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• Obsidian is relatively unstable from a geologic perspective. It is rare to find obsidian older than about 20 million years, which is very youthful in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock.
• Igneous rocks are formed from molten rock that has cooled and solidified. The inside of the Earth is very hot - hot enough to melt rocks. Molten (liquid) rock is called magma. When the magma cools.
• Big Obsidian Flow, Newberry National Volcanic Monument, Oregon. Truly, there are so many.
• Rocks with greater than 90% felsic minerals can also be called leucocratic, from the Greek words for white and dominance. Felsite is a petrologic field term used to refer to very fine-grained or aphanitic, light-colored volcanic rocks which might be later reclassified after a more detailed microscopic or chemical analysis.

1. Obsidian Rocks Near Me
2. Obsidian Rocks Oregon
3. Obsidian Rocks Uses

Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. It is an amorphous material known as a 'mineraloid.' The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo). Obsidian is usually an extrusive rock - one that solidifies above Earth's surface.

• Physical properties

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Feldspar, any of a group of aluminosilicate minerals that contain calcium, sodium, or potassium. Feldspars make up more than half of Earth’s crust, and professional literature about them constitutes a large percentage of the literature of mineralogy.

Of the more than 3,000 known mineral species, less than 0.1 percent make up the bulk of Earth’s crust and mantle. These and an additional score of minerals serve as the basis for naming most of the rocks exposed on Earth’s surface.

Each of the common rock-forming minerals can be identified on the basis of its chemical composition and its crystal structure (i.e., the arrangement of its constituent atoms and ions). The nonopaque minerals can also be identified by their optical properties. Fairly expensive equipment and sophisticated procedures, however, are required for such determinations. Therefore, it is fortunate that macroscopic examination, along with one or more tests, are sufficient to identify these minerals as they occur in most rocks. The following descriptions include basic chemical and structural data and the properties used in macroscopically based identifications. Optical data, which are not included in these descriptions, are available in mineralogy books.

Two important rock-forming materials that are not minerals are major components of a few rocks. These are glass and macerals. Glass forms when magma (molten rock material) is quenched—i.e., cooled so rapidly that the constituent atoms do not have time to arrange themselves into the regular arrays characteristic of minerals. Natural glass is the major constituent of a few volcanic rocks—e.g., obsidian. Macerals are macerated bits of organic matter, primarily plant materials; one or more of the macerals are the chief original constituents of all the diverse coals and several other organic-rich rocks such as oil shales.

In the classification of igneous rocks of the International Union of Geological Sciences (IUGS), the feldspars are treated as two groups: the alkali feldspars and the plagioclase feldspars. The alkali feldspars include orthoclase, microcline, sanidine, anorthoclase, and the two-phase intermixtures called perthite. The plagioclase feldspars include members of the albite-anorthite solid-solution series. Strictly speaking, however, albite is an alkali feldspar as well as a plagioclase feldspar.

In geology, felsic is an adjective describing igneous rocks that are relatively rich in elements that form feldspar and quartz.^[1] It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.

Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

Terminology[edit]

In modern usage, the term acid rock, although sometimes used as a synonym, normally now refers specifically to a high-silica-content (greater than 63% SiO₂ by weight) volcanic rock, such as rhyolite. Older, broader usage is now considered archaic.^{[citation needed]} That usage, with the contrasting term 'basic rock' (MgO, FeO, mafic), was based on an ancient concept, dating from the 19th century, that 'silicic acid' (H₄SiO₄ or Si(OH)₄) was the chief form of silicon occurring in siliceous rocks. Although this intuition makes sense from an acid-base perspective in aquatic chemistry considering water-rock interactions and silica dissolution, siliceous rocks are not formed by this protonated monomeric species, but by a tridimensional network of SiO₄^4– tetrahedra connected to each others. Once released in water and hydrolyzed, these silica entities can indeed form silicic acid in aqueous solution.

The term 'felsic' combines the words 'feldspar' and 'silica'. The similarity of the resulting term felsic to the German felsig, 'rocky' (from Fels, 'rock'), is purely^{[citation needed]} accidental. Feldspar is linked to German. It is a borrowing of Feldspat. The link is therefore to German Feld, meaning 'field'.^[2]

Classification of felsic rocks[edit]

In order for a rock to be classified as felsic, it generally needs to contain more than 75% felsic minerals; namely quartz, orthoclase and plagioclase. Rocks with greater than 90% felsic minerals can also be called leucocratic,^[3] from the Greek words for white and dominance.

Felsite is a petrologic field term used to refer to very fine-grained or aphanitic, light-colored volcanic rocks which might be later reclassified after a more detailed microscopic or chemical analysis.

In some cases, felsic volcanic rocks may contain phenocrysts of mafic minerals, usually hornblende, pyroxene or a feldspar mineral, and may need to be named after their phenocryst mineral, such as 'hornblende-bearing felsite'.

The chemical name of a felsic rock is given according to the TAS classification of Le Maitre (1975). However, this only applies to volcanic rocks. If the rock is analyzed and found to be felsic but is metamorphic and has no definite volcanic protolith, it may be sufficient to simply call it a 'felsic schist'. There are examples known of highly sheared granites which can be mistaken for rhyolites.

For phaneritic felsic rocks, the QAPF diagram should be used, and a name given according to the granite nomenclature. Often the species of mafic minerals is included in the name, for instance, hornblende-bearing granite, pyroxenetonalite or augite megacrystic monzonite, because the term 'granite' already assumes content with feldspar and quartz.

The rock texture thus determines the basic name of a felsic rock.

Obsidian Rocks Near Me

See also[edit]

Notes[edit]

1. ^Marshak, Stephen, 2009, Essentials of Geology, W. W. Norton & Company, 3rd ed. ISBN978-0393196566
2. ^Harper, Douglas. 'feldspar'. Online Etymology Dictionary. Retrieved 2008-02-08.
3. ^Aurora Geosciences Ltd. 'Technical report Mackenzie mountains iron-copper property, Northwest Territories, Canada'(PDF).

References[edit]

Obsidian Rocks Oregon

• Le Maitre, L. E., ed. 2002. Igneous Rocks: A Classification and Glossary of Terms 2nd edition, Cambridge

Obsidian Rocks Uses