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(Mn,Fe,Ca)2(Mg,Zn,Fe)5(Si,Al)8O22(OH)2
Monoclinic, C2/m, a = 9.606, b = 18.126, c
= 5.317 Å,
b = 102.63o, Z = 2
Tirodite is among the least common amphiboles at these deposits and among the best studied. It occurs at both Franklin and Sterling Hill, but has not been found in the marble quarries. Tirodite was first described from Franklin by Bauer and Berman (1930) and was called zinc-manganese-cummingtonite. Palache (1935) adopted the same nomenclature. Subsequent studies have shown this material to be tirodite. Tirodite from another Franklin assemblage was studied by Klein and Ito (1968), and this specimen was subsequently used for the crystal structure determination by Hawthorne and Grundy (1973, 1977).
Tirodite is light-to-dark green or light-brown and has prismatic cleavages, vitreous luster, and a density of 3.24 g/cm3. It occurs in hand-sized masses. Optically, it is biaxial, negative, 2V = large, with a = 1.634, b = 1.647, and g = 1.660; pleochroism is absent (Klein and Ito (1968). Different values were obtained by Bauer and Berman (1930); the optical data may vary considerably with composition. There is no discernible fluorescence in ultraviolet. Absent the characteristic assemblages described below, tirodite is best identified by chemical analysis.
Tirodite is a manganese magnesium silicate hydroxide mineral of the amphibole group. Representative analyses of Franklin and Sterling Hill specimens are given in Table 12. For unpublished analyses, FeO varies from 3.5 to 13.3 wt. %, MgO from 8.3 to 18 wt. %, and ZnO from 6.8 to 11.2 wt. %.
Tirodite is likely more common in the Franklin ore body than has been recognized to date; other occurrences might be found, especially in Ca-poor ore assemblages. In the original occurrence described by Bauer and Berman (1930), tirodite was associated with rhodonite, spessartine, and nelenite (the latter known to them as ferroschallerite). Specimens from this assemblage have been described in detail by Dunn and Peacor (1984).
Tirodite also occurs with gray willemite, rhodonite, apatite, and lennilenapeite (Dunn et al., 1984b). Tirodite described by Klein and Ito (1968) occurred with franklinite, calcite, tephroite, and willemite. A different tirodite assemblage from Franklin consists of sphalerite, together with willemite possibly exsolved from tirodite, but the relationship has not been studied.
At Sterling Hill, tirodite is associated with willemite, spessartine, pyroxmangite, and calcite from the 1010 stope on the 700 level.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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Link
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This
page created: January 11, 2001 |
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