Sten – Gävleborg

I am a block of greenstone that comes from the Los cobalt mine in westernmost Hälsingland, in Gävleborg county. Greenstone is a basic - silicon-poor - rock such as gabbro, diabase or basalt that has been slightly metamorphically transformed, but not very strongly; if it is more strongly metamorphosed, it is called amphibolite. During the metamorphism, some greenish minerals are formed, such as chlorite and epidote, which give the rock a dark gray-green color, hence the name greenstone. Compared to the grass below me, you might not think I look very green, but if you look a little more closely, you can probably sense a green tint in my gray surfaces. Then I look quite white on several surfaces, but they are probably fracture surfaces with some alteration or fracture fillings, and not my real color.

From the beginning I was a basic volcanic rock, basalt or perhaps andesite. That was about 1860 million years ago. It is true that there is no dating of me in particular, but the geologists have determined the age of acid volcanite and granite nearby, and they were about 1860 million years old, and I should be about the same age. Slightly younger than the volcanic rocks further south in Bergslagen. Some time later - the geologists do not know exactly when - fractures formed in me due to movements in the bedrock, and in these fractures minerals containing the metals copper and cobalt were deposited.

In the 18th century, cobalt was mined in Los, to produce the dye cobalt blue, which was used to dye fine porcelain blue. Nowadays, the cobalt mine is a tourist mine, which you can visit in the summer and learn more about cobalt mining. Los cobalt mine in westernmost HälsinglandNowadays, cobalt is used in alloys, magnets and batteries, and has become a sought-after metal again, but the deposit in Los is probably too small for it to be profitable to start mining cobalt there again.

Nickel is a metal closely related to cobalt, which also occurs in Los. When they started mining cobalt there, the metal nickel was completely unknown, and it was a Swedish chemist named Cronstedt who discovered nickel for the first time in the ore from Los in 1751. Because of that, there is a monument on the main road near the mine in the village of Los to commemorate that this is the discovery site of nickel. Incidentally, cobalt and nickel got their names from German mining trolls, which the miners, at least in Germany, believed in in the past.

Geologist Åke Johansson
Swedish Museum of Natural History