Metamorphic raw materials
In Norway most of the rocks are metamorphic. Igneous and sedimentary rocks are rare compared to the metamorphic rocks which dominate the geological landscape. A rock is constant only within a limited pressure and temperature. If these limits change, the rock itself will tranform to become stable again. By dislocation and faults rocks are exposed to enormous pressure and temperatures, and their mineral composition, structure and chemical compounds will change. This is how metamorphic rocks form.
Both igneous and sedimentary rocks can be transformed into metamorphic rocks. Sandstone and mudstone can be transformed to the metamorphic rocks quartzite, hornfels and slate. The igneous rock basalt can be transformed to greenstone. As mentioned earlier the metamorphic rocks are abundant in Norway. This is especially true of gneiss which is the signature rock of Norway. It is easier to say where gneiss is missing than where you can find it (Garmo 1983; Sigmond 1996:16-21).
The Earth´s crust as well as being intruded by magma, is from time to time subjected to stresses generated within the crust and mantle which are sufficiently great to cause it to break to form faults, and also to bend forming folds. These forces are often concentrated along relatively narrow, sinuous belts when the folding, usually combined with intrusion and extrusion of magma, gives rise to mountain chains. The rocks within a mountain chain not only sustain considerable pressures but are also heated both generally and by the large scale of intrusion of magma, with the effect that rocks are deformed and recrystallized to varying degrees. Such rocks are called metamorphic rocks. (Hamilton et al 1976, 148)
Quartzite is metamorphic quartz rich sandstone. Several different variants of quartzites exist. The high content of quartz is common for all types of quartzite. They can contain as much as 96-99% quartz in addition to iron oxide and feltspar.
- Appearance: The appearance vary a lot between different types of quartzites. They exist in many color and grain sizes. Some quartzites can be layered and tabular in shape.
- Fracture: Most variants of quartzites, both fine grained and coarse grained have a conchoidal fracture. One exception is the tabular types that fracture along these layers.
- Grain size: From fine grained to coarse grained
- Properties: The fine grained variants are homogenous, brittle and elastic while the coarse grained are robust. Quartzites do not have the same sharp edges as flint and quartz, but some are quite sharp. Quartzites are common.
- Tools: All kinds of tools
- Source: All over the country.
Colour: White, grey, reddish.
Texture: Medium-grained; usually of a granoblastic texture.
Structure: Usually massive but primary sedimentary features may be preserved, such as bedding, graded bedding or current bedding.
Mineralogy: Essentially composed of tightly interlocking grains of quartz. A little feldspar or mica may also be evident. White varieties are distinguished from marble by their greater hardness.
Field relations: Quartzites are metamorphosed quartz sandstones and are found in association with other metamorphosed sedimentary rocks such as phyllite, schist and marble." (Hamilton et al 1976, 188)
Slate is a layered rock which makes it possible to split it into parallell squares.
- Appearance: Slate can be black with shades of blue, green, brown or brownish yellow. The layering can often be seen with the naked eye.
- Fracture: Fracture along the layers
- Grain size: Fine grained
- Properties: Homogenous, brittle, available
- Tools: Projectiles, knives, amulets
- Source: Common
Colour: Black and shades of blue, green, brown and buff.
Structure: By definition, slates are characterized by a single, perfect cleavage (slaty cleavage), enabling it to be split into parallel-sided slabs. On the cleavage surfaces sedimentary structures such as bedding and graded bedding can often be seen. Fossils may be preserved but are invariably distorted. Folds are often apparent in the field.
Mineralogy: Too fine-grained to distinguish with the naked eye. Pyrite porphyroblasts often occur, usually as cubes.
Field relations: Slates are produced by low-grade regional metamorphism of pelithic sediments (shales, mudstones) or fine-grained tuffs. They may be associated with other metamorphic sedimentary or volcanic rocks." (Hamilton et al 1976, 150)
Hornfels is metamorphic mudstone.
- Appearance: Hornfels can be both light and dark with different color and bondings, most often in green or brown. The rock is massive and fine grained to such a degree that it is difficult to decide the mineral content even with a microscope.
- Fracture: Conchoidal fracture
- Grain size: Fine grained
- Properties: Homogenous, brittle, robust
- Tools: Is suitable for axe production
- Source: Hornfels is characteristic of Oslofeltet and is not commen in other parts of the country.
The main mineral in soapstone is chalk which is a very soft mineral. The rock is thus very soft and easy to form. Soapstone is common along fault zones where you can find in magnezium rich rocks.
- Appearance: Untreated soapstone is grey and seems dusty or greasy in texture.
- Fracture: Undetermined fracture
- Grain size: Medium grained
- Properties: The soft texture makes the soapstone perfect for cutting objects, for example figurines. However the texture makes it impossible to make tools by percussion.
- Tools: In the Stone Age – figurines, net sinkers. In other prehistoric periods – cooking vessels.
- Source: Soapstones are found in all counties in Norway except Vestfold and Vest-Agder.
Soapstone is massive Talc which occurs as a secondary mineral formed as a result of the alteration of olivine, pyroxene, and amphibole.
It occurs along faults in magnesium-rich rocks. It occurs less frequently as a result of thermal metamorphism of dolomitic limestone s.
It's distinguishing features are extreme softness, soapy feel, and greenish white colour. Because of its softness it has often been used for carving into artifacts such as figurines.
Schist is a coarse grained rock with a marked layering, defined by platy or elongated minerals, often finely interleaved with quartz and feldspar.
(Hamilton et al 1976, 150)
Colour: Grey or pink but with dark streaks and layers.
Texture: Medium- to coarse grained. Characterized by discontinuous, altering light and dark layers, the former usually having a coarsely granular texture while the latter, which often contains mica, may be foliated.
Structure: In addition to the gneissose texture described above, gneisses tend to be banded on a large scale with layers and streaks of darker and lighter coloured gneiss. Granite and quartz veins and pegmatites are common. May be folded.
Mineralogy: Feldspar is abundant and, together with quartz, forms the granular, lighter coloured layers. Muscovite, biotite and hornblende are commonly present, while any of the minerals characteristic of higher grades of regional metamorphism may occur.
Field relations: At the highest grades of metamorphism rocks may approach melting temperature when they are able to recrystallize freely and so produce the textures characteristic of gneisses. Thus gneisses occur, in association with migmatites and granites, in the central parts of metamorphic belts. (Hamilton et al 1976, 190).