Artinite and the Coast Ranges of California

The mineral is artinite, a magnesium carbonate that is an alteration product of high-magnesium serpentinites of the Diablo Range in central California. It’s from New Idria, a mercury (and other) mining district named for the historic Idria mercury mines now in Slovenia, which have been mined since 1490.

Tectonically, the New Idria area of California is in a fenster (German for window), a hole in a thrust sheet that is related to the collisions that produced the California Coast Ranges. Serpentine’s mechanical properties are such that it can flow – very slowly on human scales – and in places like Mt. Diablo and New Idria, it has risen buoyantly like a salt dome to breach the thrust sheet and bring the underlying serpentine to the surface.

The magnesium-rich serpentinite is part of the Coast Range Ophiolite – a word meaning “snake rock” for its often greenish color and scaly texture. Ophiolites are usually bits of oceanic crust that have been emplaced within or upon continental crust, and that’s what happened here.