Doubly terminated

Suttrop-type quartz

I’ve had this 26-mm doubly terminated milky quartz crystal since 1971, when I inherited it from my mineralogy professor, Carl Beck. Much of his material was unlabeled, which usually means you can never be absolutely sure where something comes from. But occasionally, things are so distinctive you can be perhaps 99% certain.

But you have to see other examples – which I never did, for 51 years. But now I’m as sure as I can be that this is an example of “Suttrop-type quartz,” named for classic occurrences near Suttrop in Sauerland, in North Rhine-Westphalia, Germany. It’s probably from the most famous locality, the Auf dem Stein quarry, but I’ll refrain from insisting on that. There’s another nearby locality at Kallenhardt that’s also well known for crystals like this, and in fact similar crystals come from an outcrop belt around 150 km long.

It’s just milky quartz, albeit with an excellent symmetry. The crystals probably grew in a hydrothermal system associated with late Carboniferous basaltic intrusive activity. The siliceous fluids emanating from the intrusions permeated late Devonian and early Carboniferous limestones, partially dissolving them and allowing the quartz crystals to grow in veins and pockets. The all-around lack of contact points is perhaps explained by the crystals growing in a soft marl (impure limestone) vein filling (for more information see Amir C. Akhavan, The Quartz Page).

The milkiness seems to be related to original inclusions of fluids, gas bubbles, and small bits of anhydrite (calcium sulfate) in the crystals. This began at relatively high temperatures; the anhydrite dissolved as the temperature fell to around 60-80°C (anhydrite is more soluble at lower temperatures) contributing to the volume of void spaces. It’s reported that fluid inclusions can amount to 10% or more of the crystals, enough to make for a specific gravity (a measure of the density of a substance in comparison to the density of water) that is measurably lower than that of pure quartz (Behr and others, 1979, Die Quarzmineralisation vom Typ Suttrop am N-Rand des rechtsrheinischen Schiefergebirges: in Meiburg (ed.), Geologie und Mineralogie des Warsteiner Raumes: Der Aufschluss, Sonderband 29, Heidelberg).

Quartz is actually trigonal in its crystallography, so these highly symmetrical crystals should technically be considered “pseudohexagonal” – the two different rhombohedral forms are equally developed, giving the appearance of hexagonal pyramids on the ends of the crystals.  

The Late Carboniferous igneous activity, about 305 million years ago, that sent the fluids into the older limestones may represent a very early phase of the fracturing that ultimately led to the opening of the North Atlantic Ocean, a process that didn’t really get underway big-time until the Jurassic, 100 million years later. Alternatively (and probably more likely), the igneous activity might represent the last stages of the Variscan Orogeny, a complex collision between what is now central and northern Europe and Gondwana (see, for example, Benek and others, 1996, Permo-Carboniferous magmatism of the Northeast German Basin: Tectonophysics, Volume 266, Issues 1–4, p. 379-404).

The word “quartz” (as querz) was first used in print in Germany in 1505. According to MinDat, it probably derived from a Saxon miners’ word for the veins and stringers of barren white material they found and called querklüfte, meaning “cross fissures.” Quartz, simple silicon dioxide, is the most common mineral in the earth’s crust, although feldspars (several different minerals) as groups add up to more.

My Cat. No. 161.