Sten – Jönköping

I am a block of dark gabbro from Taberg, or Småland's Taberg as this mountain is sometimes said, south of Jönköping in Jönköping County in Småland. If you want to be really precise, you can call me titanomagnetite-olivinite, because I consist mostly of the minerals titanomagnetite and olivine, but I think gabbro can do as a name for everyday life.

Gabbro is an igneous rock, just like granite, but gabbro is formed from a basic magma that comes from the mantle, which is found below the earth's crust, and which is much poorer in the element silicon. Therefore, gabbro contains very little (or nothing) of light-coloured silicon-rich minerals such as quartz, and a lot of dark silicon-poor minerals such as pyroxenes and olivine, which are instead rich in iron and magnesium. That's why we gabbros are so dark in colour.

Gabbro, diabase and basalt are closely related rocks with the same composition. Gabbro forms slightly larger bodies that have solidified deep down in the earth's crust, diabase solidifies in cracks in the crust as (usually) steep dykes, and basalt is a very fine-grained lava rock formed by volcanic eruptions on the surface of the earth. The gabbro at Småland's Taberg, where I come from, is particularly closely related to the diabase swarm that runs in a north-south direction from Skåne up through Småland to Lake Vättern. The Tabergs gabbro lies in the same shear zone in the bedrock, the so-called Protogine zone, as these diabase dykes, and we are roughly the same age. A couple of the diabase dykes have been dated to about 1220 million years, and I myself have been dated to about 1200 million years. So I'm happy to see the diabase from Gylsboda in Skåne here in the same stone circle, we are old mates.

But I'm a little extra special, because I'm not only made up of gabbro through and through, but various different rock varieties with different tricky names, like titanomagnetite-olivinite These have separated from my magma when different minerals accumulated in different places when the magma crystallized. Titanomagnetite means titanium-containing magnetite, i.e. a variant of the iron ore mineral magnetite which, in addition to iron (and oxygen), also contains the metal titanium. There is quite a lot of iron and titanium ore in Småland's Taberg, and for several hundred years iron ore was mined in the mountain, the last period from 1939 to 1960. However, I don't think any titanium was extracted. Now the mine is closed, and bats live in the old mine tunnels. The mountain has become a nature reserve. In the summers, tours are arranged in the mine tunnels, to look at the historical traces left by the mining. Welcome to Taberg

A special finesse is also that in 1830 a new unknown element, a metal named vanadium, was discovered in the iron-titanium ore from Taberg. Like titanium, vanadium is an important alloy metal that can be mixed with iron to give steel various suitable properties.

Småland's Taberg is also the second highest mountain peak in Småland and the entire Southern Swedish Highlands, with a height of 343 meters above sea level. It is steep, especially on the south-east side where the rock was carved to mine iron ore, and rises high above the surroundings, so the view from the top is very nice. If you don't want to walk, you can drive up a narrow and winding road. Up there at the top there is a restaurant or cafeteria, at least in the summer.

Geologist Åke Johansson
Swedish Museum of Natural History