Youngite

Not a mineral

You probably realize by now that I’m fond of finding things in my collection that tell complicated stories. I know (and I’m glad) many of you like that and learn from it, but it’s primarily selfishness on my part - *I* get to learn stuff! I learned essentially everything in this post, including the identity of the rock, about four years ago, after holding on to it for decades.

Youngite isn’t a mineral – it’s a rock (a rock is an aggregate of minerals, although counterintuitively you can have a monomineralic rock). And it isn’t even a general rock name, but just a local name for an interesting variety of agate or jasper found only in one place in the world: Guernsey, Wyoming USA, in the southeast part of the state.

The rock is from modern caves in the Guernsey Limestone of Devonian-Mississippian age, apparently more or less the stratigraphic equivalent of the Mississippian Madison Limestone. But at Guernsey the layers that should underlie this limestone, including Cambrian, Ordovician, Silurian, and most Devonian strata, are missing, and the Mississippian limestone lies directly on Precambrian (Archean, 2.58 billion years old, as well as younger Paleoproterozoic) rocks of the Hartville Uplift. There is lots of iron in those Precambrian rocks, and lots of silica in the layers above the limestone, so it may be that they are the sources of the pinkish “jasper” or chalcedony (fine grained quartz) and the druzy quartz (a coating of tiny, sharp crystals) that filled the cracks in the broken limestone (breccia) that we now call youngite.

This suggests a complicated tectonic history, which is not too surprising because the Hartville Uplift is near or along the southeastern margin of the Wyoming Craton, one of the main building blocks of the North American Continent. There’s some controversy as to whether the Uplift is in or adjacent to the Wyoming Craton margin. It might be within the Trans-Hudson Orogenic Belt, a 1.85-billion-year-old zone where the Wyoming Craton amalgamated with the Superior Province, the largest cratonic block in North America, named for Lake Superior. My view (at least about 70% of the time) based on the appearance of the magnetic map is that the Hartville Uplift is within the Wyoming Craton.

Regardless of those details, the absence of the pre-Mississippian rocks indicates that there was a very long period of non-deposition or erosion, or both. Then the Guernsey Limestone was deposited in a shallow sea, and THEN, even more uplift happened. We know that because the top of the Guernsey Limestone has karst in it – caves, dissolution and collapse features that are the origin of the broken breccia that we’re talking about here, the youngite. Such features develop in subaerial conditions, or at most in extremely shallow water. We know this happened before the overlying rocks (of Pennsylvanian age) were deposited, because they partly fill in the karst features, which must have existed before the Pennsylvanian sediments were laid down.

So that time of dissolution, after the Guernsey Limestone was deposited about 340 million years ago but before the overlying Pennsylvanian rocks were deposited about 310 million years ago is the time the youngite must have formed. At least the breccia formed then; the infilling quartz and chalcedony might have been quite a lot later, and some people think the source of the silica is volcanic ash in Oligocene – 30-million-year-old – rocks that are above the limestone.

The whole works is exposed at the surface today because the Hartville Uplift reflects a later breaking, a Laramide uplift from about 70 million years ago, the same time as the formation of the modern Black Hills Uplift and the Wind River and other ranges in Wyoming and adjacent states. That’s the second uplift of these rocks.

The modern cave where the youngite is found is a present-day development, or at most a few million years old, so the limestone here has been dissolved and karstified at least twice, once about 320 million years ago and once in about the past two or three million years, with an intervening uplift about 70 million years ago that may or may not have contributed to the karstification. My understanding is that today the cave is largely water-filled and inaccessible.

The youngite is fluorescent, emitting light whose color is different under ultraviolet radiation vs. normal visible light, and it reacts differently in short wave (green color) and long wave (more bluish) ultraviolet (UV) light. The fluorescent part of the rock is mostly the layered agate/chalcedony rather than the pink jasper. In many other examples of green fluorescence in chalcedony around the world, the activator, the impurity that makes the fluorescence, is the uranyl ion (uranium oxide), and according to the online database MinDat, that’s the case here, too.

I don’t know for whom this rock was named. It should not be confused with youngite, a “sulphide of lead, zinc, and other metals, probably a mixture,” described in 1877 by J. B. Hannay (Min. Mag. 1:152), who named that now discredited material for Prof. John Young of the Glasgow Museum. Scottish mineralogist/chemist Hannay (1855-1931) is the namesake of hannayite, a magnesium-ammonium phosphate that I found rarely in my time analyzing kidney stones. Hannay was the first to create artificial diamonds, in 1880.

Keep learning.