The Marble Mountain 'marbles' are spherulites enclosed in a rhyolitic obsidian. Obsidian is a volcanic glass, formed where the lava was so dry that mineral crystals were not able to grow. With a hand lens (loupe), you can see the glass clearly in between the marbles. Rhyolite is a compositional term: it's a silica-rich volcanic rock, the fine-grained equivalent of granite.
Geologists and mineralogists think that many spherulites are mineral fillings of original gas bubbles. The Marble Mountain specimens pretty clearly didn't form this way, because the individual marbles overlap and build one on the other into composite masses. They don't have the oval or smooth spherical shape of typical volcanic gas bubbles.
Solid-state recrystallization theory best fits our Marble Mountain marble marvels. After an eruption of obsidian, the glass cooled and gradually hardened. Minute mineral grains crystallized--but only a few and those widely spaced. Each of these initial crystals acted as a nucleation site for millions of other crystals. Rapidly, spheres of newly crystallized rock appeared, each embedded in uncrystallized obsidian glass.
This is exactly the same process that happens when making fudge--microscopic sugar crystals suddenly appear as the fudge 'sets up' and turns from soft and shiny (non-crystalline) to stiff and matte-surfaced (crystallized). The difference is that you stir fudge to smooth it out, whereas the obsidian isn't stirred, so it only partially crystallizes. Also, the obsidian is a very dry substance, which limits crystallization.
It's interesting that some of the 'marbles' have tiny black biotite mica crystals in them. (Crystals may actually be hornblende or even olivine.) The biotite grains might have acted as nuclei for the initial recrystallization of the hot volcanic glass. If so, each spherulite would have a biotite crystal at its center.
The tiny (mostly microscopic) crystals in the 'marbles' radiate out from a common center. This radial structure is hard to see without good light and a hand lens or binocular microscope.
The minerals in the marbles are probably high-temperature forms of quartz and feldspar--minerals such as tridymite, cristobalite, and chalcedony (all quartz relatives), and sanidine (a feldspar). X-Ray analysis would be needed to determine the exact composition.
If mineral crystals line but only partially fill cavities in volcanic rock, you get lithophysae, which are concentrically shelled structures. Marble Mountain marbles are not lithophysae.
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