Why is obsidian so dense
Author: Torsten Purle (steine-und-minerale.de) | Last update: 29.03.2021
Obsidian - properties, origin and use
English: obsidian | French: obsidienne
Table of Contents Obsidian
Obsidian - The black volcanic glass
One of the oldest descriptions of obsidian goes back to the Roman polymath Pliny (23 to 79 AD). In his work De Naturalis Historia he describes a black, glass-like stone with translucent transparency: "... quem in Aethiopia invenit Obsidius, nigerrimi coloris, aliquando et translucidi ..." and thus justifies the origin of the name obsidian - the name of the stone a Roman named Obsidius, who brought the stone from Ethiopia.
Properties of obsidian
Definition: Obsidian is an igneous rock of extrusive origin that is assigned to volcanic glasses.
Obsidian is one of the acidic, siliceous (up to 70% silicic acid) rocks. Trachytic, Adamic and phonolithic obsidians, which are characterized by a lower silica content, are basically possible, but comparatively rare compared to rhyolite, high-silica obsidian. With less than one percent, water of crystallization is also involved in the composition. Occasionally obsidian contains quartz and feldspar crystals as inclusions.
The structure of obsidian is compact and glass-like, i.e. there is no crystal structure of the minerals that build it up - which is why the German mineralogist Dietrich Ludwig Karsten (1768 to 1810) called obsidian "lava glass" at the time.
The amorphous rock, however, devitrifies over millennia by initially forming radial crystals - spherulites - which are reminiscent of white snowflakes (snowflake obsidian) and later in this way pitchstone is formed as a result of progressive aging.
The progressive crystallization or devitrification of obsidian and thus also the aging of the rock can be recognized by the spherulites, which are visible in the form of white, irregularly defined spots - the so-called snowflakes - in the obsidian.
The luster of obsidians is glass-like and with a Mohs hardness of 5 to 5.5 on the 10-point scale of the hardness of minerals according to Friedrich Mohs (German mineralogist, 1773 to 1839), obsidian is a relatively hard rock whose breakage is shell-like and very is sharp-edged. The cleavage is not given. The transparency is opaque, on the other hand the transparency is translucent on the edges of the rock. The density is 2.5 to 2.6 g / cm3
The color of obsidian
The color of obsidian is especially blackwhere Joseph Zappe describes the obsidian color 1817 in more detail as follows, which is "usually velvet black, sometimes changing into grayish black, ash and smoke gray, sometimes from pitch black to clove brown". Next to it, obsidian is in dark green, dark brown, golden brown ((rainbow obsidian / gold obsidian), silvery-gray (silver obsidian) or reddish hues common.
The Flemish botanist, chemist and mineralogist Anselmus de Boodt (1550 to 1623) compares obsidian with black marble in his book “Gemmarum et Lapidum” (gemstones and stones) because of the color.
The dark color is causally due to iron-containing minerals such as hematite and magnetite, which could be detected in the rock. The line color - the color that occurs when a mineral or stone is painted over an unglazed porcelain tablet - is white to light yellow in obsidian.
Varieties of Obsidian
- Snowflake Obsidian: black base with white, snowflake-like drawings
- Gold obsidian: Obsidian shimmering gold
- Silver obsidian: silvery-gray shimmering obsidian
- Rainbow obsidian: blue-gray, dark gray to black obsidian with rainbow-colored shimmer
Origin and distribution of obsidian
Obsidian as a rock of igneous origin arises as a result of the very rapid cooling of lava, for example when there are large differences in the temperature of lava and air or when lava suddenly cools in the water. The liquid rock melt solidifies so quickly that a crystallization of the rock-forming minerals is not possible, but that a glass-like, structureless and solid structure is nevertheless formed.
The formation of obsidians is always linked to volcanoes. For this reason, volcanic glasses also occur as a crusty coating on many lava flows - both on the surface of the earth and under the sea, as ejection from eruptions or as a cover from volcanic domes.
Numerous obsidian deposits were found in Anatolia / Turkey; Indonesia; Japan; Iceland; Lipemic Islands / Italy; Wyoming, New Mexico, Utah, Texas, Hawaii / USA; Greece; Armenia; New Zealand; Kamchatka / Russia; Ecuador; Guatemala and Hungary described.
Obsidian in Germany
The only obsidian occurrences in Germany are in the vicinity of Baden-Baden in Baden-Württemberg.
Use and meaning of obsidian
Due to the compact nature of the rock and the sharp edges of the machined obsidians, the rock glass was already known as a Weapon and tool used for cutting. Today obsidian is used as a material for handicraft items as well as for Sculptures, and is used in the construction industry in powder form as a material for the production of mineral wool.
Obsidian and jewelry
When processing obsidian into jewelry, multifaceted cuts are avoided. For the black stone with the opaque transparency and snowflake drawings, especially smooth cuts are used - cabochons: oval or round, teardrop-shaped or slightly angular.
Healing stone obsidian
Obsidian is also sold as a healing stone, from which the suppliers promise positive effects on mental and physical health.
However, the healing properties of obsidian could not be proven in clinical studies.
Obsidian and mix-ups
Due to the visual similarity - in terms of color and luster - obsidian can be confused with Pechstein, Onyx and Schörl.
Evidence of obsidian
Obsidians have no fluorescence.
Obsidian - Our recommendation *
⇒ Differentiate between pitch stone and obsidian
⇒ Black diamonds - Fancy diamonds
⇒ Black tourmaline
⇒ De Boodt, Anselmus (1609): Gemmarum et Lapidum Historia
⇒ Karsten, D. L. G. (1789): The mineral cabinet left behind by Mr. Nathanaël Gottfried Leske. First volume. Publishing house I.G. Muller bookstore
⇒ Zappe, JR (1805): Mineralogical Handlexikon, or alphabetical listing and description of all previously known fossils, according to their old and new nomenclature and characteristics, their geognostic occurrence and economic-technical use, together with the explanations of the for Characteristic made up words. Vienna. Anton Doll Verlag
⇒ Ludwig, C. F. (1803): Handbook of Mineralogy according to A. G. Werner. First part: Oryctognosia
⇒ Bauer, J .; Tvrz, F. (1993): The Cosmos Mineral Guide. Minerals rocks precious stones. An identification book with 576 color photos. Gondrom Verlag GmbH Bindlach
⇒ Pellant, C. (1994): Stones and Minerals. Ravensburger nature guide. Ravensburger Buchverlag Otto Maier GmbH
⇒ Schumann, W. (1991): Minerals rocks - characteristics, occurrence and use. FSVO nature guide. BLV Verlagsgesellschaft mbH Munich
⇒ Maresch, W., Medenbach, O .; Trochim, H.-D. (1987): The colored natural guide rocks. Mosaik Verlag GmbH Munich
⇒ Murawski, H. (1992): Geological Dictionary. Ferdinand Enke Verlag Stuttgart
⇒ Schumann, W. (1992): Precious and precious stones: all precious and precious stones in the world; 1500 unique pieces. BLV determination book, BLV Verlagsgesellschaft mbH Munich
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