Minerals of the banalsite-stronalsite series in
amygdules from the Brunlanes ultramafic
volcanic series
Sven Dahlgren & Alf Olav Larsen
Introduction
Banalsite was first described from the Benallt manganese mine, L1eyn Peninsula, Wales by
Smith et al. (1944), while stronalsite was described as an alteration mineral in veins cutting
mafic metatuff xenoliths at Rendai, Kochi, Japan (Hori et al. 1987). Their general chemical
formula is ANa2A14Si40,6, where A is Ba and Sr, respectively. The study by Liferovich et al.
(2006b) has confirmed that there is a complete solid-solution between banalsite andstronalsite. A third tectosilicate closely related to the above mentioned species, lisetite,
CaNa2A14Si40'6' has been described from the Liset eclogite pod, Selje, Norway (Smith et al.
1986). Liferovich et al. (2006b), however, has shown that lisetite has limited miscibility withbanalsite-stronalsite. The crystal structures of stronalsite and banalsite have been
determined by Liferovich et al. (2006a).
Due to its inconspicuous appearance banal site and stronalsite have hitherto been found at
only about 15 localities worldwide, typically from alkaline ultramafic rocks, rodingite, jadeite
and from metasomatic rocks enriched in Fe-Mn oxides and carbonates, and in most of thelocalities banalsite and stronalsite are only minor constituents. For an overview, see
www.mindat.org/min-504.html and www.mindat.org/min-3804.html. However, more localities
will most probably be discovered in the future.
Certain lava flows of the Brunlanes ultramafic volcanics are rich in white, porcelain-likeamygdules, which have received a minimum of attention by previous geoscientists. During an
investigation on the mineralogy of these amygdules it became clear that minerals of thebanalsite-stronalsite solid solution series are an important constituent. We hereby report thewidespread occurrence of banalsite-stronalsite in amygdules in the Brunlanes ultramaficvolcanics, and the first record of these minerals in basaltic rocks. In Norway tiny amounts ofbanalsite and stronalsite have previously been reported from the Mikkelvik alkaline stock,
RingvasslllY, Troms by Bille (2000) and Zozulya et al. (2009), respectively.
The Brunlanes ultramafic volcanicsThe Brunlanes ultramafic volcanic series is situated in the extreme southwestern most part ofthe onshore continental Carboniferous-Permian Oslo Rift. The Brunlanes volcanics belong to
the basaltic family, but they more concisely represent a series consisting of olivine melilitites,melilitites, nephelinites and basanites, i.e. they are very undersaturated, ultramafic, alkaline
magmatic rocks (Dahlgren unpubl.).
Most of the volcanics are lava flows, represented by both aa and pahoehoe lavas, but
subordinate pyroclastic deposits also occur. The phenocrystal phases include abundantclinopyroxene, magnesian olivine, apatite, Ti-magnetite and perovskite. Formerly it wasbelieved that the Skien nephelinite lava series, situated north of Brunlanes, represented theoldest volcanism in the Oslo rift (Segalstad 1979, Anthony et al. 1989). New data, however,demonstrates that the Brunlanes volcanics are slightly older. The lower part of the Brunlanes
volcanic series yield Late Carboniferous U-Pb perovskite ages of 300.2 ± 0.9 Ma (pyroclasticsurge) and 300.4 ± 0.7 Ma (olivine melilitite aa-flow) respectively, whereas perovskite from a93

,pyroclastic unit underlying the oldest lavas at Skien yield a slightly younger age of 298.9 ±0.7 Ma (Corfu and Dahlgren 2008).
The bulk part of the Brunlanes ultramafic volcanics are found in the cliffs of Saltstein east of
M01en and the adjacent peninsula of Oddane. The volcanic series strikes north-south anddips 40-50· east. The base of the volcanic series, in the west towards M0len, is not exposeddue to cover by the extensive Younger Dryas "Ra" moraine. Late Silurian sandstones areexposed at Fugl0ya, situated immediately west of M0len, but it is not yet known whether thevolcanic series rest unconformably on these Silurian strata or, as in the Skien district, on Late
Carboniferous sediments (Dahlgren & Corfu 2001).
To the east of Saltstein and north of Oddane the volcanic series has been transected by later
nepheline syenite and larvikite plutons. Consequently the volcanics have been contact
metamorphosed. The metamorphism is severe near the pluton boundary, but already a few
tens of meters away from the boundary the volcanics have been reasonably well preserved.
Fresh melilite has never been observed (only pseudomorphs are seen in thin section fromsome flows), but reasonably well preserved olivine and fresh perovskite is abundant.
Clinopyroxene appears unaltered throughout the sequence except when within a few metersfrom the syenite contact. Mapping of the entire volcanic series in scale 1: 100, detailed logs ofcontinuous stratigraphic profiles, petrography and geochemistry shows that there is nostratigraphic repetition due to faulting within the series. The total thickness of the volcanics isat least 800 meters (Dahlgren unpubl.).
The volcanics are ultramafic and silica undersaturated. That means high MgO (typically 1019 %) and low Si0 2 (typically 32-40 %). This is characteristic for magmas that have been
generated by limited partial melting deep within the upper mantle, probably at least at 100 km
depth. The Brunlanes volcanics are also Ca-rich (12-18 % CaO), Na-rich (0.5 to 3 % Na20)and characterized by high abundances of many trace elements, for example: Strontium
typically in the range 1000-2300 ppm, and barium typically in the range 500-1500 ppm(Dahlgren unpubl. whole rock analyses).

Amygdules
Many volcanic rocks are highly porous. Their porosity depends on several factors related totheir chemical composition, their content of volatiles (H 20; CO 2 etc.) on the time of eruption,
the style of eruption etc. The Brunlanes pyroclastic rocks were deposited as ignimbrites andsurges, i.e. the magmas became highly fragmented during explosive volcanism. Thesedeposits consist of lithic clasts (fragments of older lavas and fragments of already solidified
rocks from the magma chamber) and phenocrysts embedded in a matrix of glass fragments
and microlites. In the pyroclastic rocks secondary minerals are typically are found in post
emplacement vertical degassing conduits and as tiny grains in the altered glassy matrix.
Lavas generally erupt from a vent and flows more or less violently out onto the surface.
Frequently considerable amounts of volatiles are being kept within the melt during theemission of basaltic lavas, but as the lavas coolon the surface, the volatiles expands, due todecompression, and form vesicles. In the Brunlanes lavas vesicles are abundant in certain
parts of both the aa- and pahoehoe lavas. Typically these vesicles, and in the case of aalavas, also the cavities interstitial between the fragments in the upper and lower flow
breccias, have been secondarily filled with white minerals. Thus the original gas vesicles now
stands out as white spots and masses, as amygdules (i.e. vesicles now filed with secondary
minerals), within an otherwise almost black rock background.
The amygdules in the aa-Iavas typically vary from subrounded, centimeter-sized to irregular,amoeboid, up to several centimeters large (Fig 1). Commonly the cores of the aa-Iava flow

94

Fig. 1. Amygdules in aa-Iava from Saltstein, Brunlanes. The small, spherical white
amygdules are about 1 cm across. Typical for the aa-favas are the larger, amoeboid
amygdules and the mineralized tensional cracks (lower right). Photo: S. Dahlgren.

units contain few amygdules, but frequently tensional cracks that formed during the flow ofrelatively viscous lavas crosscuts the dense cores. These cracks are also filled with whiteminerals. The pahoehoe lavas are especially rich in amygdules, and may contain amygdules
across the entire thickness of a lava flow. The size, shape and distribution of the amygdules
vary different flows and may even vary considerably from bottom to top within single flow
units (Fig 2). Some characteristic features are:Spongy pahoehoe. These typically occur in the uppermost decimeters of some flows, andthe volume between of amygdules may by far exceed (up to 70%) the volume of rock matrix.
Generally these amygdules are small, from 0.1 to 3.0 mm in diameter, and are almost
spherical (Fig 3).
Pipe vesicles. These features typically are fingerlike, vertical vesicles that formed near thebase of the cooling lavas. They may be up to 15 cm long and 2 cm thick.
Horizontal, irregular vesicle "trains". These are conspicuously concentrated along certain
horizons within single lava flows. They were formed during prolonged flow, in pulses, andaccompanying inflation during the emplacement of these lavas. These trains of amygdules
thus represent vesicles formed repeatedly near the roof of increasingly growing roofs on
increasingly shrinking lava tubes and they are frequently elongated. Sometimes these
original cavities represent gas blisters of considerable size (0.5 cm long and 10 cm thick; Fig.
4).
The minerals in the amygdules are very fine-grained. In thin sections under the polarization
microscope it is difficult to distinguish the various mineral species because all possess
shades of gray and their textures are complex and not quickly interpreted. Evidently not allminerals are in equilibrium, and clearly some minerals form at the expense of others. The95

Fig. 2. Cross-section of an inflated pahoehoe lava from Saltstein, Brunlanes. Black rock inthe upper part of photo represents the upper crust of the flow (about 0.5 m thick). The zone
with amygdule trains parallel to the upper crust represent vesicles formed in a lava tube
where lava pulses accreted lava en vesicles to the lave tube roof. Big white areas in theforeground represent mineralized gas cavities (the biggest one is 20 cm long). Photo: S.
Dahlgren.
detailed sequence of mineral formation, reaction relationships etc. has, however, not yet
been established. In some of the larger amygdules fibrous zeolites may be visible by thenaked eye. Otherwise, XRD analysis is the most practical way of identifying the minerals.
This technique was the mean that lead to the discovery of the banalsite-stronalsite series
minerals in the Brunlanes basalts.
XRD analyses on amygdules from basalts at Saltstein and Oddane on Brunlanes haveshown that albite and analcime are the most common and ever-present minerals. In addition,minerals of the banalsite-stronalsite series, thomsonite, natrolite, sodalite, cancrinite andnepheline have been identified, but the amount of the accessory minerals in the different
amygdules may vary from being prominent to zero. A few very tiny grains (only a few ~m) of96

Fig. 3. Spongy pahoehoe lava, Saltstein. The amygdules constitute more than 50 % of thevolume, and each of the amygdules are about 2-3 mm in diameter. Photo: S. Dahlgren.

Fig. 4. Close-up of large mineralized gas cavity in a pahoehoe lava from Saltstein. 10 kr coinfor scale. Photo: S. Dahlgren.

97

is contact to the syenite intrusion the amygdules may contain grossularite, albite and epidote,
and this assemblage evidently a contact metamorphic one formed from already existingmineralized vesicles.

Minerals of the banalsite-stronalsite series
Several amygdules from the basalts at Saltstein near M01en were analyzed on their bulk
chemical composition using semiquantitative XRF analyses. The total amount of Ba and Sr
were calculated into percent banalsite-stronalsite, assuming negligible substitution of Ba andSr in the other minerals present. Although barite has been observed in SEM our impressionis that this is a very minor phase, but it may in some instances bias our calculations. The
result shows that the amount of banalsite-stronalsite may be up to 15 % of the total mineralcontent of the amygdules. Most amygdules, however, have a rather low content of banalsitestronalsite, probably down to zero. The minerals of the banalsite-stronalsite series occur as
irregular grains up to 100 IJm across, often concentrated in certain parts of the amygdules.
Semiquantitative chemical analysis (SEM/EDS) show that the composition, in mol-%banalsite (Bns) and stronalsite (Sns), varies from approximately 55%Bns/45%Sns (i.e.
banalsite) to 40%Bns/60%Sns (i. e. stronalsite). Our study indicates that stronalsite seems to
be more common than banalsite.

Fig. 5. Back-scatter SEM image showing minerals of the bana/site-stronalsite series (white)in analcime and sodalite (dark grey, black). Photo: A.a. Larsen.

Origin of the amygdule minerals
The minerals in the amygdules, as well as within the aa-flow tops and cracks are similar andthus require a common mode of formation. The mineral association albite + analcime ±nepheline ± natrolite ± sodalite ± cancrinite ± calcite ± banalsite ± stronalsite ± barite ±thomsonite is clearly very rich in Na, and relatively speaking also Ba and Sr. These
components must also have been present in the fluids that precipitated the amygdule
minerals.

98The great original volume of vesicles makes pahoehoe lavas very porous and permeable
rocks. Aa-Iavas may be more dense, but their upper and lower flow-breccia zones obviously
also are very porous and permeable. The Brunlanes volcanics series most likely was prone
to post eruption infiltration of H20-C0 2 fluids. These fluids may have originated as meteoric
water which percolated the volcanic pile, leached the glassy lavas and became especiallyenriched in Na, Sr and Ba. Subsequently these solutions precipitated albite, nepheline,calcite, analcime, stronalsite and banalsite, as well as a few other minerals, within thevesicles, to form the amygdules, as well as on cracks and in flow-top breccias. Later theseamygdules were subjected to contact metamorphism near the ascending syenite intrusionand grossularite, epidote, albite assemblages formed.

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Referanse til denne artikke/:
Dahlgren, S. & Larsen, A.D. (2012): Minerals of the banalsite-stronalsite series in amygdules
from the Brunlanes ultramafic volcanic series. Norwegian Mining Museum, Publication No. 49,93·100.
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