samples is given in Table I, from which it is apparent that the composition is variable within each specimen, the CrzO 3 content lying between 4 and 8~o. The latter is high for a muscovite, as most analyses in the literature do not exceed 5 ~ Cr203 unless the mica is phengitic. Thus Rumyantseva et a...
| Other Authors: | |
|---|---|
| Format: | Text |
| Language: | English |
| Subjects: | |
| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.616.2821 http://www.minersoc.org/pages/Archive-MM/Volume_50/50-358-724.pdf |
| Summary: | samples is given in Table I, from which it is apparent that the composition is variable within each specimen, the CrzO 3 content lying between 4 and 8~o. The latter is high for a muscovite, as most analyses in the literature do not exceed 5 ~ Cr203 unless the mica is phengitic. Thus Rumyantseva et al. (1984) report 17.93 ~ Cr20 3 but with only 4.00 ~o A1203 and 51.70 ~o SiO2 (i.e. Si: A122: 1) in a chromium phengite. Deer et al. (1962) report the highest Cr20 3 on record up to that date as 6.08 ~o (Whitmore et al., 1964). The association of the present fuchsite matches best with the ankerite-quartz-sulphide-gold grouping of Whitmore t al. (1946) although the carbonate isnot ankerite. The high content of K in the fushsite means that even if the modal proportions of the mineral were only 0.5 ~, the K content of the rock, assuming no other mineral contains K, would be 0.055 ~ K20, which is too high to be derived isochemically from most ultramafic igneous rocks (Wedepohl, 1969). The implication is that some K was introduced in the fluids that carbonated the original igneous rock. The relatively high Ni content of the fuchsite, reaching a maximum of over 1 ~o NiO is also unusual and is linked with a small phengitic substitution giving the highest ratio of Si:A1 of 1.60: 1 which is still a long way from the 3: I lowest ratio of phengite (Deer et at., 1962). Ni is a relatively unusual component of muscovites but was pre-sumably derived from the original ultramafic rock. The variegated green colour of the virginite therefore largely derives from the composition of the breunnerite with a small contribution from the unusually high, but varied, Cr content of the fuchsite. It seems likely that a more extensive s arch in the Baie Verte fracture zone would identify fuchsite with even more Cr and Ni. |
|---|