Linnajavri
Mineral resources
Mineralogy and chemistry
Soapstone is a talc-carbonate rock. The soapstone in the Linnajavri area is of the same type as known from other areas with abundant ultramafics in the Scandinavian Caledonides. Talc derived from alteration of ultramafics to a certain extent inherits the trace element chemistry of the unaltered precursor ultramafic minerals (foremost olivine and serpentine).
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Talc in thin section, x nichols Photo: L. P. Nilsson |
The content of talc in the soapstone is about 50 %, but not seldom it locally may reach as much as 70 – 80% of the bulk rock composition. The soapstone is coarse grained with coarse magnesite crystals set in a more fine-grained matrix of talc with some chlorite intermixed with it. In addition fine-grained magnetite is disseminated in the soapstone.
The soapstone texture is mostly massive, but locally it may show a pronounced foliation or banding.
The soapstone has formed during pervasive metasomatic infiltration of CO2 rich fluids through the serpentine dominated ultramafics. The introduction of CO2 into the serpentinite has instantly trigged the formation of talc and carbonates at the prevailing pressure and temperature ranges (upper greenschist to lower amphibolite facies conditions). The carbonates formed are magnesite and subordinate dolomite, both with a varying iron content in the lattice structure (i.e. breunnerite and ferrodolomite respectively in the carbonate nomenclature). The FeO content in the magnesite varies between 1.5 and 17 wt-% and in the dolomite between 1.5 and 4 wt-%. The magnesite may locally exhibit a substantial crystal zoning, always with the more Mg-rich and pristine compositions in the center of crystals. In a case studied in detail a compositional span from 8 to 16 % FeO has been recorded in a large and early-formed magnesite crystal from center to rim.
The talc has a composition typical for ultramafite-derived talc, i.e. with lattice bound FeO in the range 1.0 – 4.5 wt-% and in addition variable tenors of nickel, also lattice bound, at the expense of Mg and Fe. Trace amounts of Co and Cr also enter the talc lattice.
In addition to the major components talc and carbonates varying amounts of chlorite of slightly different composition occur in the Linnajavri soapstones. Viewed together chlorite analyses tend to cluster into two groups, one enriched in Al and Fe relative to the other that in turn is enriched in Si and Mg relative to the former. The Si and Mg enriched group is the more primitive of the two.
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| MgO, Al2O3, FeO and NiO vs SiO2 |
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| NiO/MgO, Cr2O3/Al2O3, Cr2O3/SiO2. |
Further, sulfides as well as eventual other trace constituents as sulpharsenides, arsenides, alloys, etc. are invariably extremely rarely observed in these deposits. The dominating opaque minerals are spinels, mostly magnetite, but also chromite and its hydrothermal alteration products ferrichromite and chrome-magnetite. Towards the contact with serpentinite late formed serpentine may occasionally be seen, and then preferably as very minor needles, etc. in larger magnesite crystals, and more rarely also in talc. It is worth noting here that this serpentine is not formed from talc, but from chlorite that is structurally more closely resembles serpentine than talc does. SEM-analyses have revealed a span in chlorite compositions from stable Mg-Fe chlorite through intermediate meta-stable hybrid compositions (Al + Cr depleted pseudo-chlorites and Al + Cr enriched serpentine) to late formed serpentine, a feature not at all seen in talc.
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| Percentage of Ni and Cr in talc and chlorite. |
Where soapstone borders calcite and dolomite marble units a pale brown tremolite is locally developed close to the contact, e.g. in the Boarta-2 lens. However, occurrence of tremolite only in a very subordinately affects the soapstones in the Linnajavri area, and tremolite is not observed at all to the southeast and south of the Kvitfjell body.
A large number of bulk chemical analyses as well as mineral analyses (SEM-analyses) are performed. These are partly confidential and partly available in open file NGU reports. Cf. reference list.
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Backscatter electron image of soapstone. Photo: A. Korneliussen |
Discovery and recognition of deposits
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Exploitation in the Linnajavrí area