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Under the microscope the spherulites are of circular outline and are composed of thin divergent fibers that are crystalline as verified with polarized light. Between crossed Nicols, a black cross appears in the spherulite; its axes are usually perpendicular to one another and parallel to the crosshairs; as the microscope stage is rotated the cross remains steady; between the black arms there ...
Examples of aphanitic igneous rock include basalt, andesite, and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian.
Rhyolite was mined there starting 11,500 years ago. [32] Tons of rhyolite were traded across the Delmarva Peninsula, [32] because the rhyolite kept a sharp point when knapped and was used to make spear points and arrowheads. [33] Obsidian is usually of rhyolitic composition, and it has been used for tools since prehistoric times. [34]
Geologic map of the Sonoma Volcanics. The Sonoma Volcanics are a geologic formation of volcanic origin that is widespread in Napa and Sonoma counties, California.Most of the formation is Pliocene in age and includes obsidian, perlitic glass, diatomaceous mud, pyroclastic tuff, pumice, rhyolite tuffs, andesite breccias and interbedded volcanic lava flows.
Farther north of these craters are five lava domes, including Deadman Creek Dome, Glass Creek Dome, Obsidian Dome, and Wilson Butte. [11] These domes are composed of gray rhyolite, frothy pumice, and black obsidian. The Inyo volcanic chain extends into Long Valley Caldera but is not related to the caldera's volcanism. [8]: 290
These rocks are usually found within obsidian or rhyolite lava flows. [2] Lavas low in feldspar minerals may produce a version known as snowflake obsidian . These cavities are believed to be caused by expanding gases in tuffs and rhyolitic lavas before solidification. [ 1 ]
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Llanite – Type of mineral – A hypabyssal rhyolite with microcline and blue quartz phenocrysts from the Llano Uplift in Texas; Luxullianite – Rare type of granite; Mangerite – Plutonic intrusive igneous rock, that is essentially a hypersthene-bearing monzonite; Minette – A variety of lamprophyre