In 1903 a company was founded, and contracts for the exploitation of the deposits were signed. In 1905 a processing plant was finished, though the necessary machinery hand't arrived yet. The next 60 years magnesite was mined on a more or less contineously basis from a number of deposits.

The magnesite was used for the production of heat resistant tiles and for special beton. Typically, the magnesite was supposed to contain up to 30% serpentine in order to get an optimal quality of the tiles. Production fall back sharply afyer the Second World War, and in 1963 the very last load of raw magnesite was sold.

In the Modum area occur a number of different rock-types: amphibolites, gabbros, banded gneisses, quartzites, sillimanite-granites, micaschichts, rocks containing either scapolite or albite and pegmatites.

The magnesite/serpentine bodies are mainly found in an anthophyllite-containing quartzite, but also amphibolite, an albitised breccia and sillimanite-containing quartzite are also observed host rock for the the magnesite/serpentine bodies.

The magnesite/serpentine deposits are understood to be a magmatic carbonate rock, and as such they must be seen as dunite pegmatites (Schetelig, 1925).

About 20 magnesite/serpentine bodies are known to occur in the Modum. All rhe deposits are rather small. The smallest have a thickness of 2-3 meter, while the largest measure are about 18-20 meters. The maximum observed lenght of the deposits is 100 meters, but most are under 30-40 meters in length.

DYPINGITE is magnesium-carbonate, described as a new species in 1971 by Gunnar Raade from the Dypingdal deposit. Dypingite is closely related to hydromagnesite, and is at 150 oC altered to this mineral. Dypingite is found at the surface of thr pits, and is considered to be a post-mining mineral formed at low temperatures.

HEMATITE is found in many of the deposits. The best crystals are found in the Dypingdal quarry, where they occur as rhombohedral to pseudo-octahedral crystals embedded in serpentine. The crystals have a good metallic luster and are relatively easily prepared outof the soft serpentine, resulting in attractive display specimens. The largest known crystals were probably around 15 cm in diameter.

HENEUITE was described in 1986 as a new species from the Tingelstadtjern deposit by Raade. Heneuite occurs as pale blue-green, nodular masses measuring several cm accross, usually surrounded by a rim of althausite whivh, in turn, is surrounded by apatite. Heneuite contains microscopic blue inclusions of yhe phosphate analogue of ellenberite. Heneuite has good cleavage along (010).

HOLTEDAHLITE was described as a new species in 1979 from the Tingelstadtjern deposit by Raade and Mladeck. Holtedahlite occurs exclusively as a colorless massive mineral, associated with althausite and apatite. Holtedahlite is not easily disguishable from apatite.

HYDROTALCITE was described as a new species in 1842 by Hochstetter. Hydrotalcite is a common alteration product of serpentine. However unesthetic in itself, in combination with the nice green serpentine and black metallic hematite crystals, as found in Dypingdal, it still has a certain quality.

LEUCHTENBERGITE is a commonly occurring clinochlore variety in the magnesite/serpentine deposits.

HYDROMAGNESITE has been found as a thin, white covering on serpentine in the Tingelstadtjern deposit.

MAGNESITE occurs as colorless, white, pale yellow or pale pink fine to coarse crystalline. Cleavage planes in coarse crystalline magnesite can be up to 20-30 cm.

MANASSEITE was described simulteneously from Snarum and Amity, new York as a new species in 1941 by Frondel. Manasseite is frequently found intergrown with hydrotalcite at the Dypingdal deposit, and are virtually not distinguishable from each other. The color is white to bluish-white.

SZAIBELYITE has been found at both the Nedre and Øvre Dypingdal deposits, and at the Tingelstadtjern deposit. At the Nedre Dypingdal deposit szaibelyite has been found as veins in magnesite. At the Tingelstadtjern deposit szaibelyite has been found as thin veins in althausite.

SERPENTINE occurs as antigorite, lizardite and chrysotile in the deposits. It has not been specified which type(s) of chrysotile has been found: ortho-, para- or clinochrysotile. Antigorite is by far the most abundantly occurring type, but chrysotile is common. To what extent lizardite occurs has not been described. The serpentine can be found as yellow to green masses of considerable size, or as more fine grained mixtures with magnesite. The purity of the serpentine varies considerably, from quarry to quarry, but also in one and the same deposit. Remarkable is the occurring of serpentine as pseudomorphoses after forsterite (olivin) crystals. In the Dypingdal deposit are altered crystals up to 10 cm found, often with a core of unaltered forsterite. In the Øvre Langerudsgruve are pseudomorphs found as 20 cm long needles with a diameter of 1-2 cm. These needles are randomly orientated in layers parallel to the layering of magnesite and serpentine.

FORSTERITE CHRYSOTILE-ASBEST

ARAGONITE, BRUCITE, CALCITE, DOLOMITE, ILMENITE, MAGNESIOFERRITE, MAGNETITE, MUSCOVITE, PYRITE, SPINEL, TALCUM,

For novice rockhounds and cutters the quarries and dumps represent an enormous stockpile of serpentine and magnesite for many future generations.

The deposits are all situated on private land, and permission to collect must be obtained from the respective landowner. In practice this is basically a formality, because the landowners have been so far very positive to small-scale hobby collecting. Dealers are definitely obliged to contact the respective landowner and will be asked to pay a small fee per kilo.