The Solumsåsen Quarry near Holmestrand

by Ronald Werner

INTRODUCTION

In the Oslo area occur massive lava flows. These volcanics are certainly not the most mineral-rich rock-types in the area. However, a small number of excellent occurrences is known. Among these, a minor stone-quarry near Holmestrand is probably the best known in Norway. The good news for foreign micromount collectors is, that material is plentiful and truly exciting. I visited the quarry in april 1995 shortly after a blast in the most mineralized zone. Beware of world's richest anatase occurrence! (well, by numbers of crystals, maybe...)

GEOLOGY

In connection with large-scale tectonics some 300 m.Y. ago, a pre-existing system of fractures in the Oslo area was re-activated. As a result, the Oslo-area subsided between 1 and 2 km and widespreadth volcanic activity occurred. A first series of dark basalts was followed by lighter rhombenprophyry lava's, containing large and clearly visible feldspar crystals. In the Holmestrand-Horten-Tønsberg district (70 km south of Oslo) extensive lava and rhombenporphyre flows are conserved, notwithstanding fierce erosion during the last ice-age (till 10.000 BC). In a tiny stone-quarry -the Solumsåsen Pukkverk- near Holmestrand (fig. 1) some parts of the rhombenporphyry rock are very rich in vesicles, containing an interesting suit of Ti-, Be- and REE-minerals.

MINERALOGY

The mineralogy of the vesicles in the Holmestrand rhombenporphyry is relatively simple: no more than about 20 different minerals have been observed. Of these, only 5 are of real relevance to the mineral-collector. These will be described into some detail, while the others will be mentioned for completeness' sake. A comprehensive article about this occurrence has been written by Larsen (1994) in STEIN.

Anatase is incredibly abundant in the vesicles: hundreds, thousands, no, millions of small, but very well-formed crystals can be found! Each individual vesicle can contain all from 1 sad, lonely crystal up to a crowd of over a hundred anatase crystals.
The development of the crystals is relatively simple, and many have only one single bi-pyramidal form: (101). In addition have been observed a prism (110), and more sub-ordinate a (103) bi-pyramidal form and (001) basis form.
The crystals can be found either isolated, or not uncommonly as aggregates. These sometimes resemble a porcupine, while others are randomly grouped (fig. 2).
In most vesicles the crystals lie randomly, but in rare cases the crystals lie in a straight line, reflecting the flow of mineralizing gasses/fluids.
Very nice (101) twins are not uncommon in certain zones, but are otherwise very rare. The color of anatase varies from olive green-brown to blue with high, un-metallic luster, to black with metallic luster.
The crystals are small, and 1,5 millimeters must be considered to be a really huge crystal from this occurrence. However, due to their sheer abundance, the well-formed, shiny crystals on a white matrix of albite or illite make very fine specimens for the micromount-collector.

Bertrandite is rare, and seems to be concentrated in random zones throughout the flow. During my last visit (april '95) my companion found several pieces with bertrandite, while I found so far only a single specimen.
The crystals are colorless and completely clear, and therefore easily overseen. Their form can be described as lineal-shaped, and twinning (fig. 3) is abundant. The size is about 1-3 mm.

Lanthanite-(Ce) is extremely rare and so far I didn't find a single specimen. The crystals are described as being colorless to pale rose, flat prismatic up to 2-3 mm long. The luster is pearly (Larsen '94).

Parisite-(Ce) and synchysite-(Ce) are described together, while these minerals occur either intergrown as polycrystals, or as crystals that are impossible to distinguish. It has been said that parisite-(Ce) dominates over synchysite-(Ce), and for the sake of simplicity trade-material in Norway is labeled as parisite-(Ce). I propose not to follow this policy, but rather emphasize that synchysite-(Ce) and parisite-(Ce) are both present as individual crystals and as polycrystals.
The usually 6-sided crystals vary from tabular till long-prismatic. As can be seen on fig. 5 are prism faces often ill-developed. The basis however, is usually smooth and shiny.
The larger long-prismatic crystals are more commonly seen to be grown individually in the vesicles, while smaller long-prismatic crystals and the tabular crystals more commonly occur in aggregates. The tabular crystals are sometimes found as nice rose-like groups.
The most exciting feature of this occurrence are the polycrystals of parisite-(Ce) and synchysite-(Ce) in a variety of combinations. "Pure" parisite-(Ce) crystals are in itself not uncommon (fig. 4), but polycrystals are probably far more abundant. Often seen are thick, tabular crystals, where the thick mid-section consists of the one mineral, while the thinner outer plates are the other (fig. 5). Also common are barrel-shaped parisite-(Ce) crystals with a thin middle section of synchysite-(Ce) (fig.6). The polycrystals can occur as single, isolated crystals, and as aggregates of several crystals (fig. 7).

In a personal communication, Alf O. Larsen informed me that also synchysite-(Ce) occurs as "pure" crystals. A simple test to distinguish parisite-(Ce) from synchysite-(Ce), has been described by Donnay & Donnay (1953). While synchysite-(Ce) dissolves very much faster in acid than parisite-(Ce), it should be possible to establish their identity by observing differences in solubility. The color of both the parisite-(Ce) and synchysite-(Ce) varies from pale yellow, brown to orange-brown. The maximum size of the crystals is about 2-3 mm.

Other minerals: albite, apatite, calcite, epidote, fluorite, illite, manganese-oxide, markasite(altered), pyrite(altered to goethite/lepidocrocite/hematite), quartz and titanite.

CONCLUSION

The vesicle minerals of the Solumsåsen quarry near Holmestrand belong with one notable exception (lanthanite) to the more common species as far as micromount-collectors are concerned. However, I have never seen nor heard of a similar occurrence on earth, and there is a good chance it is an absolutely unique one. And more important, the material is so incredibly much fun, that I consider a couple of samples in your collection a must! I supplied some US collectors (MN-readers!) with plenty material, so all interested micromount collectors should be able to get hold of some pieces! For those U.S. collectors visiting Norway it might be interesting to know that the quarry-owner allows collecting! A helmet is obligatory. The exact location of the quarry can be found on the topographical map (1 : 50.000) of the Holmestrand area. Or just contact me.

ACKNOWLEDGMENTS

I am very much indebted to Mr. Alf Olav Larsen for allowing me the use of some very informative SEM-pictures. Mr. Larsen's article in STEIN 2/94 has obviously been of enormous help in writing this article! Together with his article there are several addition (SEM-) pictures of parisite-(Ce) and/or synchysite-(Ce) and of many of the other minerals.

REFERENCES

-Donnay, G. & Donnay, J.H.D. (1953): The crystallography of bastnäsite, parisite, röntgenite and synchysite. American Mineralogist, 38, 932-963.
-Larsen, A. O. (1994): Drusemineraler fra Solumsåsen pukkverk, Holmestrand. Stein, 2/94, 136-141.
-Werner, R (1993): Om synchysitt i Norge. Stein, 3/93, 221-226