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vealed microcrystalline plate-like and spherulitic structures (figure 7). The turquoise matrix was com-posed mainly of carbonaceous material, limonite, and a clay mineral. Secondary quartz is locally pres-ent in the matrix as elongate bladed aggregates. The slate host rock shows a platy, fine-grained crystal-loblastic texture.
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• Spherulitic texture: – In silicic volcanic rocks in which needles of quartz and alkali feldspar grow radially from a common center • Variolitic texture: – Radiating plagioclase laths in some basalts are probably the result of nucleation of later crystals on the first nuclei to form during devitrification of glass.
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Spherulitic aggregate (pherulites) are radiating arrays of fibrous, needle-like or acicular, crystals that are common in glassy felsic volcanic rocks. Early descriptions of spherulites are by Cross and Iddings in 1891 and by Harker in 1901.
Oct 25, 2014 · After having studied the thin sections of basalt under the microscope, I have come across many spherulitic structures occurring in clusters. How can this happen? Is it due to devitrification or is it something else?
- Petrology & Petrography
- Introduction to Igneous Rocks
- Gases in Magmas
- Plutons
- Volcanic Eruptions
- Explosive Eruptions
- Nonexplosive Eruptions
- Volcanic Landforms
- Tephra Cones (also called Cinder Cones)
- Maars
- Lava Domes (also called Volcanic Domes)
- Textures of Igneous Rocks
Petrology - The branch of geology dealing with the origin, occurrence, structure, and history of rocks. Petrography - The branch of geology dealing with the description and systematic classification of rocks, especially by microscopic examination of thin sections. Petrography is a subfield of Petrology. In this course, most of the lecture material ...
An igneous rock is any crystalline or glassy rock that forms from cooling of a magma. A magma consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase. Magma can cool to form an igneous rock either on the surface of ...
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, li...
Plutons are generally much larger intrusive bodies that have intruded much deeper in the crust. Although they may show sharp contacts with the surrounding rocks into which they intruded, at deeper levels in the crust the contacts are often gradational. Lopoliths are relatively small plutons that usually show a concave downward upper surface. ...
In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks. As they rise they may encounter a depth or pressure where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (i.e. it makes bubbles just like in a bottle o...
Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas). z Explosive bursting of bubbles will fragment the magma into clots of liquid that will cool as they fall through the air. These solid particles become pyroclasts (meaning - hot fragments) and tephra or volcanic ash, which refer to sand- sized o...
Non explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas). If the viscosity is low, nonexplosive eruptions usually begin with fire fountains due to release of dissolved gases. Lava flows are produced on the surface, and these run like liquids down slope, along the lowest areas they can find. Lava...
somewhat steeper lower slopes (about 10o). Shield volcanoes are composed almost entirely of thin lava flows built up over a central vent. Most shields are formed by low viscosity basaltic magma that flows easily down slope away form a summit vent. The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it coo...
Tephra cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions. They usually consist of basaltic to andesitic material. They are actually fall deposits that are built surrounding the eruptive vent. Slopes of the cones are controlled by the angle of repose (angle of stable slope for loose unconsolidate...
z Maars result from phreatic or phreatomagmatic activity, wherein magma heats up water in the groundwater system, pressure builds as the water to turns to steam, and then the water and preexisting rock (and some new magma if the eruption is phreatomagmatic) are blasted out of the ground to form a tephra cone with gentle slopes. Parts of the crater ...
Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent. Blocks of nearly solid lava break off the outer surface of the dome and roll done its flanks to form a breccia around the margins of dome...
The main factor that determines the texture of an igneous rock is the cooling rate (dT/dt) Other factors involved are: The diffusion rate - the rate at which atoms or molecules can move (diffuse) through the liquid. The rate of nucleation of new crystals - the rate at which enough of the chemical constituents of a crystal can come together in one p...
It is found that the emergence of mesh-like canals may distinguish representatives of complex and robust clades, and the differences in polyp-canal connections suggest distinct evolutionary trajectories among coral species.
People also ask
What type of structures were found in a turquoise spherulitic rock?
Which rhyolites have a spherulitic texture?
What is a spherulitic aggregate?
Are spherulites Confocally radiating crystals?
Spherulites are clusters of confocally radiating crystals that, contrary to the implication of their name, are not commonly spherical but usually resemble sheaves of wheat tied at the center.
