N. W. Dunbar; V.T. McLemore
New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM, 87801
Numerous expanded spherulites are found in rhyolitic lava flows in a volcanic sequence in the
Florida Mountains, near Deming, New Mexico, USA. The spherulites range in size from less than 1 mm to greater than 30 cm, and are composed largely of concentrically-zoned dark gray to pinkish crystalline material. Microprobe examination of the spherulites shows a distinct core, surrounded by a zone of crystals very similar to crystal forms described by Lofgren (1970, 1971), grown rapidly in an undercooled melt. This “quench” zone segues into the outer spherulite, which is composed of intergrown crystals of quartz and two compositions of alkali feldspar, formed as quartz-rich bands systematically interlayered with bands of intergrown feldspar and quartz. The bands range in width from around 100-200 microns and produce the spherulite’s concentric structure. The distribution and regularity of the bands are interpreted to be controlled by diffusion processes at a crystal-melt interface during crystal growth. Similar spherulitic forms, with similar internal growth features were observed in an artificial melt that was rapidly cooled (Jacobs et al., 1992; Dunbar et al., 1995).
Thermal monitoring of the melt during the cooling showed a period of exothermic crystallization at high temperature (1100 o C), and this is when the spherulites are thought to have crystallized. The outward appearance and internal structure of these are indistinguishable from natural spherulites.
This similarity, plus banding, morphology and chemical composition of feldspar all suggest that the
Florida Mountain spherulites formed at high temperatures early in the cooling history of the rhyolite lava.