New data on the forsterite–tephroite series

Carl A. Francis

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Other| June 01, 1985

New data on the forsterite–tephroite series

Carl A. Francis

Harvard Mineral. Mus., Cambridge, MA, United States

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Author and Article Information

Carl A. Francis

Harvard Mineral. Mus., Cambridge, MA, United States

Publisher: Mineralogical Society of America

First Online: 02 Mar 2017

Online ISSN: 1945-3027

Print ISSN: 0003-004X

American Mineralogist (1985) 70 (5-6): 568–575.

Abstract

New data including electron microprobe analyses, unit cell parameters, and infrared spectra are reported for a suite of fourteen metamorphic samples which span the entire forsterite–tephroite series. Regression equations relating composition to unit cell parameters, and infrared band position were calculated. Perhaps the most useful determinative curve is the relationship:

{% Mg/M}^{2 +} = - 31981.250 + 23707.779 d_{130} - 4378.2695 {(d_{130})}^{2}

Huggins’ (1973) method of calculating the degree of cation order from infrared spectra was tested. The results suggest that IR spectroscopy may indeed be a rapid and reliable method for determining the mole fraction of Mg on the M(1) site when sensitivity numbers (S) are known better.

Zincian olivines ranging in composition from (Mg_1.33Mn_0.41Zn_0.25Fe_0.01) to (Mn_1.89Mg_0.05Zn_0.05Fe_0.01) occur in the metamorphosed Zn–Mn–Fe deposits at Franklin and Ogdensburg, Sussex Co, New Jersey. Individual crystals contain oriented lamellae of willemite that evince an exsolution origin from tephroite which originally contained ~20 mol.% Zn₂SiO₄ components. Unit cell parameters vary nonlinearly as a function of mean octahedral cation radius calculated from microprobe analyses and strongly imply the ordering of cations. Zinc is thought to have a moderate preference for the M(1) site.