Collaborative Research: The Crystal Chemistry of Rock-Forming Chlorite

变质岩石学的主要目标是了解结晶条件(温度、压力、体积成分、挥发性组分的化学活性等)之间的相互关系。在变质过程中。尽管绿泥石是一种常见的成岩变质矿物，其组成和晶体结构被认为与这些结晶条件有关，但还没有对绿泥石的结构进行研究，以确定岩石学因素对结构的影响。在这个项目中，将使用单晶X射线衍射方法来研究特征良好的绿泥石系列。除了白云岩套外，所有的样品都含有石墨，因此铁的氧化态受到限制。缅因州将要研究的变质片岩具有以下特征：(1)地层压力接近3.5kbar，(2)铝饱和，(3)中到高的镁含量，(4)不同的地层温度。为了进行比较，选择了缅因州从变质的铅镁铁质到超镁铁质岩体的一套，因为它富含镁，但不饱和铝。此外，还将检查一套铁含量更高的套房。在进行化学分析后，将比较不同品级和不同品级的绿泥石晶体的结构细化情况。研究结果将有助于确定Al、Mg、Fe2和空位在绿泥石的各个晶位上的分布情况，这是结晶压力和温度以及绿泥石组成的函数。这些结果将有助于解释为什么变质绿泥石的允许成分范围存在较尖锐的成分边界。此外，对阳离子有序中涉及的变量的理解可能有助于进一步理解限制钛、铁、锰、镍、铬等取代的控制因素。一个主要的目标是了解这些因素如何影响或控制八面体薄片和绿泥石中2：1层之间的氢键。最重要的是，这些数据将消除先前试图建立与天然岩石直接相关的绿泥石热力学溶液模型的特殊方面。

Guggenheim and Guidotti0308588 and 0308546A major goal of metamorphic petrology is to understand the interrelationships among the crystallization conditions (temperature, pressure, bulk composition, chemical activities of volatile constituents, etc.) during metamorphic processes. Although chlorite is a common rock-forming metamorphic mineral believed to have a composition and crystal structure that relates to these conditions of crystallization, no structural studies of chlorite have been made to identify how petrologic factors affect the structure.In this project, single-crystal X-ray diffraction methods will be used to examine suites of well-characterized chlorite. For each suite except the Plumbago pluton suite, all samples contain graphite, so that the iron oxidation state is constrained. The metapelitic schists to be examined from Maine will have the following characteristics: (1) formation pressures at near 3.5 kbar, (2) Al saturated, (3) intermediate-to-high Mg2+-contents, and (4) varying formation temperatures. For comparison, a suite from the metamorphosed Plumbago mafic to ultramafic pluton in Maine, is selected because it is Mg-rich, but not Al-saturated. In addition, a more Fe-rich suite will be examined. After chemical analyses are made, structure refinements of chlorite crystals within each grade and between grades will be compared.The results of the study will help determine how Al, Mg, Fe2+, and vacancies are distributed in the various crystallographic sites of chlorite as a function of the pressure and temperature of crystallization and chlorite composition. These results will help to explain why there are relatively sharp compositional boundaries for the permissible composition range for metamorphic chlorites. In addition, an understanding of the variables involved in cation ordering may provide additional understanding of the controls limiting Ti, Fe3+, Mn, Ni, Cr, etc. substitutions. A major objective is to understand how these factors affect or control hydrogen bonding between the octahedral sheet and the 2:1 layer in chlorite. Most importantly, these data will eliminate the ad hoc aspect to previous attempts to formulate thermodynamic solution models for chlorite that are directly relevant to natural rocks.