Larvikite has for more than a hundred years been appreciated as one of the world's most attractive dimension-stones. last year this beautiful blue-coloured rock was selected as the norwegian national rock, and at present, its production and use is more extensive than ever.
The main reason for the continuous success of this unique Norwegian rock on the world market is the blue iridescence displayed on polished surfaces. These spectacular colours are caused by optical interference in microscopic lamellae within the feldspars.
Through extensive mapping it has been possible to delineate various types of larvikite deposits that are considered to have commercial value in both a short and long term perspective. Reserve estimates for the different types have been calculated, uncovering that some of the most attractive types have rather limited known reserves if the present level of production is maintained or increased.
The name ‘laurvikite’ was first applied by the famous Norwegian geologist and politician Waldemar Brøgger in his descriptions of the southern part of the Carboniferous-Permian Oslo Igneous Province.
The rock name was later changed to Larvikite, having its origin in the small coastal town of Larvik (at that time Laurvik) and situated almost right in the centre of the main plutonic complex of larvikite.
The larvikites form a series of semi-circular intrusions, varying from quartz-bearing monzonites in the east (earliest phases) towards nepheline-bearing monzonites and nepheline syenite ( both monzointe and nephelin syenite are rocks dominated by a special variety of the mineral feldspar) in the west (latest phases). This could result from either multiple caldera collapses or a system of multiple ring intrusions from a deep-seated parental magma chamber. This theory of ring complexes was introduced by the Danish geologist Jon Steen Petersen and is still considered valid.
Actually, the larvikites are the plutonic equivalents to the once overlying massive sequences of romb porphyry lava flows exposed in the surrounding areas. Recent dating of the larvikite confirmed the progressive evolution of the ring intrusions through time, giving a range of ages between 297±1.2 Ma and 292±0.8 Ma (Early Permian).
From a geologist’s point of view, the larvikites are important in understanding the tectonomagmatic processes responsible for the formation of the Oslo Rift. However, most other people see larvikite as a particularly beautiful rock. The reason for this is the abundance of ternary feldspar displaying bluish to silvery play of colours or iridescence. This aesthetic uniqueness has caused larvikite to become one of the world’s most used and appreciated dimension-stones, cladding hundreds of prestigious buildings and thousands of kitchen tops world-wide, and lately also the approval of larvikite as the ‘National rock’ of Norway.
Production started already in the 1880’s, and at present, the export value of rough blocks of dimension-stone from the Larvik region is 500 million Norwegian kroner (60 million Euro), distributed on approximately 30 individual quarries. Different types of larvikite have different market value, and the customers can choose between a range of types and qualities under attractive trade names such as ‘Blue Pearl’, ‘Emerald Pearl’, «Royal Blue» and ‘Marina Pearl’.
The first record of larvikite as a source for dimension-stone is a letter dated October 1811, from a certain Ohlsen to ‘Headmaster Floor’, where the former suggests extraction of larvikite, among other purposes, for a castle in Copenhagen. However, nothing happened until 1884, when quarrying was initiated by Ferdinand Narvesen close to the town of Stavern, southwest of Larvik. A few years later, Theodor Kjerulf, then director of the Geological Survey of Norway, took part in the further development of the industry by recommending the deposits of dark blue larvikite southeast of Larvik. The geologist John Oxaal did the first survey of the quarried larvikites and described the quarrying activity in the area. Reid Kvien, a geological consultant in Larvik, was the first to apply Jon Steen Petersen’s model in his interpretation of commercial larvikite types and in compiling the first resource map for the municipality. In recent years, NGU has carried out surveys of the larvikite deposits.
Quarrying larvikite- aspects of industrial quality
Larvikite is extracted in rectangular blocks aimed for export markets. To obtain the highest market value, the blocks must be large (preferably larger than 4 m3) and homogeneous. If the blocks contain flaws, veins, discolouration or other features reducing the uniformity of colour and quality, the market price for the blocks is reduced significantly, if they can be sold at all.
The average block yield in the larvikite quarries is close to 10%, which means that nine out of ten cubic metres of extracted rock are not utilised as dimension-stone. However, the leftover rocks are to an increasing extent used for other purposes. Large «waste» blocks are shipped to the UK and other North European countries to be used for coastal protection, and some of it is crushed to rock aggregate. The rest is deposited in designated landfill areas close to the quarries.
Attractive and uniform colour is the most important issue besides the block size. In general, larvikites displaying strong blue play of colours (iridescence) are more attractive and valued than the ones with weaker blue and/or silvery colour-play. The variation in colour essentially follows the ring-pattern identified by Petersen.
The spacing of joints and fractures determines the block size, and is thus of great importance. Such features are controlled by the physical properties of the larvikite and structures such as fault systems. In parts of the area, ductile shear zones in the larvikite similarly reduce the block potential. Moreover, modal layering and planar alignment of the feldspars («cleavage and planes showing iridescence) are important for establishing quarrying and sawing directions of the rock.
In addition to the iridescence, the larvikite subtypes vary in overall ‘background’ colour, from dark grey (almost black) to light grey, and there are also systematic variations in maximum grain size (mostly from 1 to 6 cm) and grain-size distributions. Variations may also be seen in the degree of alignment of the feldspar crystals and modal layering.
The future well-being of the larvikite industry depends on several factors. Most importantly is the international market. In particular, it is crucial to maintain a high price level to compensate for high production costs. This depends on the competition between the production companies and the aesthetic quality of the commercial blocks.
The second issue relates to production techniques and strategies, efficient quarrying technology, waste handling and use (environmental aspects) and resource management (legal aspects). It is therefore vital for the industry to continue to invest in state-of-the-art technology for improving block yield and reducing costs, and simultaneously meet more and more strict demands for environment-friendly development.
The area around Larvik is experiencing rapid population growth and expansion of industry, tourism and infrastructure. Furthermore, since basically the whole area consist of larvikite bedrock, it is impossible to preserve all larvikite with iridescence for future exploitation without getting into conflict with other land use in the region. This conflict between resources and infrastructure has caused a need for delineating the most important deposits in the area that can realistically be preserved for the future. By combining the field observations we have of larvikite types, colour, fracturing, alteration and morphology, we have delineated what we believe are the most important resources. Such estimates contribute to a generally raised awareness on the importance of the larvikite resources and the use of area. Additionally the survey underlines the responsibility for building a good management regime for a national resource of global significance.
Unique larvikite types
The Klåstad subtype (also called ‘Emerald Pearl’) is one of the most attractive larvikite varieties, quarried since the 1880’s. It is one of the darkest varieties of larvikite, and iridescence varies from dark blue to silver/bronze, of which the former variety is by far the most valuable. There are large, potential reserves of this subtype beneath the farmland between Larvik and Sandefjord, but so far we have little information about the quality. Minor occurrences, although hardly of commercial value, are found west of Larvik.
Several thin zones of dark and relatively fine-grained (most grains smaller than 1 cm) larvikite are found along the eastern margin of the Klåstad subtype. These are collectively named the Bergan type. Blue iridescence can be seen in the feldspars, but it is not the most distinct feature of the rock. Thus, this subtype is the only larvikite in production which predominantly is applied for non-polished workings, such as paving slabs and other outdoor uses.
The Stålaker subtype is light-coloured with bluish iridescense, and has generally a higher content of mafic minerals than other light-coloured subtypes. There are a few large quarries in this subtype, marketed under the commercial name «Marina Pearl».
In addition to the Klåstad subtype, the light-coloured Tvedalen subtype («Blue Pearl»), displaying strong, blue iridescence, is the best known and at present, the larvikite sybtype with the highest production rates. Although this subtype can be followed laterally for almost 20 km, only the present quarry areas in Tvedalen and at Auen have proven to contain homogeneous quality in sufficiently large volumes to secure long-term sustainable production.
The Malerød subtype (commercial name «Royal Blue») was introduced commercially as late as in the 1970’s and the area has subsequent become an important production site. It is coarser grained than other light-coloured larvikite types and displays bright blue iridescence. The subtype can be followed both to the east and west of the present production area, and even east of Farrisvann.
Written by Gurli Meyer, Idunn Kjølle, Sven Dahlgren, Tom Heldal