Show Notes
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.
More from History of the Earth
The mineral here is just calcite (even though it’s mostly almost black), but it shows interesting features. Cone-in-cone structures are nested cones, seen here in cross section. The inset shows them a little better – in the main photo, they are represented by very narrow vertical triangles.
Mineral Monday + Tectonic Tuesday. Blue quartz is uncommon and is usually colored by inclusions of unusual minerals like crocidolite, tourmaline, or dumortierite. The purplish-blue quartz here, from north of Llano, Texas, is colored by inclusions of ilmenite (iron-titanium oxide). This rock is called llanite for its occurrence in the Llano Uplift of central Texas, and although similar rocks are found in other parts of the world, the variety name llanite really only applies to this location. On a sunny day, the blue quartz in the rocks has an opalescent sheen that sometimes seems to “wink” at you from the outcrop.
Epitaxy, from Greek words meaning “upon” or “above” and “ordered arrangement,” in minerals means crystals of one (or the same) mineral growing in a particular crystallographic position on another (or the same) mineral. It happens because the molecular spacings and orientations happen to be similar, allowing, even encouraging the crystal structure of the second, later mineral to mesh with that of the first. Some mineralogists might say that epitaxy requires the two minerals to be different minerals, but I do not – just two distinct generations of crystallization.
In the far northwest corner of the flat, flat Texas panhandle, extending into New Mexico, there’s a narrow, elongate magnetic low. The intensity of the anomaly – 250 nanoTesla or more – says it’s fundamentally the expression of a lithologic change rather than a structure; i.e. it represents something pretty strongly magnetic. Its long narrow geometry is that of a dike. And its negative value suggests that it’s reversely polarized, solidifying from magma during a time when the earth’s magnetic field was in the orientation opposite to that today.