Syenite pegmatite dikes are encountered throughout the whole Larvik plutonic complex and in the basaltic rocks close to the border. The syenite pegmatites in the southern part of the Oslo Region approaches agpaitic mineralogy, specially along the south-western border. The pegmatites occur as fissure filling dikes with more or less sharp borders against the wall-rock. Brøgger (1890) distinguished between two types of pegmatites: A western type occurring in the Langesundsfjord area, and an eastern type occurring in Brunlanes-Larvik-Tjølling-Sandefjord area. The latter type was previously called Stavern-type dikes (Fredriksvärn-type by Brøgger). These pegmatites may attain large dimensions. Dikes with a thickness of 1 m are quite common, and some dikes are 10-20 m thick and 120 m long. They have sharp borders against the wall-rock, and are usually coarse- to giant-grained, with feldspar individuals up to 2 m in size. The main minerals are greyish to reddish microclin (microperthite to cryptoperthite, often schillerising) and black amphibole (hastingsite, magnesiohastingsite or magnesiokatophorite) ± nepheline (often altered to spreustein) ± magnetite ± biotite ± a suite of accessory minerals. The amount of accessory minerals is, with few exceptions, rather limited. The pegmatites of the western type, i. e. in the border zone on the island in the Langesundsfjord and on the mainland in the immediate vicinity of the fjord, occur as more or less irregular veins, often not particularly coarse-grained. This type of pegmatites show a more agpaitic mineralogy than the previous type, and are classified as nepheline syenite pegmatites. The main minerals are white or greyish microcline, nepheline (often more or less altered to spreustein) ± aegirine ± ferro-edenite (barkevikite) ± magnetite ± biotite. In addition, a large variety of accessory minerals may be present, and the abundance of Zr-, Ti-, Nb-, REE- and Be-minerals are conspicuous. The basaltic rocks close to the border of the larvikite massif are locally transected by huge pegmatite dikes, mainly of the western type. In most syenite pegmatite dikes, apart from the primary, magmatic stage, a secondary, hydrothermal stage is discernible. The hydrothermal stage is usually characterised by extensive zeolitisation and alteration of the magmatic minerals, and with crystallisation of low-temperature hydroxides and hydrous silicates. Many of the rare REE-minerals and Be-minerals belong to this stage of pegmatite formation. A few minerals have crystallized as the result of supergene prosesses, but is never the less part of the complete history of the syenite pegmatite dikes in the Larvik plutonic complex.
Pegmatite dikes can be found in the whole Larvik
plutonic complex, but is specially abundant in the Langesundsfjord and
Tvedalen areas. Collection of minerals is possible nearly everywhere, except
at protected areas and when prohibited by the land owner or quarry operator.
Låven
The name Låven means the barn, because
the island looks like a barn with a rounded roof. The length is about 80
m, the width about 40 m, and the height about 8.5 m (Fig. x). The "roof"
slightly slopes towards south and south-west, while the northern and north-eastern
sides of the island are very steep. The "roof" is furrowed by several north
- south running parallel glacial grooves. Several places on the "roof"
are bruised by blasting for rare minerals, and rock fragments from earlier
collecting activities fill portions of the central glacial groove. The
"roof" of the Låven consists of syenite pegmatite with a thickness
of a few meter overlaying a base of basaltic rock. The main pegmatite minerals
on Låven are white microcline, reddish to brownish nepheline, grey
sodalite, aegirine-augite, biotite and magnetite. Nepheline and sodalite
are usually altered into spreusteins of different colours. Nearly
50 different minerals have been found on Låven, and the island is
the type locality for 6 species (aegirine, mosandrite, tritomite-(Ce),
catapleiite, låvenite and leucophanite). Because of its diversity
of minerals and its importance as a type locality, Låven is protected
by law since 1970.
The southern part of Stokkøya
The southern part of Stokkøya consists mainly of basaltic rocks and covered by a huge pegmatite, approximately 45-50 m wide and 100 m long in north - south direction. The pegmatite consists of a platy dike, slightly dipping towards south. The dike is up to 10 m thick at the southern end, but gradually wedging out towards north. From the huge pegmatite there is a large, vertical pegmatite dike up to 8-10 m wide which extends 125 m towards west (Figs. x and x). The huge pegmatite consists mainly of microcline, nepheline, spreustein, aegirine, biotite and magnetite. The minerals occur in relatively large individuals. Accessory minerals like zircon, mosandrite, tritomite-(Ce), leucophanite, fluorite and albite have been found, but the occurrence is much less abundant than at the neighbouring island Låven. A large number of smaller pegmatite dikes and apophysis from the main pegmatites intrude the basaltic rock on the southern part of Stokkøya. In one of these apophysis from the vertical pegmatite dike, chevkinite-(Ce) was described by Segalstad & Larsen (1978b).
Another mineral rich pegmatite dike occurs near
the peak of the highest hill on Stokkøya (see Fig. x). The dike
is horizontal and approximately 25-40 cm thick. The dike has a limited
extension. The pegmatite dike was known already in the mid 19. century
as a locality for a black garnet, and Brøgger (1890) described the
locality and its minerals. The pegmatite dike was specially rich in zircon.
This aspect made the locality interesting as a source for zirconium during
World War Two. Prospecting and blasting at the site was carried out in
autumn 1942 (Larsen 1996).
Skutesundskjær (Skutesund skerry)
Skutesundskjær is in fact two neighbouring
islets, almost connected at low tide (Fig. x). The main syenite pegmatite
dike on Skutesundskjær (N) is approximately 2 m thick. The main mineral
is greyish white microcline which in the middle section of the dike occurs
in tabular individuals up to 10 cm thick. Skutesundskjær (N) has
been the main locality for wöhlerite, which previously was found in
excellent crystals several cm in length. Other accessory minerals are also
quite abundant at the locality. Skutesundskjær is also the type locality
for rosenbuschite. Barkevikite (ferro-edenite) was described by
Brøgger (1890) from the Barkevik area. Although the exact type locality
is unknown, Skutesundskjær was essential in his description of the
mineral. Extensive mineral collecting from the late 19. century until the
1930’s, including heavy blasting, has almost completely destroyed the main
pegmatite dikes on Skutesundskjær. Both islets were protected by
law in 1973.
Saga I quarry
The Saga I larvikite quarry is situated on the south-western side of Strandåsen (also called Sagaåsen) at Mørje, close to Telemark-Vestfold county border at Mørjefjorden (Fig. x). The quarry was opened in 1970 and was finally closed down in 1994. In the sole of the quarry, a huge syenite pegmatite accompanied the quarrying process. The pegmatite was a plate-formed body, varying from 2 m to 4 m in thickness. The longest extension in north-west direction is 60 m, in north-east direction 120 m. Approximately 50 000 t of pegmatite material has been quarried.
The Saga I syenite pegmatite dike is unique in
many respects. The dike was one of the largest pegmatite bodies in the
Oslo Region, and probably never again will a similar pegmatite dike be
almost completely uncovered by blasting. More than 70 different species
have been found in the Saga I pegmatite, approximately 40% of all minerals
found in the syenite pegmatites in the whole Oslo Region. In addition,
the Saga I pegmatite was specially rich in Be-minerals, and 12 different
species have been identified. The pegmatite also showed interesting features
on the hydrothermal alteration of nepheline associated with crystallization
of cancrinite, sodalite, zeolites, Al-hydroxides (böhmite, diaspore
and gibbsite) and low temperature Be-minerals. The pegmatite also became
famous for huge individuals of certain minerals: Biotite in crystals up
to 50-70 kg, leucophanite in cleavable masses to several kg, mosandrite
in coarse crystals up to ¾ m in length, thorite in masses to several
kg, nepheline in masses to several hundreds of kg, böhmite crystals
up to 3 mm in length (Larsen 1981), and extremely rare minerals like behoite
in excellent penetration twins, berborite in crystals up to 10 mm in length
(Giuseppetti et al. 1990), bromellite (Larsen et al. 1987), ephesite and
tadzhikite-(Ce).
The larvikite quarries in Tvedalen
Exploitation of larvikite for ornamental purposes started in 1888 in the Tjølling area. The industry has slowly grown, and represents today 80% of all Norwegian stone export. In the Tvedalen-Tjølling area there are 30 quarries in operation. During quarrying for larvikite, a large number of syenite pegmatites dikes are blasted. Most of the dikes are typically fissure filling dikes with sharp borders against the wall-rock. Typically main minerals are greyish microcline, nepheline, spreustein, ferro-edenite, magnetite and biotite. Most pegmatite dikes, however, are rather scarce in rare minerals. Some quarries have shown to be specially productive regarding the occurrence of interesting pegmatite dikes. Notable localities in the Mørje-Tvedalen-Brunlanes areas are Torbjørnsås quarry, Bjørndalen quarry, Vevja quarry, Thorstein quarry, Treschow quarry (Almenningen), Saga Pearl quarry, Heia quarry and Tuften quarry. A few of them are briefly mentioned here:
Vevja quarry was previously one of the most rewarding larvikite quarry in the Tvedalen area, despite its relatively small size. During several years of operation, mineral rich syenite pegmatites were constantly encountered. The quarry was closed down in 1984. Several pegmatite dikes were rich in vugs containing chiavennite and natrolite. Well crystallised gonnardite have been described from the locality (Mazzi et al. 1986), and the quarry is type locality for tvedalite (Larsen et al. 1992).
Treschow (Almenningen) and Saga Pearl quarries are two neighbouring quarries in the eastern part of the Tvedalen area. The quarries have been the rich localities for Be-minerals: helvite, epididymite and chiavennite (both orange and yellow). Attractive samples of natrolite and greenish yellow fluorapophyllite have also been found at the localities.
Tuften quarry (previously also called Svensken
quarry) is one of the major larvikite quarries in Tvedalen. During the
last 20 years, the quarry has yielded a lot of outstanding finds: Microcline
crystals up to 30 cm in length, hambergite crystal aggregates up to up
20 cm in length together with huge, square prismatic aegirine crystals,
rich amounts of chiavennite, bastnäsite-(Ce) as thick tabular crystals
up to 20 mm across, thorite in irregular masses to 10 cm across, and transparent
natrolite crystals intergrown with fluorapophyllite, and abundant calcio-ancylite-(Ce).
The alleged new minerals UK-7 and UK-10 have been found in Tuften quarry.
Bratthagen
The Bratthagen syenite pegmatite locality is situated
in a roadcut on main road RV-8, 200 m south-east of Bratthagen farm in
Lågendalen, 10 km north highway E-18 (Bommestad bridge). A description
of the locality was given by Sæbø (1965;1966b). The locality
consists of several nepheline syenite pegmatites, usually 10-30 cm thick,
often rather irregularly cross-cutting. Nearly 60 different mineral species
have been found in the dikes, and some of the minerals have been subject
to special studies: Chrysoberyl (Sæbø 1965), barylite (Sæbø
1966b), parakeldyshite (Raade & Mladeck 1977), lorenzenite (Larsen
et al. 1992) and loparite-(Ce) (Lumpkin et al. 1998). Because of its complex
and scientifically interesting mineralogy, the locality has been protected
by law since 1984.
Ula
The syenite pegmatite locality at Ula became
known in the late 1880’s because of its occurrence of beautifully schillerising
feldspar. The locality is situated close to the walkway, 100-150 m south-west
of the harbour at Ula. Brøgger (1890) described this feldspar in
detail. The Ula locality includes several pegmatite dikes, one of which
is the classic "moonstone" locality. The main mineral in the syenite pegmatites
at Ula is a grey cryptoperthite which occurs in individuals up to 20 cm
long and often shows sky-blue to greenish yellow schillerising colours.
Accessory minerals include magnesiohastingsite, magnetite, zircon, pyrochlore
and fluorapatite. The latter may show alteration to bastnäsite. The
Ula pegmatites is a classic locality for schillerising feldspar. The main
locality was protected by law in 1980.
Vøra
The Vøra pegmatite dikes are situated in
the western slope of a steep cliff (Husefjellet) immediately east of the
entrance of Vøra Camping on Vesterøya, approximately 8 km
south-south-east of Sandefjord. The Vøra pegmatites are two very
coarse-grained syenite pegmatites dikes exposed in a quartz/nepheline-free
to slightly undersaturated larvikite. The two neighbouring pegmatites form
steeply dipping, irregular dikes, less than 1 m wide, which can be followed
for a distance of 30-40 m. Apart from greyish to brownish microcline, aegirine
and magnesiokatophorite as the main minerals, the pegmatites host interesting
mineral paragenesis, among others quartz, arfvedsonite, catapleiite, monazite-(Ce),
astrophyllite, eudidymite, epididymite, aenigmatite, catapleiite, elpidite
and polylithionite (Raade & Larsen 1980). The Vøra syenite pegmatites
are rather unique among the pegmatites associated with larvikite, both
mineralogically and geochemically. Therefore, the locality has been protected
by law since 1984.
References
Brøgger, W. C. 1890: Die Mineralien der Syenitpegmatitgänge der südnorwegischen Augit- und Nephelinsyenite. Zeitschrift für Krystallographie, 16, 1-235 + 1-663.
Giuseppetti, G., Mazzi, F., Tadini, C., Larsen, A. O., Åsheim, A. & Raade, G. 1990: Berborite polytypes. Neues Jahrbuch für Mineralogie, Abhandlungen, 162, 101-116.
Larsen, A. O. 1981: Boehmite from syenite pegmatites in the Oslo Region, Norway. Mineralogical Record, 12, 227-230
Larsen, A. O. 1988: Contribution to the mineralogy of Norway, No. 68. Helvite group minerals from syenite pegmatites in the Oslo Region, Norway. Norsk Geologisk Tidsskrift, 68, 119-124.
Larsen, A O. 1996b: Zirkondriften på Stokkøya. Nesjar, 1995-1996, 57-59.
Larsen, A. O., Åsheim, A. & Berge, S. A. 1987: Bromellite from syenite pegmatite, southern Oslo Region, Norway. Canadian Mineralogist, 25, 425-428.
Larsen, A. O., Åsheim, A., Raade, G. & Taftø, J. 1992: Tvedalite, (Ca,Mn)4Be3Si6O17(OH)4·3H2O, a new mineral from syenite pegmatite in the Oslo Region, Norway. American Mineralogist, 77, 438-443.
Larsen, A. O., Raade, G. & Sæbø, P. C. 1992: Lorenzenite from the Bratthagen nepheline syenite pegmatites, Lågendalen, Oslo Region, Norway. Norsk Geologisk Tidsskrift, 72, 381-384.
Mazzi, F., Larsen, A. O., Gottardi, G. & Galli, E. 1986: Gonnardite has the tetrahedral framework of natrolite: experimental proof with a sample from Norway. Neues Jahrbuch für Mineralogie, Monatshefte, 1986, 219-228.
Raade, G. & Larsen, A. O. 1980: Contribution to the mineralogy of Norway, No. 65. Polylithionite from syenite pegmatite at Vøra, Sandefjord, Oslo Region, Norway. Norsk Geologisk Tidsskrift, 60, 117-124.
Raade, G. & Mladeck, M. H. 1977: Parakeldyshite from Norway. Canadian Mineralogist, 15, 102-107.
Sæbø, P. C. 1965: Contribution to the mineralogy of Norway. 1: The first occurrence of the rare mineral barylite, Be2BaSi2O7, in Norway. 2: Note on a new occurrence of chrysoberyl in Norway. 3: The first occurrence of ramsayite, Na2Ti2Si2O9,, in Norway. 38 pp. Unpublished thesis, University of Oslo.
Sæbø, P. C. 1966b: Contribution to the mineralogy of Norway, No. 35. The first occurrence of the rare mineral barylite, Be2BaSi2O7, in Norway. Norsk Geologisk Tidsskrift, 46, 335-348.
Segalstad, T. V. & Larsen, A. O. 1978b: Chevkinite and perrierite from the Oslo Region, Norway. American Mineralogist, 63, 499-505.