The crystal structure of proto‐enstatite, MgSiO3
The crystal structure of proto‐enstatite, MgSiO3
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原顽辉石 MgSiO3 的晶体结构
DOI：
10.1107/s0365110x59001554
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发表时间：
1959
期刊：
影响因子：
--
通讯作者：
J. Smith
中科院分区：
文献类型：
--
作者：
J. Smith
室温下，顽火辉石为正交晶系，晶胞尺寸为a-9.25 B-8.74，c = 5.32，赝空间群为Pbcn。赝空间群的原子坐标由X射线粉末数据确定。SiO 3链完全伸展。其中一个Mg原子具有不规则的配位，这可能是低温下原顽火辉石不稳定的原因。结构与Atlas（1952）预测的非常相似。原顽火辉石是迄今为止描述的MgSiO 3的五种形式之一（文献综述见Atlas，1952）。五种中只有三种被认为有稳定的磁场--菱形顽火辉石、斜顽火辉石和原顽火辉石。由于斜方顽火辉石与其他两种晶型之间的转变缓慢，而且原晶型与斜晶型之间的转化容易，所以确定它们的相关系是困难的。斜顽火辉石或原顽火辉石在高温下是否是稳定的形式，或者两者是否都有一个稳定场，这一点还有待商榷（Atlas，1952; Foster，1951; Boyd & Schairer，1957），但可以肯定的是，菱形顽火辉石在低温下是稳定的形式，而原顽火辉石在这种结构确定的温度下是不稳定的。在1400 ℃加热时，原顽火辉石转变为斜顽火辉石，在室温下长时间静置和研磨时。因此，这两种形式的结构必须是相关的，事实上，阿特拉斯已经使用这一点作为预测原顽火辉石晶体结构的基础。本研究有三个目的：建立可靠的原生顽火辉石晶体学数据，提供MgSiO_3相变过程中原子运动的信息，寻找应力显著影响原生顽火辉石向斜顽火辉石转变的结构原因。本文将介绍第一个目标，第二个和第三个目标将在稍后与N。Morimoto，他目前正致力于斜顽火辉石结构的改进。作者感谢F博士。1~.博伊德的地球物理实验室，华盛顿，哥伦比亚特区的尝试生长大晶体的原顽火辉石。尽管付出了相当大的劳动，但所有的尝试都是不成功的，因此必须对粉末样品进行结构测定。结晶条件为：
: Proto-enstatite, at room temperature, is orthorhombie with 8 units of MgSiO3 in a cell of dimensions a----9.25, b----8.74, c= 5.32 A, pseudo space group Pbcn. The atomic coordinates of the pseudo space group were determined from X-ray powder data. The SiO3 chain is fully extended. One of the Mg atoms has an irregular coordination, which may be the reason for the instability of proto-enstatite at low temperature. The structure is very similar to that predicted by Atlas (1952).Proto-enstatite is one of the five forms of MgSi03 that have so far been described (see Atlas, 1952 for literature survey). Only three of the five have been thought to have a field of stability--rhombic enstatite, clino-enstatite and proto-enstatite. Determination of the phase relations is difficult because of the slug-gishness of the transformations between rhombic enstatite and the other two forms, and because of the ease of the inversion between the proto-and clino-forms. Whether clino-or proto-enstatite is the stable form at high temperature or whether both have a field of stability is still open to doubt,(Atlas, 1952; Foster, 1951; Boyd & Schairer, 1957), but it is certain that rhombic enstatite is the form stable at low tempera-tures and that proto-enstatite is unstable at the tem-perature of this structure determination. Proto-enstatite changes into clino-enstatite upon heating at 1400 C., upon long standing at room temperature, and upon grinding. Thus the structure of the two forms must be related and, indeed, Atlas has used this as a basis for predicting a crystal structure for proto-enstatite. This investigation ha~ three objectives: establishing reliable crystallographic data for proto-enstatite, providing information on the atomic movements that occur during the phase transformations of MgSi03 and looking for a structural reason why stress can mar-kedly effect the transformation for proto-to clino-enstatite. In this paper the first objective will be described: the second and third will be discussed later in collaboration with Dr N. Morimoto, who is cur-rently engaged on a refinement of the structure of clino-enstatite. The author is indebted to Dr F. 1~. Boyd of the Geophysical Laboratory, Washington, DC for at-tempting to grow large crystals of proto-enstatite. In spite of considerable labour, all attempts were un-successful and the structure determination has had, perforce, to be carried out on a powder sample. The conditions of crystallization were: treatment of an