Crystal chemistry of the humite minerals

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Virginia Polytechnic Institute

The minerals of the humite group have the formula nM₂SiO₄• M₁₋ₓTiₓ(OH₃F)₂₋₂ₓ0ₓ₂ where M is Mg, Fe, Mn, Ca, Zn in decreasing order of abundance, x < 1, and n=1 for norbergite, n=2 for chondrodite, n= 3 for humite, and n=4 for clinohumite.

Description of the humites as alternating layers of forsterite (Mg₂SiO₄) and sellaite and/or brucite (MgF₂ and Mg(OH)₂) composition is incorrect because no layers with these compositions are present in the structure. The humites are structurally analogous to olivine in that both consist of a hexagonal close-packed array of anions (O, OH,F), zigzag chains of edge-sharing, M-filled octahedra and isolated Si04 tetrahedra. They differ from olivine in that none of the available tetrahedral sites coordinated by one or more OH or F anions is occupied by Si; this results in a stagger of the chains of octahedra. Substitution of Fe, Mn, and Ca for Mg has the same effect on the unit cell volume (normalized to half the mean anion-anion distance along the normal to (001)) in minerals with the humite structure as it does in minerals with the olivine structure.

Fifty-five humite mineral samples have been analyzed by microprobe x-ray emission methods for F, Mg, Si, Ca, Ti, Mn, Fe, and Zn. Al, P, Cl, Cr, Co, Ni, Cu, and Pb were not detected by analysis or wavelength scanning at the less than 0.03 weight percent level. The OH⁻¹ concentrations were calculated using atomic proportions and assuming electro- static charge balance in the M₁₋ₓTiₓ(OH,F)₂₋₂ₓ0₂ₓ part of the formulae.

Epitaxial intergrowths between members of the humite group have been predicted on the basis of their similar structures and wet chemical analyses. Most chemical analyses of humite minerals show substantial anion deficiencies which lead to non-stoichiometric Si:(OH + F + OTi) ratios; many also indicate non-stoichiometry in the olivine equivalent portion of the formula. In contrast, the microprobe analyses indicate that the humite minerals are stoichiometric. They also suggest that most bulk chemical analyses are deficient in H₂O and were made with material which contained minor amounts of other mineral impurities.

Single crystal x-ray diffraction studies were made on nine microprobe analyzed humites to determine the effects of minor chemical substitution on the unit cell parameters. These studies show that the substitution of (Fe + Mn) for Mg increases the normalized cell volume; that Ti+ 2(O) for Mg + 2(OH,F) has little effect on the cell volume; and that (OH,F) + tetrahedral vacancies for 0 + Si results in a decrease in cell volume.