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What the schist?Oh, sorry, gneiss rock specimen.The layers are formed in a metamorphic rock, probably a schist. Original rock (and layering) probably shale with sandy layers. As the rock is heated and subjected to intense pressure, the minerals are changed, or metamorphosed. In a schist, the clay minerals that make up the original mudstone are changed into mica (most commonly muscovite or biotite) and the sandy layers are changed to quartzite. The minerals form in a preferred direction- especially the mica, which will be perpendicular to the direction of greatest pressure. Schist can be recognized by the abundance of shiny mica.With more heat and pressure, the minerals begin to segregate into bands of light and dark material. The alignment of minerals causes foliation, or metamorphic layering, which may or may not be the same as the original sedimantary layering in the rock.The folding of the layers makes it obvious that the rock has been subjected to some intense pressure. The thick white band in the lower center is probably quartz, again indicating heat and pressure.Another possibility is metamorphosed muddy limestones, with the white bands being marble segregations- but to me the bottom of the specimen looks micaceous, which suggest schist or gneiss.
The light stripes were originally the skeletons of microscopic creatures (algae), sinking to the bottom of ocean or lake.Changes in the concentration of these creatures over time has produced different thicknesses of white stripes, e.g. in response to cyclical changes in climate.The thicker brown (sand) stripes could represent longer periods when the algae were absent, or cataclysmic events when more sand than usual was deposited, e.g. tsunami.
It's a bit of prehistoric sea-side rock (candy) dating from the time before the wheel and other round things were invented, and before they learned how to put the name of the town through the middle.
WOW I would have said Petrified wood.. how does one tell the difference Bass?
Addendum for Bass:gneiss (nies) n. 1. a metamorphic rock, generally made up of bands that differ in color and composition, some bands being rich in feldspar and quartz, others rich in hornblende or mica. [1750-60; < G Gneis, ult. der. of OHG gneisto spark] Derived words --gneiss'ic, gneiss'oid, adj. ___________________________________________
It is really difficult assuming the role of the geological martyr that I seem to be cast into at times. Ah, the responsibilities of age and wisdom.
Based on the photo, I would still favor schist (on the verge of becoming gneissic): 1. Breaks along well developed micaceous layer 2. No visible crystals in laminations- suspect that laminations are original texture instead of chemical segregations of minerals
Gneiss - Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes ...SCHIST The schists form a group of medium-grade metamorphic rocks ...NOW, BASSIf you can show that the rock that started this discussion was the result of high-grade regional metamorphic processes, resulting in metamorphic differentiation, I'll be very glad to cede the point.