The Crystal Caves at Grimsel

After Prof. Hans Anton Stalder

large crystals on display within the Crystal Caves Discovery and exposure
The forward section of the crystal cave was discovered on 4th October 1974 during excavation of the access tunnel to the Grimsel II power station, which is operated by the company Kraftwerke Oberhasli AG (KWO).

After careful recovery of the first crystals, it soon became apparent that the cave was considerably more extensive than originally assumed. This was seen as a unique opportunity to place it under state protection as a natural geological monument and a governmental resolution to this effect came into force on 11th December 1974.

Work on exposing the cave began once the KWO tunnel construction had been completed. The first step was to remove the huge slab of rock which was obscuring most of the cave. This slab of crystal, which weighs 875 kilograms, is now located in the administration building of KWO in Innertkirchen. After removal, it became clear that, beyond this, there was yet another cavity filled with crystals. During the winter of 1985/86, the observation gallery was constructed to provide access to this rear section.

The minerals
The crystal cave contains mainly rock crystal (= quartz, silica). Many of the crystals, which are up to 20 centimetres long, are remarkable for their complete transparency. In addition to the normal hexagonal forms, so-called "Gwindel" (rotated quartzes) can also be found.

Chlorite (a magnesium-/iron-aluminosilicate) is the second most commonly occurring mineral. This dark-green scaly mineral completely filled a large part of the rear section of the cave and more than a cubic metre was removed to provide a better view of the underlying rock crystals. Calcite (calcium carbonate) occurs mainly in the rear section, the green chlorite mass being covered more or less everywhere by thin white tabular calcite crystals. In the rear section of the cave there is also a large rhombohedral calcite crystal. Fluorite (calcium fluoride) is loosely scattered over the entire cave. The pale pink octahedral crystals are generally located on top of rock crystals and are overgrown in some locations with white calcite. In a side-fracture, brassy yellow pyrite (iron sulphide) and galena (lead sulphide) are exposed in addition to chlorite and calcite.

A total of 12 different types of mineral have been identified, including adularia (potassium feldspar) epidote (calcium silicate), titanite (titanium silicate), apatite (calcium phosphate), biotite (dark mica) and milarite (a rare beryllium silicate). The latter six are, however, very restricted in their occurrence and are mainly obscured in the altered rock surrounding the cave.

large band of crystals on display within the Crystal CavesThe origin of the crystal cave
As is the case for all the alpine crystal caves of the Aar Massif, this feature was formed around 16 million years ago, towards the close of the alpine orogeny.

At that time, the following conditions prevailed: both small and large fissures (fractures) formed in the rock and were immediately filled with a hot aqueous salt solution. With a temperature in excess of 400°C, the solution dissolved various minerals (mainly quartz and biotite) out of the surrounding rock. Traces of this process can be seen in the form of bleaching halos around the fissures. These processes occurred at least 10 kilometres below what was then ground surface.

The solution later cooled slowly and the minerals precipitated out in the cave in the form of the beautiful crystals we see today. Since then, the entire rock formation has gradually been uplifted, a process which has been balanced by constant erosion. The crystal cave thus arrived at the location where we can admire it today.

Appraisal of the cave
Every alpine mineral cave is unique and represents a special case. This is particularly true of the cave at Gerstenegg. It is large - but not so large that the crystals have been loosened from the rock by earth tremors. Its location also means that it is not subject to the effects of surface weathering. It has a relatively large aperture - most mineral caves are much narrower. Its location in the KWO tunnel means that it is in a controlled environment which can be sealed off. The fact that the cave is protected in this way, and that anyone is free to visit it, is a unique situation.

To arrange a visit to the Crystal Caves see the Visit the GTS page.


The GTS underground facilities are also available to interested 3rd parties for underground testing and research. The GTS offers cost-effective access to a fully developed, well characterised underground research facility with round the year logistical support - please contact Dr. Ingo Blechschmidt, Head of the Grimsel Test Site, for further details.
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