Humite

The effect of small valence and large coordination number is further shown by the observation that silicon tetrahedra, which share comers only with aluminum octahedra, share edges with magnesium octahedra (in olivine, chondrodite, humite, clinohumite) and with zirconium polyhedra with coordination number eight (in zircon). 

Epidote-clinozoisite Ca2(Al, Fe )3(Si04)3(0H) Piedmontite Ca2(Al,Mn+ Fe+= )3(Si04)3(0H) Zoisite Ca2Al3(Si04)3(0H) dimorph clinozoisite Humite (morphotrophic series with olivine)... 

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...

The cation arrays in the humite series are intergrowths of Ni2ln with unit lamellae of ResB. A similar series can be generated in which the cation arrays are intergrowths of Niiln with unit lamellae of CrB In the case of silicates, the only known example is leucophoenicite, a dimorph of manganhumite 3 Mn2Si04 Mn(OH)2 . 

At the second level the question arises as to why such simple and well-known cation arrangements appear in oxides. It seems to us plausible that this is a consequence of important non-bonded repulsions between the cations. Om belief is that these are frequently greater than anion... anion repulsions. And certainly this is consistent with the observed regularity of the cation arrays - which is often greater than that of the anion arrays. (Good examples are provided by various olivines and humites, an extreme case being that of the chondrodite type 2Cd2Si04 Cd(OH)2 which is shown in Fig. 25.)... 

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. 

Magnesium is a bright, silver-white alkaline earth metallic element (symbol Mg atomic no. 12). It is widely found as different salts in minerals such as boracite (borate), ankerite and dolomite (carbonates), carnallite (chloride), brucite (hydroxide), periclase (oxide), lazulite (phosphate), amphibole, cummingtonite, enstatite, gamierite, hornblende, humite, hypersthenite, iolite, jade, meUlite, monticellite, olivine, Pennine, saponite, sapphire, serpentine, talc, and tremolite (silicates), and epsomite, kainite, kieserite, and polyhalite (sulfates). 

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