inter-layers. The calcite of compact layer is rhombohedral，colorless and transparent，growing vertically downwards without lateral expansion. The calcite of porous layer is in the form of fine particles，milky white and opaque，and aggregates in a "coating" structure， with a certain degree of lateral expansion， which accounts for the expansion of the base and trunk of the breast-like stalactite. The dating results suggest that the compact and porous layers do not grow continuously， but existing significant hiatus. The 232Th content of the porous layer is generally one to two orders of magnitude higher than that of the compact layer，indicating that the content of water-insoluble impurities in the porous layer is significantly higher than that of the compact layer. Based on the experimental results and the observations that the "coating" depositions are also on the walls and roofs of the cave，we speculate that the solution recharging "coating" depositions seeps from surrounding rocks or solid sediments （stalactites）. The porous structure of "coating" deposition may be due to the diversion of underground river， the deterioration of the cave sealing， the decrease of CO₂ concentration in the cave， and resultant acceleration of calcite crystallization rate. These factors make calcite crystal more defects and messy accumulation.