bazzite from Tørdal

Bazzite from a pegmatite near Tørdal, Telemark, South Norway: by Ronald Werner

Introduction

The Tørdal quarries in Nissedal are among Norway`s most popular mineral-occurrences, and offer both beginning and advanced collectors excellent possibilities to collect pegmatite minerals. These vary from simple minerals like for instance amazonite, lepidolite, topaz and beryl, to rarities like yttrotantalite, fluocerite-(Ce), and tveitite-(Y) (type-locality).

However, in the Nissedal area several hundred more pegmatites occur, but were never taken into exploitation. It was during a recent prospecting program for tin, that in one of these pegmatites - referred to as the "Heftetjern pegmatite"- a spectacular find of bazzite and other scandium-rich minerals was made.

Geology

The Nissedal area consists outof a series of volcano-sedimentary supra-crustal rocks (1300-1200 My / amphibolites, gabbros, tuffs etc.: the so-called "Nissedal outlier"), overlying and bordering a basement of granitic gneisses (1520-850 My). These have been intruded by the Tørdal Granite (960-850 My), the source of the pegmatitic fluids. In a theory advanced by Bergstøl & Juve (1988), it is assumed that these fluids, while moving upwards through the rocks of the "outlier", were contaminated with scandium, which occurs in relatively high percentages (ppm) specifically in the mafic rock- types of the "outlier". Both the basement rocks and the Tørdal Granite show normal scandium contents.

The Pegmatites

The Heftetjern pegmatite is part of a large system of pegmatite veins emanating from the Tørdal Granite. The veins consist in the first place of reddish microcline, albite and quartz, and contain locally bodies of cleavelandite-amazonite pegmatite. Most of the cleavelandite-amazonite pegmatites consist of two generations of crystallisation.

In some cases the pegmatites show well-developed zoning, with an amazonite-dominated zone against the wall-rock, and a quartz/cleavelandite dominated core. In other cases the two stages of crystallisation occur intimately mixed, without zoning.

The amazonite of the first generation is often developed as "graphic granite" against the wall-rock. In the case zones are lacking, the amazonite is generally pervaded by thin veins of microcline/cleavelandite along crystal-faces and fractures.

The second generation pegmatite usually hosts the most interesting minerals. The Heftetjern pegmatite shows well-developed zoning. Outcrops of bazzite-bearing pegmatite occur over an area of approximately 0,5 km2. The thickness of the outcropping veins varies between 3 and 40 meters.

Mineralogy

I concentrate myself on the scandium minerals, and will only mention some of the more relevant specifics of the other minerals

Bazzite is the most spectacular scandium-containg mineral from the Heftetjern pegmatite. The crystals occur as nice blue prisms with a maximum observed length of 3 centimeters. The average crystal-size is in between 0,1 and 0,5 centimeters.

Crystal-development is typically simple: a combination of a hexagonal prism and basis. Other forms have not been observed.

bazzite The bazzite occurs embedded in feldspar and quartz, in close association with yellow, ordinary beryl. In some cases the bazzite crystals occur as parallel-epitaxy along the C-axis on the beryl crystals. The beryl is in all cases to a certain extent altered. The paragenetical relationship between bazzite and beryl has so far not been examined. The bazzite also occurs as subhedral plates in feldspar and quartz, or as small crystals in fibrous aggregates of bavenite, an alteration product of beryl.

The bazzite contains on average 14,5 % Sc2O3. As can be seen in table 1, the bazzite contains almost 3 weight-percents of Cs2O. This means that caesium exceeds 0.1 in the formula unit, and as a caesium-rich variety, the mineral should be called caesian bazzite.

Many of the bazzite crystals are fractured, possibly due to internal stress caused by the presence of inclusions. Another factor seems to be later regional metamorphosis to which the Nissedal area has been subjected.

Bazzite cannot be considered to be occurring abundtly in the pegmatite veins of Heftetjern. Nevertheless, this locality seems so far to be among the richest known occurrences in the world for bazzite.

Ixiolite occurs as scandium-rich variety: (Ta,Nb,Sc,Sn,Fe,Mn,Ti)2O4. The crystals are shiny black with a brownish tint, with rectangular or square outline, and with a maximum size of 0,5 cm. Samples with up to 18.8% Sc2O3 have been found. Of 8 analyzed ixiolite-samples, 6 turned out to be very scandium-rich, and were called scandian ixiolite. Since scandium represents a major constituent in the unit-cell formula, these samples could be submitted to the IMA/CNMMN as a potential new species. However, in a personal communication to me, Mr. Bergstøl informed me that no such steps are considered for the timebeing.

Many of the crystals are fractured. Occasionally a rust-brown halo around the ixiolite indicates the presence of radioactive constituents. Partly or complete alteration of ixiolite to pyrochlore-group minerals, due to late-stage hydrothermal activity, is not uncommon.

Pyrochlore-group minerals from the Heftetjern pegmatite have the general formula: (Ca,Sc,Y,Sn,U)2O6(O,OH,F). Analysis has revealed the presence of pyrochlore, microlite and betafite. They are all metamict alteration products of ixiolite. Microlite is by far the most common of the three.

A microlite-sample containing 3.4% Sc2O3 could be submitted to the IMA/CNMMN as a potential new species. However, due to the very small amounts of material available for examination, it is very unlikely that it will ever come so far.

The other minerals

Bavenite occurs as massive, radiating aggregates of very thin, fibrous crystals, as an alteration product of beryl.

Bertrandite occurs as very small, colourless crystals in cavities of altered beryl.

Beryl occurs as greyish to greenish-yellow, hexagonal, prismatic crystals up to a length of 30 cm with a diameter of 20 cm. Most of the beryls show obvious signs of alteration.

Feldspar-group minerals are represented by: anhedral, reddish microcline; anhedral, colourless albite-oligoclase with "etched-out" cavities; subhedral, white to pale bluish cleavelandite, in platy aggregates; green amazonite as euhedral crystals up to 40 cm embedded in quartz, and as "graphic granite" in contact with the wall-rock.

Other observed minerals are: allanite, cassiterite, fluorite, gadolinite, garnet, magnetite, mica-group minerals, monazite, quartz and zircon.

Some geochemical considerations

In the earth`s crust scandium is one thousand times more abundant than gold, but enrichments of scandium in mineral-deposits are probably reciprocally that much rarer. Any significant enrichment of Scandium in a mineral-deposit deserves therefore extra attention. Study of the Heftetjern pegmatite has given support to recent theories about the geochemical behaviour of scandium. During magmatic differentiation-processes scandium is enriched in the more mafic rock-types and/or minerals. Contamination of fluids crossing scandium-enriched rocks seems to be an important mechanism for getting significant scandium enrichments in a mineral-deposit. A similar mechanism was advanced by V.M. Goldschmidt (1934) for the thortveitite-bearing pegmatites of the Iveland/Evje district in Aust-Agder.

Further geochemical study of the Heftetjern pegmatite veins concerning the relation of scandium on the one hand, and tin, beryllium and lithium on the other, may reveal important information for the prospecting for this metal with potential high-technology applications.

Conclusion

The discovery of a pegmatite where all the blue beryls turn out to be bazzite instead of aquamarine, should make us suspicious of any "blue beryl". Nevertheless, the pegmatite veins of Heftetjern are so far known the only in a very large pegmatite distrcit yielding bazzite, and the mineral is even in these veins rare. Scandium minerals will probably remain rare and much sought after.

The Heftetjern occurrence will hopefully make bazzite available to a somewhat broader public. Top-quality specimens will probably remain very rare. Another important point is, that exploration-blasts into the Heftetjern pegmatite have made bazzite-containing rock clearly visible and easily accessible for collectors. Doubtleslly there have already been some persons collecting here. There is a good chance that in some collections bazzite from Heftetjern is labelled as aquamarine. Collectors should be aware of this mistake. Finally, it should be noted that in this area hundreds of pegmatite outcrops occur. It is very likely that a more thorough and conscientious examination of these pegmatites will result in new finds of interesting minerals.

Acknowledgements

I would like to thank Mr. Bergstøl and Mr. Juve, the discoverers of the bazzite, for their kind willingness to supply me with vital information on the subject. Mr. Juve`s critical reading of my manuscript has led to certain important impovements, for which I much indebted to him.

References

-Anonymous contributors: collecting experiences as narrated by some Norwegian mineral-collectors.

-Bergstøl, S. & Juve, G. (1988) : "Scandian Ixiolite, Pyrochlore and Bazzite in Granite Pegmatite in Tørdal, Telemark, Norway. A contribution to the Mineralogy and Geochemistry of Scandium and Tin"; Mineralogy and Petrology 38: 229-243; Springer-Verlag

-Chistyakova NB, Moleva VA, Razmanova SP (1966): "The first find of bazzite in the USSR"; Dokl Akad Nauk SSSR 169: 1421-1424

-Fleischer, M. & Mandarino, J.A. (1991): "Glossary of Mineral Species"; The Mineralogical Record Inc.

-Goldschmidt, V.M. (1934): "Drei Vortrage über Geochemie"; Geol Fören Stockh Förh 56: 385-427

-Juve, G. & Bergstøl, S. (1990): "Caesian Bazzite in Granite Pegmatite in Tørdal, Telemark, Norway"; Mineralogy and Petrology 43: 131-136; Springer-Verlag

-Ramdohr & Strunz (1978): "Klockmanns Lehrbuch der Mineralogie; Enke-Verlag

-Wilke, H.-J. (1976): "Mineral-Fundstellen Band 4, Skandinavien"; Weise-Verlag