Updated November 27, 2000
Blinking light indicates updates
Eudialyte
In the recent years, the eudialyte group of minerals
has been subject to thorough studies, mainly by the efforts done by mineralogists
at the Canadian Museum of Nature (CMN) and at the Geological Museum (GM)
in Copenhagen. This has resulted in the description of four new members
of the eudialyte group: kentbrooksite, khomyakovite, manganokhomyakovite
and oneillite. In order to see where the eudialytes from the syenite pegmatites
in the Oslo Region fall within the mineral group, ten different eudialytes
from ten different localities have been analysed at CMN. The samples came
from Vesle Arøya, Brønnebukta (Siktesøya), Barkevik
strand, Bjørndalen quarry (Tvedalen), Kariåsen (Sandefjord),
Ramberg strand (Bjønnes), Eikaholmen, Nordholmen, Stokkøya,
and Buer (Vesterøya). The results show that the Norwegian eudialytes
fall into the eudialyte-kentbrooksite species range. There were only minor
variations among the different samples, and all of them should be termed
eudialyte. My sincere thanks go to Dr. Ole Johnsen (Copenhagen) who proposed
the study, and to Dr. Bob Gault (Ottawa) who did the complex analytical
work.
March 2000
Tainiolite
The Li-Mg mica tainiolite was found as a new mineral
for the syenite pegmatites in the Olso Region by Frode Andersen in 1998.
The locality was a relatively mineral rich pegmatite dike at the southern
part of Siktesøya. Only minute amounts of the mineral was found.
During investigation of pegmatite material from the dumps at the outlet
of a water tunell at Valleråsen near Porsgrunn, tainiolite was identified
as thin, colourless plates up to 5 mm across in small vugs. The mineral
was identified from its powder diffraction pattern and a semiquantitative
XRF analysis. Thanks to Arne Åsheim for the wet chemical Li analysis.
March 2000
Graphite
Graphite, as a new mineral from the syenite pegmatites
in the Oslo Region, was identified as very small scales (<0.1 mm) in
microscopic vugs in feldspar from the upper part of Tuften larvikite quarry
in Tvedalen. The graphite gives the feldspar a greyish hue.
March 2000
Chiavennite
An excellent find of chiavennite was done in April
2000 in the upper part of Tuften quarry, Tvedalen. A huge pegmatite had
been been blasted, and parts of the pegmatite was rich in vugs. In a limited
area of the pegmatite, abundance of red-orange chiavennite was found in
vugs in analcime. Associated minerals include hambergite, fibrous aegirine,
natrolite, calcite and chlorite. During June 2000 the pegmatite dike was
completely blasted and transported away. Thanks to I. Burvald for information
about the find.
May 2000
Thomsonite
Thomsonite with an uncommon habit was found in
Spring 2000 in a small syenite pegmatite dike at the upper part of Tuften
quarry, Tvedalen. The mineral occurred as very thin blades in fan-shaped
aggregates. Often two and two blades were intergrown as interpenetration
twins, and thus show a cross when viewed along the c-axis. Accessory minerals
include natrolite and analcime. Thanks to I. Burvald for information about
the find.
April 2000
Helvite
Brown to reddish brown helvite has been found
quite abundant in Håkestad larvikite quarry in Tjølling during
the spring 2000. The composition of the helvite is approximately 60 mol.%
helvite, 30 mol.% danalite and 10 mol.% genthelvite. The mineral has been
found as well developed tetrahedra from millimeter-sized up to 2 cm along
the edge. The mineral is often "frozen" in analcime. Associated minerals
include natrolite and sulphides. Hydrothermally altered meliphanite is
observed among the primary pegmatite minerals, and this mineral is probably
the source of Be in the helvite. Thanks to K. Eldjarn for the first information
about the find.
August 2000
Pyrochlore, polymignite and gemmy moonstone
In June 2000 a pegmatite dike was blasted in Håkestad
larvikite quarry in Tjølling. The pegmatite was strongly hydrothermally
altered, and most of the primary minerals (feldspars, amphiboles, biotite)
were gone on the expence of huge amounts of natrolite, analcime and chlorite.
A few magmatic minerals had survived; pyrochlore and polymignite, which
were found in well developed crystals. Pyrochlore crystals measured up
to 2 cm across, while polymignite occurred as black prismatic crystals
up to 5 cm long and 1 cm across. Both minerals, however, were quite brittle
and very easily broke apart when taken out. An exceptional piece of gemmy
"moonstone" (5x3x3 cm) was found in the pegmatite by I. Burvald. The mineral
was very pale bluish grey in colour and practically transparent with an
internal moonstone (opalescence) effect. Part of the mineral has been made
into an emerald-cut gem.
August 2000
Copper
Native copper from the Heia quarry in Tvedalen
was found and identified by B. Swensen, Porsgrunn, in 1968. It has not
previously been considered as a mineral belonging to the paragenesis of
the syenite pegmatite because it occurred as millimeter-sized, thin flakes
on cleavage planes of feldspar in larvikite. Re-investigation of the original
sample, however, reveals that the copper occurs in very close proximity
to a syenite pegmatite. Most probably, the copper has resulted from reduction
of cupric solutions of either hydrothermal or supergene origin, and related
to the syenite pegmatite formation. Hydrothermal copper minerals are well
known from syenite pegmatites in the Tvedalen area. Thanks to B. Swensen
for the mineral sample, one of the two existing samples.
November 2000
Natrolite
Two excellent finds of natrolite have been done
in the Treschow quarry in Tvedalen in October 2000 by I. Burvald. The vugs
measuring 1x0.5x0.5 m were lined with natrolite crystals up to 20 mm in
length. An interesting fact was that the natrolite occurred as two generations.
The second generation natrolite occurs as thick prismatic crystals, often
somewhat tapering, which protrudes from the base of the first generation
of natrolite which occurs as tiny crystals covering the wall of the vugs,
including large plates of calcite. Thanks to I. Burvald for presenting
the finds.
November 2000
Melanocerite (or what?), and more
An extremely coarsely crystalline syenite pegmatite
has recently been blasted in the Røyås quarry in the eastern
part of the Tvedalen area. Except for the main pegmatite minerals like
feldspar, biotite and nepheline, the pegmatite contained up to arm-thick
aegirine crystals and huge aggregates of "spreustein" with lots of small
vugs with böhmite. In addition, the pegmatite contained minor amounts
of astrophyllite, molybdenite, sphalerite and fluorite. Dark brown aggregates
of a radioactive mineral, up to several centimeter in diameter, was found
relatively abundant in the pegmatite. Preliminay analysis shows that the
mineral is a silicate of REE and Th, probably aikin to Brøgger's
caryocerite
or today thorian melanocerite. However, more study is needed to really
identify this and similar minerals from the Tvedalen area.
November 2000