An epitaxy of rutile on hematite from Vinstra, Oppland, Norway

by Ronald Werner

INTRODUCTION

Ever since the discovery of Anatase and Brookite in the Hardangervidda, and Anatase in Valdres, it is reasonable to say that Norway is an important source of high-quality specimens of two of the three titanium-dioxide minerals. The third member of this polymorphous substance -rutile-, has been reported from numerous occurrences throughout Norway. But none of these finds ever caused any commotion among mineral-collectors. However, a very small occurrence near Vinstra, 80 kilometers north of Lillehammer, has over the years yielded a good quantity of an exceptionally beautiful epitaxy of rutile on hematite. Eventhough mainly of interest to micro-mineral collectors, I would hereby like to propose that Norwegian localities have yielded all three titanium-dioxides in specimens of outstanding quality.

GEOLOGY

The Vinstra area consists entirely outof weakly metamorphosed rocks of sedimentary origin, the so-called "Hedmark group". These rocks are dominated by sandstone’s, pelites and phyllites, which were deposited in basins or faulted throughs in the Pre-Cambrian shield during the late Pre-Cambrian and Eocambrian. These sedimentary formations must have reached an accumulated thickness of many kilometers. The rocks of the Vinstra area have been affected by four separate periods of deformation, causing folding and faulting. During the first stage of deformation low-temperature veins of Quartz, Feldspar and occasionally hematite were emplaced. The exact origin of the fluids has not been described, but their appearance suggests that they may very well be classified as Alpine type of "klufts". In this context it is interesting to know, that just 10 km north of Vinstra, near Sjoa, at the western bank of the Låagen river, in a similar looking occurrence, nice anatase crystals can be collected at a fee-collecting site. Brookite has so far not been reported to occur in this area.

MINERALOGY

The mineralogy of the veins near Vinstra is very simple. The veins consist in the first place outof massive, milky-white quartz. In cavities good clear quartz crystals up to 5 cm can be found. Occassionally the crystals have a pale smoky color. Tabular crystals of white, translucent Albite is the second mineral to occur in the veins. Less common are small, white orthoclase crystals in the "adularia" habit. Monazite occurs very rarely as orange, transparent crystals up to 2-3 mm. These resemble the monazite crystals I have in my collection from several Alpine occurrences. Chlorite occurs widespreadth in minor quantities. Rutile and hematite are the minerals of principal interest to collectors. They both occur alone and as a very beautiful star-like epitaxy of thin golden-brown rutile needles on the prism-planes of pseudo-hexagonal, tabular, lustrous black hematite crystals. Hematite is abundantly present in the veins. The crystals are generally of a simple development: one or not uncommonly two hexagonal prism (1120) and/or (1010), and a basispinacoid (0001). However, much more complex crystals, with several additional forms, are not uncommon. Remarkable are square or rectangular hematite crystals with epitaxial upgrowth of rutile. The maximum size of the hematite crystals is approximately 1 cm. Very rarely lens-shaped crystals are found. Whether these are hematite or Ilmenite is unknown. The hematite from Vinstra contains a high percentage of titanium, and was until recently traded as Ilmenite. So far, only hematite has been positively identified. A vast majority of the hematite crystals occurs without association of rutile. Rutile can sparingly be found as "Sagenite" in aggregates of very small needles. The parallel orientated needles of rutile on the prism faces of the hematite crystals can reach a length of 0.5 cm. Eventhough very much more uncommon than the hematite crystals, the epitaxy of rutile on hematite may considered to be relatively common.

CONCLUSION

The epitaxy of rutile on hematite from Vinstra is of great beauty, and matches anything coming from the Alps. And eventhough size-wise incomparable with similar Brazilian material, its sober perfection and delicateness are hard to surpass.

REFERENCES

Englund, Jens-Olaf; Stratigraphy and Structure of the Ringebu-Vinstra District, Gudbrandsdalen; with a short Analysis of the Western Part of the Sparagmite Region in Southern Norway; Norges Geologiske Undersøkelse 1973; Nr. 293

Selbekk, Rune S.; Stein, Nr. 3, 1991: "Meldinger", blz 10.