Garnets in cavities in volcanic rocks are cool, in part because there’s a nice contrast in color between the garnet and the background. Garnets are usually metamorphic minerals, so why do they form in volcanics?
The slightly pinkish volcanic rocks at Ely, Nevada, are tuffs, solidified ash falls from silicic (rhyolitic) eruptions. When warm ash falls, it sometimes remains warm enough that the ash almost melts to itself to make a rock called a welded tuff, or ignimbrite (from words that mean “fiery shower”).
For about 18 million years, from 36 to 18 million years ago, central Nevada was the site of one of the largest periods of explosive volcanism known. According to Best and others (2013, The 36–18 Ma southern Great Basin, USA, ignimbrite province and flareup: Geosphere 9 (2): 260-274) there are at least 42 volcanic calderas as much as 60 km in diameter. Both the size and longevity of this volcanic event are poorly understood, but it is probably related to the thinness of the crust in the Great Basin, a relatively low angle of subduction, and tear faults breaking the crust.
The welded tuff near Ely (called Garnet Hill) is probably toward the older end of the eruptions, around 36 million years old. The ash and associated lavas contained abundant gases that became trapped in the solidifying tuff, forming roughly circular cavities in the rock. The garnets did not crystallize from molten solutions as many minerals do, nor did they grow under heat and pressure in a metamorphic setting. Rather, they deposited directly from somewhat later mineral-rich vapors that passed through the rocks, cooling when they encountered the cavities.
I find it amazing and fascinating that garnets can crystallize directly from vapor.
There are six common types of garnets defined by their chemical composition. The garnets at Ely have often been called spessartine (named for the Spessart Mountains of Germany), manganese aluminum silicate, but analysis shows these garnets to have consistent iron to manganese ratios of around 2.5 to 1, which makes them almandine, closer to the iron end member in a continuous chemical series extending to spessartine.
My larger specimen here above, which I collected in 2016, has some imperfections, but it is close to a centimeter across. Cat. No. 1199. The other, more perfect crystal at the top of this post, also from Garnet Hill near Ely, is one I bought many years ago for $3; it’s about 4 mm across and shows the isometric trapezohedron form nicely. Cat. No. 409.
The name ‘almandine’ is an alteration of older alabandine, for Alabanda, a town of Asia Minor (Turkey) where such material was cut into gems in the Middle Ages. Apparently the first use of the name was by Georgius Agricola (Georg Bauer) in 1546.
Garnets were one of the first crystals that whetted my interest in minerals, rocks and eventually Geology. As I look back it was initially the color (almandine) and later the uniqueness of the dodecahedrial crystal morphology. It is almost "magical" that nature can produces these crystal shapes, although in true reductive reasoning, its all chemistry and physics in the long-run.
I grew up in the SE US (Atlanta, Georgia area) near the foothills of the Appalachian Mountains. In that heavily weathered region of metamorphic rocks we occasionally came across anthills with tiny garnets incorporated in the sand-sized gains of the hills. These little ruby red pieces of garnet were instantly interesting to us kids.
Garnets common in some metamorphic rocks of the Scottish Highlands (best I've seen are on a mountain called Beinn Oss). Legend is that they're formed from blood dripping off the 'sluaigh' or fairy host carrying away battlefield victims through the sky. There's also an alternative legend to do with 'Barrow zones'...