Pegmatite at level 1, A/S Granit quarry

Just before Christmas 2007 a large section of a pegmatite vein was exposed in the A/S Granite quarry, Tvedalen. This pegmatite was also known from earlier in the springtime 2007. Unfortunately it wasn’t well exposed because of either ice or snow most of the time during the Christmas holidays 2007/New year 2008, but in a short time around New years eve, it were visited by several collectors, and a good variety and several good samples where rescued.




Already in the spring 2007 it was known that this pegmatite contained beryllium minerals. Then hambergite and chiavennite were found, together with thorite and others. Around New Year, several more exciting Be-minerals were found, beside hambergite and chiavennite, also eudidymite, epididymite, helvite, behoite and berborite were found. A large number of other interesting minerals were found during this few days too. Also later in January 2008 it was possible to find interesting minerals, butt less of the Be-minerals.

The Be-minerals were occurring in mainly three/four different “paragenesises” in this pegmatite. First it was the primary Be-mineral, which is hambergite, occurring as fibrous masses or as crystals surrounded by other minerals. Then there are minerals occurring in vugs in coarse crystalline feldspar, here nice crystals and aggregates of epididymite were found, crystals to 6 mm long. Also helvite was found in vugs like this. Then there were the secondary Be-minerals, formed by hydrothermal alteration of the pegmatite and the primary Be-minerals. As fillings in open fractures in feldspar, chiavennite and epididymite was found together with natrolite and fluorapophyllite. Also occurring as secondary Be-minerals; behoite and berborite was found in vugs in “spreustein” – altered sodalite/cancrinite/nepheline, replaced by zeolites, mostly natrolite, but also possibly thomsonite-Ca. Both behoite (occurring in twinned crystal aggregates) and berborite on natrolite lined vugs.

The pegmatite contained as said a lot of other interesting minerals as well, here shortly described from the various environments in the pegmatite they where found.

As primary rock forming minerals, feldspar, mainly microcline and then some albite were the main minerals, then the feltspatoides, nepheline, sodalite and cancrinite were present. Main mafic minerals were; annite, aegirine and a probably ferro-edenitic amphibole where common, and in the hydrothermal parts, natrolite and analcime where the common minerals. In some annite rich parts zircon could be considered rock forming as well. Of the rock forming minerals it’s worth mentioning annite and aegirine. Annite crystals with a diameter to 7 cm were found. A lot smaller but nice double terminated crystals to 2 cm long, was found in matrix. Huge sections of aegirine was found too, some larger than 15 cm long. As in many other beryllium rich pegmatites, also manganese minerals was present, and in this pegmatite not only as secondary oxides!

Alf Olav Larsen have analysed a carbonate mineral occurring in the Be-rich part of the pegmatite, a pretty hard weathered, but a primary mineral in the pegmatite. It resulted in Ca 65% MnCO3, 30% FeCO3 and 5% CaCO3 – an iron rich rhodochrosite. The “crystals” of rhodochrosites are frozen into the feldspar, indicating that it’s a primary mineral in the pegmatite. It’s pretty altered, only the larger “crystals” being pink in the centre and of the then loosing colour ending up in a heavy black oxide rim. In nearby parts of the pegmatite, secondary Mn-oxides/oxides were abundant. Non of these have been analysed, but compared to other Mn-oxides/oxides occurring in the area it would be good “qualified” guesses to say that the major part of some amorphous transparent brown Mn-oxides/oxides are neotocite. In cavities in the neotocite rich material there is some brown botryoidal aggregates that looks very similar to gonyerite found in Sagåsen Quarry. Beside these two minerals there are lots of black Mn-oxides/oxides, both dendritic and botryoidal. This might be both hisingerite and todorokite.



The “spreustein” also contained other minerals than behoite, berborite and zeolites. Most common after natrolite, is böhmite crystals, usually rather small (ca 0,2 mm). Also gibbsite is pretty common in the spreustein.

In some parts of the pegmatite, as mentioned, zircon crystals where occurring as a major part of the rock. What were curious about some of these zircons were that some of them was hollow. The crystals are like shells, perfect on the outside – bipyramids without any prismatic faces, then inside there is an open hole.

On a later trip a lot of eudialyte-group members where found in pretty large masses, up to fist size. Most of the nodules are pretty altered, and having a rather dark brown colour. Looking pretty much like the ferrokentbrooksite from Vesterøya, Sandefjord. Some are a lot lighter in colour, almost yellow, and can very well be kentbrooksite.



List of the minerals found so far in the “level 1 pegmatite”:

Aegirine
Albite
Analcime
Annite
Arsenopyrite
Behoite
Berborite
Böhmite
Calcite
Cancrinite
Chiavennite
Epididymite
Eudialyte-group
Eudidymite
Fluorapophyllite
Galena
Gibbsite
Hambergite
Helvite
Magnetite
Microcline
Molybdenite
Natrolite
Neotocite
Nepheline
Rhodochrosite
Sodalite
Sphalerite
Sulphur
Wulfenite
Zircon

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