“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.” — Isaac Newton
Today’s smooth pebble comes from the Michigan shore of Lake Superior, USA.
Syenite is a granite-like rock that has very little quartz (or none). Most of the silica is in feldspars, with the iron and manganese in amphiboles and/or pyroxenes, but they are usually not especially abundant. Syenite probably forms by partial melting of continental crustal rocks, usually inferred to indicate only minor, almost incipient partial melting. When rocks melt, potassium (as in the potassium feldspars in syenite) and sodium are liberated first. More extensive melting frees calcium, which makes other minerals.
Partial melting can make for silica deficiencies that prevent even feldspar from forming, and feldspathoids such as nepheline, analcime, and sodalite crystallize instead. That’s what has happened in this rock, with sodalite, Na4(Si3Al3)O12Cl, abundant enough we can call it sodalite-syenite.
The rocks crossed Lake Superior to Michigan with the glaciers. The most likely origin is an igneous body, the Coldwell Alkaline Complex, near Marathon, Ontario, on the north side of the lake. The intrusion happened about 1,100 million years ago as part of the formation of the Mid-Continent Rift System, a failed break in the North American continent that extended southwest from Lake Superior through Iowa and Kansas to Oklahoma, and south through lower Michigan (Walker and others, 1993, Precambrian geology of the Coldwell Alkalic Complex: Ontario Geological Survey Open File Report 5868, 30 p.). The Coldwell Complex has been studied intensely because of its copper and platinum-group element ores (Good and others, 2021, A new model for the Coldwell Complex and associated dykes of the Midcontinent Rift, Canada: J. Petrology, 62:7).
Yellow to yellow-orange fluorescence of sodalite in this rock makes a pretty specimen, given the tradename Yooperlite™ for its occurrence on the shores of Lake Superior in the Upper Peninsula (the U.P., pronounced yoo-pee) of Michigan.
Sodalite is often fluorescent, but some, the variety hackmanite, is also tenebrescent. Tenebrescent materials have a color on freshly exposed surfaces (purple-pink for hackmanite) that fades in natural light, but that can be restored by exposure to ultraviolet (UV) light, and the color remains after the UV source is removed—but it fades over time in normal light, in time frames ranging from seconds to weeks.
The fluorescent and tenebrescent effects in sodalite have been explained as arising from disulfide ions, S2, replacing some of the chlorine at the end of the sodalite formula. Sulfur distorts the crystal lattice to produce the optical effects. Variable expressions of fluorescence with or without tenebrescence are not fully understood (Zahoransky and others, 2016, Luminescence and tenebrescence of natural sodalites: a chemical and structural study: Phys. and Chem. of Minerals 43:7).
As far as I can tell without breaking it, my specimen is only fluorescent, and is not the tenebrescent variety hackmanite.
Another engaging and informative piece, Richard. Thanks for sharing amazing stuff via your column. “Tenebrescent” was a great find for this avid collector of interesting words.
Your discussion of partial melting brings to mind the one time controversy concerning the formation of granite: Was it granitization of sedimentary deposits, proceeding along a metamorphic pathway, or was it purely igneous, proceeding through differentiation, along an igneous path from batholiths.
The disc shape of your pebble implies it spent time going back and forth in the waves of some beach.
I enjoy your posts very much.