Humite structure

In this way we find that, in oxides for example, many cation arrangements are identical to the arrangements of the atoms in known (or plausible) alloy structures. (In some cases the cations and atoms are identical in the two cases.) This is particularly helpful in those oxide examples where it has not previously been possible to describe the structure in any simple terms, often because the anion array is not regular in any simple way, for instance in many metal sulphates. But it is also revealing in some cases that are describable in conventional terms, e.g. the humites. A long (but still incomplete) list of examples is given in Table 3. Sufficient specific cases are described and discussed in detail in the text in order to expose the principles and some of the advantages of this unfamiliar approach. 

Fig. 19. The structure of humite, 3Mg2Si04 Mg(OH, F)2, projected on (001) (Pnma). Atoms represented by circles as in Figs. 16-18. Left, SiMg trigonal prisms only right, bottom, conventional depiction as Si04 tetrahedra and MgX octahedra top, Si04 tetrahedra and Mgt trigonal prisms and (OH, F) atoms. Again note the distortion of the MgX octahedra, and h.c.p. anion array...

We shall now present studies relating to the micro-twin structure performed by B. G. Hyde, who is a main proposer of the concepts of shear and micro-twin structures. One study is on the mineral humite family,the composition of which is generally expressed by nMg2SiO4-Mg(OH)2 (this has to be altered to nMg2SiO4-Mg(OH, F)2, because (OH) is often replaced by F in the minerals). In Table 2.4, the mineral name, together with the compositions and micro-twin structures proposed by the authors, are collected, the space group is Pmcn (orthorhombic) for n = odd and... 

White and Hyde extended their observations to the manganohumite family (Mg is replaced by Mn in humite) and leucophoenicite 3Mn2SiO4-Mn(OH)2 and obtained fruitful results from the viewpoint of the micro-twin structure. 

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