Abstract:Fuchsite and other Cr-rich phyllosilicates, paragenetic with dolomite, are present in some ultramafic enclaves from the ‘frailesca’ rock (a lapilli- to block-size pyroclastic lithic-tuff), in the Almadén mercury mining district, Spain. Analyses (EMPA and TEM) of fuchsite and Cr-chlorite showed a relatively large range in levels of Cr2O3. Petrographic relationships between these phyllosilicates and primary relics of Cr-spinel crystals, as well as their high Cr content, indicate that these Cr-rich minerals origi… Show more
“…In most of the quartzites worldwide, occurrence of a variety of minerals such as zircon, rutile, tourmaline, Cr-spinels, pyrope, sphene, magnetite, hematite, chlorite, biotite, pyrophyllie, andalucite, silliminite, kyanite, corundum, microcline, epidote, and zoicite have been reported (Whitemore et al 1946;Clifford 1957;Ramiengar et al 1978;Raase et al 1983;Sinha-Roy and Ravindra Kumar 1984). Two types of origins are commonly envisaged for the formation of green-mica quartzites, namely: hydrothermal alteration, in which, mica is formed either due to replacement of pre-existing rocks or due to hydrothermal solution emanating from magmatic intrusions (Whitemore et al 1946;Geijer 1963; Morata et al 2001;Arif and Moon 2007) or by metamorphism of chromium-rich minerals in the source rock (Leo et al 1965;Argast 1995). Green mica quartzites are known to occur in Archaean rocks, e.g., Montana, USA (Heinrich 1965), Outokumpu, Finland (Treloar 1987), Dharwar Craton, India (Argast 1995).…”
Green mica (fuchsite or chromian-muscovite) is reported worldwide in the Archaean metasedimentary rocks, especially quartzites. They are generally associated with a suite of heavy minerals and a range of phyllosilicates. We report the occurrence of green-mica quartzites in the Saigaon-Palasgaon area within Bastar Craton in central India. Mineralogical study has shown that there are two types of muscovites; the chromium-containing muscovite (Cr 2 O 3 0.84-1.84%) and muscovite (Cr 2 O 3 0.00-0.22%). Chlorites are chromium-containing chlorites (Cr 2 O 3 3.66-5.39%) and low-chromium-containing chlorites (Cr 2 O 3 0.56-2.62%), and as such represent ripidolite-brunsvigite varieties. Back scattered electron images and EPMA data has revealed that chlorite occurs in two forms, viz., parallel to subparallel stacks in the form of intergrowth with muscovite and independent crystals within the matrix. The present study indicates that the replacement of chromium-containing chlorite by chromium-containing muscovite is found to be due to increasing grade of metamorphism of chromium-rich sediments. However, the absence of significant compositional gap between aforementioned varieties indicates disparate substitution of cations, especially chromium, within matrix chlorites. The chromium-containing muscovite and muscovite are two separate varieties having distinct paragenesis.
“…In most of the quartzites worldwide, occurrence of a variety of minerals such as zircon, rutile, tourmaline, Cr-spinels, pyrope, sphene, magnetite, hematite, chlorite, biotite, pyrophyllie, andalucite, silliminite, kyanite, corundum, microcline, epidote, and zoicite have been reported (Whitemore et al 1946;Clifford 1957;Ramiengar et al 1978;Raase et al 1983;Sinha-Roy and Ravindra Kumar 1984). Two types of origins are commonly envisaged for the formation of green-mica quartzites, namely: hydrothermal alteration, in which, mica is formed either due to replacement of pre-existing rocks or due to hydrothermal solution emanating from magmatic intrusions (Whitemore et al 1946;Geijer 1963; Morata et al 2001;Arif and Moon 2007) or by metamorphism of chromium-rich minerals in the source rock (Leo et al 1965;Argast 1995). Green mica quartzites are known to occur in Archaean rocks, e.g., Montana, USA (Heinrich 1965), Outokumpu, Finland (Treloar 1987), Dharwar Craton, India (Argast 1995).…”
Green mica (fuchsite or chromian-muscovite) is reported worldwide in the Archaean metasedimentary rocks, especially quartzites. They are generally associated with a suite of heavy minerals and a range of phyllosilicates. We report the occurrence of green-mica quartzites in the Saigaon-Palasgaon area within Bastar Craton in central India. Mineralogical study has shown that there are two types of muscovites; the chromium-containing muscovite (Cr 2 O 3 0.84-1.84%) and muscovite (Cr 2 O 3 0.00-0.22%). Chlorites are chromium-containing chlorites (Cr 2 O 3 3.66-5.39%) and low-chromium-containing chlorites (Cr 2 O 3 0.56-2.62%), and as such represent ripidolite-brunsvigite varieties. Back scattered electron images and EPMA data has revealed that chlorite occurs in two forms, viz., parallel to subparallel stacks in the form of intergrowth with muscovite and independent crystals within the matrix. The present study indicates that the replacement of chromium-containing chlorite by chromium-containing muscovite is found to be due to increasing grade of metamorphism of chromium-rich sediments. However, the absence of significant compositional gap between aforementioned varieties indicates disparate substitution of cations, especially chromium, within matrix chlorites. The chromium-containing muscovite and muscovite are two separate varieties having distinct paragenesis.
“…In another comparable situation in Spain, Morata et al . (2001) record the generation of fuchsite and Cr-rich chlorites and illites in hydrothermally-altered ultramafic enclaves in which former Cr-spinels had reacted under highly acid conditions (pH < 0.3), losing Al, Cr, Fe and Mg. The liberated Cr 3+ was accommodated in octahedral suites in the clay lattices.…”
Section: Discussionmentioning
confidence: 99%
“…This suggests that the spinels probably preserve their primary mantle composition. Spinel has been reported to show comparable resistance to complete dissolution in the altered peridotites of Crommyonia (Greece) (Mitsis et al ., 2018) and also in the ultramafic enclaves at Almaden (Spain) (Morata et al ., 2001). Fig.…”
The Weaklaw vent in SE Scotland (East Lothian coast), inferred to be Namurian, produced lava spatter and volcanic bombs. The latter commonly contained ultramafic xenoliths. All were metasomatised by carbonic fluids rich in incompatible elements. The lavas and xenoliths are inferred to have been basanites and lherzolites prior to metasomatism. The abundance and size of (carbonated) peridotite xenoliths at Weaklaw denotes unusual rapidity of magma ascent and high-energy eruption making Weaklaw exceptional in the British Isles. The lavas and xenoliths were altered subsequently by low-temperature (<200°C) carbo-hydrous fluids to carbonate, clay and quartz assemblages. A small irregular tuffisite ‘dyke’ that transects the ejecta is also composed dominantly of carbonates and clays. The peridotitic xenoliths are typically foliated, interpreted as originating as pre-entrainment mantle shear-planes.Analyses of the relic spinels shows them to be compositionally similar to spinels in local unaltered lherzolites from near-by basanitic occurrences. Chromium showed neither significant loss nor gain but was concentrated in a di-octahedral smectite allied to volkonskoite. It is in the complex association of smectite with other clays, chlorite and possibly fuchsite that the diverse incompatible elements are concentrated.We conclude that late Palaeozoic trans-tensional fault movement caused mantle shearing. Rapid ascent of basanite magma entrained large quantities of sheared lithospheric mantle. This was followed by ascent of an aggressive carbonate-/ hydroxyl-rich fluid causing pervasive metasomatism. The vent is unique in several ways: in its remarkable clay mineralogy and in displaying such high Cr-clays in a continental intra-plate setting; in being more productive in terms of its ‘cargo’ of peridotite xenoliths; in presenting an essentially un-eroded sequence of Namurian extrusives; and, not least, for giving evidence for post-eruptive, surface release of small-melt, deep-source fluids.
“…ion in the M4 crystal site are determined not only by Cr-O distances but also by the chemical nature of all units. It is generally accepted that trioctahedral chlorites have a significant number of octahedral vacancies (Foster 1962, Morata et al 2001. Foster (1962) stated that low octahedral totals are induced by the method of calculation and need not indicate the vacancies, and that octahedral totals\11.5 are common.…”
For the first time ever, the luminescence spectra of Cr 3? centers in two chlorite crystals are presented. Chromium ions occupy the strong crystal-field site M4 in the brucite sheet and the intermediate crystal-field site in the inner octahedral sheet for purple and green chlorite, respectively. We discuss the influence of an effective positive charge on the Cr 3? ion and an effective negative charge of ligands on the differences in the values of the Dq and B parameters. It is concluded that the presence of Fe 2? ions and other point defects, as well as concentration quenching, causes the very short luminescence lifetimes of chromium ions.
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