Genesis of apatite in phosphate stromatolites
Genesis of apatite in phosphate stromatolites
复制标题
磷酸盐叠层石中磷灰石的成因
DOI：
--
复制
发表时间：
2001
期刊：
影响因子：
--
通讯作者：
A. Martín
中科院分区：
文献类型：
--
作者：
A. Sánchez;A. Martín
阿尔卑斯-地中海古边缘岩中与凝聚远洋沉积有关的上侏罗统磷质叠层石主要由微生物沉淀型方沸石组成，这是一种结晶度较低的碳酸盐氟磷灰石。微生物磷酸盐片层的扫描电子显微镜图像显示，高度多孔的胶体结构是由微米大小的椭圆形、卵形和香肠状物体致密堆积形成的，这些物体在某些地方形成串状，类似细菌生长结构。透射电子显微镜观察表明，这些结构主要由直径在0.1~1μm之间的六方磷酸钙微晶组成，其形状和大小与成熟牙釉质中观察到的相同。磷酸盐微晶包裹在富Fe-Al-Si的贫晶相(自生富铁蒙脱石+富Fe-Si-Al非晶相)中，这使它们具有通常在扫描电子显微镜下观察到的圆形外部形貌。透射电子显微镜研究还发现，在这些方沸石微晶中存在直径为5-50 nm的圆形电子发光中心，这是由于挥发性化合物在电子轰击下释放而产生的。对方沸石晶体和粘土之间某些区域的HRTEM研究表明，存在：(1)极小的、结晶性差的磷酸钙晶体池，其大小有两个范围(基面上的20-50 nm和5-15 nm)，其中一些显示出调制的对比度，被认为是生长步骤；以及(2)无定形磷酸钙。无定形磷酸盐相形成一层围绕微晶的云膜，有时形成大小在2-5纳米之间的球体群。低结晶度富铁蒙脱石、富Fe-Si-Al非晶相、无定形磷酸钙和方沸石似乎紧密地相互生长，因此必须对它们的成因关系进行推断。观察到的结构和结构关系表明，在磷酸盐叠层石中，细菌介导的无定形磷酸钙前驱体的方沸石晶体与自生蒙皂石的富Fe-Si-Al凝胶前驱体有关。异养细菌降解富磷有机物后，非晶相的析出在动力学上是有利的，并与富含粘性有机物的菌垫和菌鞘密切相关。在早期成岩作用中，无定形磷酸盐前驱体转化为方沸石。
Upper Jurassic phosphatic stromatolites associated with condensed pelagic sediments in Alpine-Mediterranean paleomargins mainly consist of microbially precipitated francolite, a low crystallinity carbonate fluorapatite. SEM images of microbial phosphate laminae show highly porous colloform textures formed by dense accumulations of micrometre-size spheroidal, ovoidal and sausage-shaped bodies that in places form strings and resemble bacterial growth structures. TEM observation reveals that these structures are mostly constituted by an arrangement of hexagonal crystallites of calcium phosphate, with sizes ranging between 0.1 and 1 μm in diameter, of the same shape and size as those observed in mature dental enamel. The phosphate crystallites are wrapped in Fe-Al-Si-rich poorly crystalline phases (authigenic Fe-rich smectite plus Fe-Si-Al-rich amorphous phases) that confer them the rounded external morphologies usually observed under SEM. TEM study also reveals the existence of rounded electron-lucent centres of 5–50 nm in diameter within these francolite crystallites, produced by the release of volatile compounds under electron bombardment. HRTEM study of some areas between francolite crystals and clays shows the presence of: (1) pools of extremely small, poorly crystalline calcium phosphate crystals with two ranges of sizes (20–50 nm and 5–15 nm in basal sections), some of them showing modulated contrast interpreted as growth steps; and (2) of an amorphous calcium phosphate. The amorphous phosphate phase constitutes a cloudy film that surrounds crystallites and sometimes forms groups of spherules with sizes ranging between 2–5 nm. Low crystallinity Fe-rich smectite, Fe-Si-Al-rich non-crystalline phases, amorphous calcium phosphate, and francolite appear so closely intergrown with each other that a genetic relation must be inferred for all of them. The observed textural and structural relationships demonstrate that, in the phosphate stromatolites, bacterially-mediated precipitation of an amorphous calcium phosphate precursory of the francolite crystals was associated with that of Fe-Si-Al-rich gels precursory of authigenic smectite. The precipitation of amorphous phases was kinetically favoured, and took place in close relation to bacterial mats and sheaths rich in mucilaginous organic substances, after heterotrophic bacterial degradation of P-rich organic matter. During early diagenesis amorphous phosphate precursors transformed into francolite.