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Na2(Mg,Fe3+)5Si8O22(OH)2
Monoclinic, C2/m, a = 9.760, b = 18.031, c
= 5.306 Å,
b = 103.79o, Z = 2
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| Figure 17-40. Matted and felty magnesioriebeckite with calcite (gray on left; colored by magnesioriebeckite inclusions) and a rounded fragment of sphalerite from Franklin. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #C6156. Photo by the author. | ||
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Magnesioriebeckite was first reported from Franklin by Palache (1928a) as crocidolite; it has not been reported from Sterling Hill. A subsequent description of different material, providing much of the data given here, was presented by Klein and Ito (1968).
The magnesioriebeckite described by Klein and Ito (1968) is dark green, with a density of 3.26 g/cm3. Optically, it is biaxial, negative, with a = 1.666, b = 1.669, and g = 1.670. The bulk of the known magnesioriebeckite specimens, however, are from the occurrence reported by Palache (1935). In this assemblage, magnesioriebeckite is gray-blue with a felty texture and fibrous appearance and occurs in hand-sized masses.
Magnesioriebeckite is a sodium magnesium ferric-iron silicate hydroxide mineral of the amphibole group. The only reliable analysis of magnesioriebeckite is that given by Klein and Ito (1968) and reported in Table 12. Although there is no reliable analysis of the blue felty material, Palache’s (1935) recalculation of Bauer’s analysis, deleting Zn, should be carefully evaluated. As shown herein, all the Franklin amphiboles from the orebody host significant quantities of zinc, and this one might also.
The material described by Klein and Ito (1968) occurs as 4-cm dark green prismatic crystals associated with rhodonite, calcite, and franklinite.
The assemblage described as crocidolite by Palache (1935) was found on the Trotter Dump in 1906, and much was preserved; the mineral was probably subject to preferential retention by miners because of its attractive delicate blue color, fine willemite inclusions, and uncommon appearance (Figure 17-40). A note in Bauer’s catalogue indicates that [some of] this material was found in 1920 on the 300 level.
Examination of a large number of such specimens permitted the recognition of an association with willemite and one with sphalerite and blue calcite (Figure 21-25). Some specimens have small corroded remants of aegirine which may have provided the Na, Fe3+, and Si for the formation of magnesioriebeckite, but aegirine is otherwise uncommon at Franklin. The in-situ relationships, as with most Franklin minerals, are obscure, and the available specimens do not permit a definitive interpretation.
The assemblage is clearly a late-stage one; some specimens have serpentine present, and much of the magnesioriebeckite occurs, together with lennilenapeite, as a breccia-cement or late-stage coating on rounded fragments of willemite, calcite, and sphalerite. This assemblage, uncommon because of its high Mg and Fe3+ concentration, is also host to the more abundant occurrences of lennilenapeite and has provided rare and superb crystal groups of hematite. The fine elongate twisted quartz crystals illustrated by Palache (1926, 1935) are also from this assemblage.
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| Copyright © 1995 by Pete J. Dunn |
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by Herb Yeates
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