It is used a railroad ballast( to provide stability). There are different varieties with similar physical properties which can only be differentiated in the laboratories. The chemical composition of hornblende varies depending on the specific minerals in the group. [8], Hornblende is an inosilicate (chain silicate) mineral, built around double chains of silica tetrahedra. Chemical Formula. Dark green Hornblende was found in Malmberget, Gllivare, Sweden; and doubly terminated floater crystals come from im (Schima), Teplice, Bohemia, in the Czech Republic. These are prismatic crystals with a diamond-shaped cross-section. The name Hornblende was originally given in 1789 by Abraham Gottlieb Werner. Probably common worldwide in "hornblende" granites and shists. Amphibole-Hornblende OVERVIEW Amphiboles are found principally in metamorphic and igneous In crystalline structures, fluoride is more common than hydroxide. Hornblende Hornblende Authors: Dalton Zook, Rachael Dolan, and Nicole LeFebvre Mineral Name: Hornblende Chemical Composition: Ca2 (Mg, Fe, Al)5 (Al, Si)8O22 (OH)2 Color: Black to a dark green. (Ca,Na) 2-3 (Mg,Fe,Al) 5 (Si,Al) 8 O 22 (OH,F) 2 Note that calcium, sodium, magnesium, iron, aluminum, silicon, fluorine and hydroxyl can all vary in abundance. Pyroxenes differ in their cleavage planes, which intersect at 87 and 93. Main minerals in amphibolite are plagioclase (white) and hornblende. The cummingtonite-grunerite series happens in different shades of light brown. What is the mineral composition of pumice? As discussed above, it forms a major constituent of rocks called amphibolite. Intrusive rocks are those that form from magma that cools and solidifies below the Earths surface. High Mg, Si and Mg/Fe and low Fe, Al, Ti, Na and particularly low K, amphiboles giving low Z values retained Ar more firmly and gained Ar more readily than compositions which had higher Z values, which gave younger ages. Andesite is the name of a family of fine-grained, extrusive igneous rocks that are usually light to dark gray in color. The Y site is typically occupied by aluminum, iron, magnesium, manganese, or calcium. Isotropy/Anisotropy - Anisotropic i.e. Although the change in the chemical composition of magmatism at ~70 Ma occurs before the . Amphibolite is the name given to metamorphic rocks that are mainly composed of amphibole minerals. In the book "Rock-forming minerals, volume 2b, double chain silicates" by Deer, Howie & Zussman (1997), the term "hornblende" is used as a group name for all aluminous amphiboles in the calcium amphibole subgroup of the Amphibole Supergroup. Hornblende is part of the calcium-amphibole group of amphibole minerals. A miscibility gap exists at lower temperatures, and, as a result, hornblende often contains exsolution lamellae of grunerite. Charge balance is preserved by the substitution of oxygen ions for hydroxide. Halides also tend to have a highly ordered molecular structure and a high degree of symmetry. Chromium, titanium, nickel, manganese, and potassium can also be part of the complex composition and further indicates the generalization of the formula given above. These minerals vary in chemical composition but are all double-chain inosilicates with very similar physical properties. Let us know if you have suggestions to improve this article (requires login). Let us further look into the physical and chemical properties of hornblende. Diorite is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and sometimes pyroxene.The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. Biotite hornblende granite: Hornblende is an important constituent in many igneous rocks. These minerals vary in chemical composition but are all double-chain inosilicates with very similar physical properties. (mineral) A green to black amphibole mineral, of complex structure, formed in the late stages of cooling in igneous rock. As mentioned above, it is a monoclinic type of crystal. The iron, magnesium, and aluminum ions can freely provide the alternative for each other and form what has been distinctive as separate minerals. These chains extend the length of the crystal and are bonded to their neighbors by additional metal ions to form the complete crystal structure.[9]. Syenite Plutonic Holocrystalline Quartz, orthoclase Hornblende, magnetite, biotite . Sodium, potassium are often present. It is found in many intrusive igneous rocks ranging in composition from granites to diorite to gabbros to syenites. These crystals are typically black, green, or brown in color. Sample Minerals Texture Formation Rock type Sample 1 plagioclase, quartz, hornblende, pyroxene Crystals, visible to naked eye Magma cooled slowly Diorite Sample 2 plagioclase, hornblende, pyroxene Crystals are tiny or microscopic Magma erupted and cooled quickly Andesite As seen in Table above, these two rocks have the same chemical composition . The chemical composition of granite is typically 70-77% silica, 11-13% alumina, 3-5% potassium oxide, 3-5% soda, 1% lime, 2-3% total iron, and less than 1% magnesia and titania. Hornblende can be found on every continent, but it is most common in North America and Europe. [13], It is the principal mineral of amphibolites, which form during medium- to high-grade metamorphism of mafic to intermediate igneous rock (igneous rocks with relative low silica content) in the presence of pore water. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0. On the other way out, (X)7Si8O22 (OH)2 is the main chemical formula of the Amphibole, where X represents iron, magnesium and calcium. Answer: It is not very difficult to determine the mineral with this easy technique. Hornblende present in it provides desirable back streaks to the granite. While every effort has been made to follow citation style rules, there may be some discrepancies. It is found in many intrusive igneous rocks ranging in composition from granites to diorite to gabbros to syenites. The ratio of sodium to calcium is about 7:3. Hornblende is a complex inosilicate group of minerals (chain silicates). Since Hornblende is around a 5 to 6 on the Mohs Hardness Scale, a scratch test with a fingernail alone will not give you an indication of its presence. Extensive solid solution occurs, and each end-member has iron-rich equivalents; minor elements, including manganese, titanium, chromium, potassium, fluorine, and yttrium, are usually present. Experimental studies indicate the Andesitic magmas that precipitate Hornblende should normally carry at least 3%H2O and such magmas are usually relatively rich in Alkali and have a high Fe2O3/FeO ratio. It is an isomorphous mixture of three molecules and the hornblende chemical composition is as stated: These minerals are difficult to distinguish by physical means. The group was so named by Hay in allusion to the protean variety, in composition and appearance, assumed by its minerals. Many of them are very soluble in water. AMPHIBOLE-HORNBLENDE WHAT ARE AMPHIBOLES? A generalized composition for the hornblende group is shown below. Its atomic structure is similar to other amphiboles, except that there is a large vacant site, which is occupied by K or Na. Hornblendite is an ultramafic rock (dominantly dark minerals). All members of the hornblende group share a similar structure and similar properties. If this liquid containing olivine is erupted and the rest of the liquid quenches to a glass, then this will produce a rock with phenocrysts of olivine in a glassy groundmass. Minerals are defined by geologists as naturally occurring inorganic solids that have a crystalline structure and a distinct chemical composition. It has a vitreous to dull luster and pale gray to colorless mineral streak color. Hornblende is a common mineral in many types of igneous and metamorphic rocks. This is in reference to the fact that hornblende-rich rocks are very tough and hard to break. Z is controlled by hornblende chemical composition. They have a mineral composition that is intermediate between granite and basalt. Hornblende minerals are common in igneous and metamorphic rocks. This information is useful in understanding the crystallization of magma and also useful for mineral exploration. This creates a huge number of compositional variants. Oxyhornblende is a variety in which most of the iron has been oxidized to the ferric state, Fe+3. In extrusive rocks, hornblende sometimes crystallizes below the ground, in the magma, before eruption. Edenite is a rare variety of hornblende that has 5% iron oxide and thus appears white to gray. (Ca,Na)Al1-2Si3-2O8. Hornblende is a mineral, because it is a naturally-occurring, inorganic solid with a definite chemical composition and crystal structure. Aphanitic minerals have very small crystals that can only be seen with a microscope. It is reduce for use as size stone. (Ca,Na) 2-3 (Mg,Fe,Al) 5 (Si,Al) 8 O 22 (OH,F) 2 Note that calcium, sodium, magnesium, iron, aluminum, silicon, fluorine and hydroxyl can all vary in abundance. We are obsessed with the outdoors, and work constantly to bring you the information you need to enjoy the rocks and minerals you've added to your collection. Basic glass is rare so rocks named, except scoria, will normally be silicic. Primarily hornblende has a few uses. This creates a huge number of compositional variants. Diamonds may be a girls best friend, but what is that minerals closest relative? Augite noun. Most samples have a relatively simple composition: hornblende + plagioclase. Hornblende is the most common of the clinoamphiboles. Further details about the crystal form of hornblende are mentioned below. Senja, Norway. dimension stone. If it feels smooth, then it is probably vitreous. Hornblende is also used in the manufacturing of ceramics and glass. A few rocks consist almost entirely of hornblende. Hornblende is an aluminous amphibole with a composition that contains predominantly a pargasite component. Let us look further into the chemical nature of the mineral. Cooling to anywhere on the liquidus will result in the crystallization of Fo-rich olivine. A rare variety of hornblende contains less than 5% of iron oxide, is gray to white in color, and is named edenite from its locality in Edenville, Orange County, New York. Image by NASA. What are the 3 main types of igneous rocks? Metamorphic Rocks - These are rocks formed from some other type of previously present rocks. Green to brown Hornblende is the typical amphibole found in Andesite, it normally does not found in the groundmass but it is found as a Phenocryst. Igneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and . With higher concentrations of hornblende, the gabbro in the picture is called hornblende gabbro. Image by NASA. Example - granite, diorite, syenite, gabbro. Examples- Gneiss, schist. It is found almost exclusively in volcanic rock and is sometimes called basaltic hornblende. Figure 10d-5: Galena. hornblende, calcium-rich amphibole mineral that is monoclinic in crystal structure. Hornblende is a silicate of aluminium, calcium, magnesium and iron with sodium. What is the main composition of rocks? Oxyhornblende is also typically enriched in titanium. Hornblende minerals as a group are relatively easy to identify. Chemical Composition of Hornblende. In this article, youll obtain answers to common questions about Hornblende. Artificial weathering of hornblende particles was conducted in buffered aqueous solutions at room temperature (25 C) over a pH range of 3.4-4.6. Hornblende is present in acidic and intermediate igneous rocks. Manganese and titanium can also be present. Andesite is a rock typically found in volcanoes above convergent plate boundaries between continental and oceanic plates. Hornblende is present in acidic and intermediate igneous rocks. It is overwhelmed and used for dual carriageway construction and as railroad ballast. Recall what are they? What is the parent rock of hornblende? This can cause the hornblende to recrystallize and form new minerals. Hornblende has been used to estimate the depth of crystallization of plutonic rocks. Quartz is a compound of one part silicon and two parts of oxygen, silicon dioxide, SiO 2. There are some rocks that consist of almost entirely hornblende, known as Amphibole. Hornblende: Hornblende with a typical black granular to fibrous appearance from Faraday Township, Ontario, Canada. Studsdalen, near Kragero, Telemark, Norway. Wondering what is a Gangue material? Hornblende is mafic because it contains a high percentage of iron and magnesium. Chemical composition: Hornblende is a member of the amphibole family. Its primary use might be as a mineral specimen. Composition. Hornblende is non-foliated because it does not have a layered or sheet-like structure. [13] Hornblende alters easily to chlorite, biotite, or other mafic minerals.[14]. The hardness of hornblende is in the range of 5.0 to 6.0 on the Mohs scale. Hornblende is a group name used to describe Ferro-hornblende and Magnesio-hornblende, but the term is generally more inclusive for all calcium aluminum amphibole s. (Hornblende is frequently also used to describe any dark, opaque amphibole mineral without individual analysis.) The influence of microstructures on the relationship between argon retentivity and chemical composition of hornblende. . The finely stringy and enormous assortment of actinolite-tremolite known as nephrite jade reaches from green to dark. However, hornblende is the most plentiful mineral in a rock known as amphibolite which has a huge number of uses. Pyroxene minerals mainly found in igneous and metamorphic rocks. Until now, holmquistite has only been discovered in metasomatically altered mafic rocks near spodumene or other Li-aluminosilicate rich pegmatites. Igneous Rocks - These rocks, also known as the magmatic rock, are formed by the cooling and solidification of the lava or the magma. View Amphibole-Hornblende.pdf from GEO 011 at Visayas State University Main Campus - Baybay City, Leyte. However, hornblende is the most abundant mineral in a rock known as amphibolite which has a large number of uses. Hornblende can, however, be a part of a rock, such as an amphibolite. Obsidian-bearing volcanoes are typically . During metamorphism, hornblende-rich rocks are exposed to high temperatures and pressures. It is the main component of many igneous and metamorphic rocks, such as gabbros and amphibolites. This piece of hornblende andesite is an example. The general chemical formula for the hornblende group is XYZ2-3(Si,Al)4O11(OH,F)2. The different chemical types are almost impossible to distinguish even by optical or X-ray methods, and detailed chemical analysis using an electron microprobe is required. Amphibole minerals possess compositional variants that have similar physical properties. Hornblende is not a rock, because it is not made up of multiple minerals. It is highly variable in composition, and includes at least five solid solution series: Magnesiohornblende-ferrohornblende, Ca 2 . Width of sample 9 cm. It has a hexagonal/granular crystal habitat where the fracture of this mineral is uneven. [10] It is highly variable in composition, and includes at least five solid solution series: In addition, titanium, manganese, or chromium can substitute for some of the cations and oxygen, fluorine, or chlorine for some of the hydroxide (OH). Hornblende is a field and classroom name used for a group of dark-colored amphibole minerals found in many types of igneous and metamorphic rocks. The U.S. Department of Energy's Office of Scientific and Technical Information The iron, magnesium, and aluminum ions can freely provide the alternative for each other and form what has been distinctive as separate minerals . Greenschist Facies Chemical weathering results from chemical changes to minerals that become unstable when they are exposed to surface conditions. Metamorphic amphibolites are a more widespread and variable group of rocks formed The mineral hornblende has only a few makes use of. Hornblende is a complex mineral and can be difficult to identify without a microscope. These minerals make up more than 80% of the rock. As some of these recycled crystals could have originated as entrained restitic or peritectic plagioclase, and others through reactions between melt and other Ca-bearing minerals (e.g., hornblende, garnet and clinopyroxene), there is ample opportunity for crystal cores to be out of textural, chemical and isotopic equilibrium with enclosing . The other main mineral that might be a portion of gabbro is. Hornblende is a very common mineral found in many geologic environments. Hornblende is dark in hand specimen and green to brown under the microscope. Tetrahedral substitution of aluminum for silicon is common and provides charge balance for trivalent and univalent substitution in the metal sites. This element of silicate mineral is a key constituent of igneous and metamorphic rocks. Amphibole, e.g., Hornblende, NaCa 2 (Mg,Fe,Al) 5 (Si,Al) 8 O 22 (OH) 2: Sheets, (Si,Al) 4 O 10-4 to -6: Micas . Hornblende can be found in both intrusive and extrusive rocks. Good Hornblende crystals also come from upstate New York regions in St. Lawrence Co. at Edwards, Pierrepont, Gouverneur, and Russel. Category - Silicate mineral; inosilicates; amphibole group, Mohs hardness- 5-6 ( Mohs scale 1 is talc, while 10 is for diamond), Streak - pale gray, gray-white, colorless, Diagnostic Properties - Cleavage (intersect at 124-56 degrees), color. Silicate mineral; inosilicates; amphibole group, CBSE Previous Year Question Paper for Class 10, CBSE Previous Year Question Paper for Class 12. What is the composition of the hornblende group? Metamorphic rocks with abundant hornblende are called amphibolites. Etymology derived from the Greek word amphibolos Hornblende has a Mohs scale hardness rating that ranges from just 5 to 6, which is similar to turquoise.Its specific gravity is roughly comparable to spodumene and diaspore, but depending on its exact chemical composition, its density can range from 2.9 to 3.4.Hornblende forms in the monoclinic crystal system with translucent to near-opaque clarity. Classification of Igneous Rocks. [11], There is a solid solution series between hornblende and the closely related amphibole minerals, tremolite-actinolite, at elevated temperature. Chemical composition is defined as the type of atoms bound within the molecules of chemical substances and their arrangements and ratios. It is a secondary mineral that forms as the result of alteration of hornblende, actinolite and biotite. They are an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an aluminium-iron-magnesium silicate, and an iron-magnesium silicate.The name hornblende is applied to a group of minerals that can be distinguished from each other only by detailed chemical analysis. Much of the pore water comes from the breakdown of micas or other hydrous minerals. Of course, the minerals found in the Earth's rocks are produced by a variety of different arrangements of chemical elements. The name amphibole (Greek - amphibolos meaning 'ambiguous') was used by Ren Just Hay to include tremolite, actinolite, tourmaline, and hornblende. The presence of cleavage can be used to distinguish it from black tourmaline that often occurs in the same rocks. The data are listed in e- Table 4, where each sample has both bulk-rock chemical composition and hornblende chemical composition (i.e., Al (apfu), total aluminum atom per formula unit calculated . Pumice Composition Pumice is primarily Silicon Dioxide, some Aluminum Oxide and trace amounts pf other oxide. Hornblende is usually black, green, or brown in color. These are worthless materials in which valuable metals or gems occur. Subscribe to Ask Any Difference! The X site can be occupied by sodium, potassium, calcium, or rare-earth elements. The highest quality pieces are cut, polished, and sold under the name "black granite" for use as building facing, floor tiles, countertops, and other architectural uses. Hey there! Image copyright iStockphoto / Anna Usova. Those with low aluminum content are associated with shallow depths of crystallization, while those with higher aluminum content are associated with greater depths of crystallization. Hornblende-rich rocks, such as gabbros and amphibolites, are used in the construction of buildings, sidewalks, and driveways. Full Record; Other . The kinds of changes that take place are highly specific to the mineral and the environmental conditions. Although sometimes, it can also be used as a gemstone. Chemical Composition: Felsic: Color: Predominantly white: Mineral Composition: Potassium Feldspar, Quartz, Sodium Plagioclase, Biotite, Hornblende: Tectonic Environment: Convergent Boundary - Intruded into batholiths above Andean-type Subduction Zone The general formula is (Ca,Na)2-3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2. Name Origin: Named for the composition and its relationship to hornblende. Pargasite occurs in high-grade metamorphic rocks that tend to have more aluminum. In the gem industry, hornblende is mostly famous for being an impurity included in other gemstones , but on rare occasion, some especially fine hornblende crystals may be cut and polished . It is mostly present as a gangue material. Let's imagine first a composition in the silica-undersaturated field. Hornblendes exhibit typical amphibole structures; these are based on double tetrahedral chains between which four metal sites are located. Lamprophyre is an igneous rock that is mainly composed of amphibole and biotite with a feldspar ground mass. Plagioclase contains significant amounts of an anorthite component. They are formed due to factors such as extreme pressures, temperature, or a mixture of both. View Amphibole Hornblende.pdf from GEO 011 at Visayas State University Main Campus - Baybay City, Leyte. Olivine is a silicate mineral with a general chemical composition of (Mg, Fe)2 SO4, however, magnesium and iron can be replaced with calcium, manganese, and nickel. Test your knowledge of rocks, minerals, and all things "yabba dabba doo" in this quiz. The angle between the cleavage planes and hornblende's elongate habit can be used to distinguish it from augite and other pyroxene minerals that have a short blocky habit and cleavage angles intersecting at about 90 degrees. A small list of the hornblende minerals is given below with their chemical compositions. The chemical composition of hornblende varies depending on the specific minerals in the group. Hornblende's generalized chemical formula is (Ca,Na) 2 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH) 2. . Amphibolite has a Large Number of Uses Which are: It is crushed and used for construction purposes. These elements are often used in the fertilizer and agriculture industries. It forms both igneous and metamorphic rocks. Hornblende thin section in PPL ( plane-polarized light) image ranges from green to dark brown. Hornblende is most often confused with the pyroxene series and biotite mica, which are also dark minerals found in granite and charnockite. Thus it is an important mineral ore that forms a constituent of various rocks used for day-to-day purposes. [15][16], The word hornblende is derived from German Horn ('horn') and blende ('deceive'), in allusion to its similar appearance to metal-bearing ore minerals. The mineral hornblende has very few uses. The introductory student or the beginning mineral collector can be satisfied to assign the name of "hornblende" to a specimen. Chemical Formula: Ca2[Mg4(Al,Fe+++)]Si7AlO22(OH)2 : Composition: . For detailed physical properties of hornblende, see amphibole (table). These rocks have been altered by heat and pressure, causing the hornblende to recrystallize. Omissions? Some minerals, like quartz, are virtually unaffected by chemical weathering, while . They have a vitreous to pearly luster and a Mohs hardness of 5 to 6. Thus it is notable that the hornblende mineral has a varied composition. Hornblende can also be found in some sedimentary rocks, such as shale and sandstone. Hornblende can also be used as a filler in paints, plastics, and rubber. Hornblende can be distinguished from different minerals as explained below-The angle between the cleavage planes and hornblende's elongate crystal habit can be used to distinguish it from augite and dark-colored pyroxenes. The plagioclase-hornblende-phyric andesite is a coherent unit. At this locality it also forms as the result of alteration of cummingtonite. Its chemical composition (and the element silicon, Si) was discovered by the Swedish chemist Jns Jakob Berzelius in 1823. hornblende Chemical composition - Ca 2 (Mg, Fe, Al) 5 (Al, Si) 8 O 22 (OH) 2 Hardness - 5-6 Specific gravity - 2.9-3.4 Transparency - Opaque As noted above, hornblende is a name used for a number of dark-colored amphibole minerals that are compositional variants with similar physical properties. Please refer to the appropriate style manual or other sources if you have any questions. feldspar (potassium or sodium), hornblende, biotite, muscovite. Temperature Inversion - Types, Conditions, Effects and FAQs, Jet Stream - Layers, Formation, Types, Facts and FAQs, Forest Fire - Types, Effects, Natural Disaster and Management, Fly Ash - Concrete, Bricks, Sources, Relation and Facts, Rivers - Origin, History, Formation and Uses, Typhoon - Formation, Structure, Differences and FAQs, Relation Between Temperature and Humidity, Hornblende chemical formula is as given: (Ca,Na). [6] It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Corrections? Hornblende is not a rare mineral, but it is not as common as some other rock-forming minerals, such as quartz and feldspar. Other common minerals include mica (muscovite and biotite) and hornblende (see amphibole). Hornblende is a silicate of aluminium, calcium, magnesium and iron with sodium. 5.2 Chemical Weathering. When molten rock, or melted rock, solidifies, igneous rocks are formed. Extrusive rocks are those that form from lava that cools and solidifies on the Earths surface. Obsidian is relatively soft with a typical hardness of 5 to 5.5 on the mineral hardness scale. The minerals are assigned the names Magnesio-hornblende, Ferrohornblende, Aluminio-ferro-hornblende and Aluminum-magnesio-hornblende. Hornblende noun. Hornblende is a product of magmatic and metamorphic processes. Link to MinDat.org Location Data. The name hornblende is derived from German words horn and blenden which refers to its similarity in appearance to the metal-bearing mineral ores. It has very long and thin crystals. This piece of biotite hornblende granite is an example. The general chemical formula for the hornblende group is XYZ2-3(Si,Al)4O11(OH,F)2. The four end-members and the cation content of their respective compositions are as follows: hornblende, Ca 2 (Mg 4 Al) (Si 7 Al); tschermakite, Ca 2 (Mg 3 Al 2 ) (Si 6 Al 2 ); edenite, NaCa 2 (Mg) 5 (Si 7 Al); pargasite, NaCa 2 (Mg 4 Al) (Si 6 Al 2 ). It is an isomorphous mixture of three molecules and the hornblende chemical composition is as stated: These minerals are difficult to distinguish by physical means. This granite as you can recall is commonly used in buildings for making tiles, countertops, and other architectural purposes. These minerals are typically black, green, or brown in color. A generalized composition for the hornblende group is shown below. These are rocks formed from some other type of previously present rocks. Chemical composition or chemical makeup is usually . What are the Different Uses of the Hornblende Mineral? This specimen is approximately 3 inches (7.6 centimeters) across. It is distinguished from gabbro on the basis of the composition of the plagioclase species; the plagioclase in . It occurs as phenocrysts in several varieties of extrusive igneous rocks, such an andesite. Hornblende occurs widely in metamorphic and igneous rocks. Now you must be thinking about what amphibolites are? Hornblende (Amphibole) Group: Chemically, the most significant differences between the pyroxene and amphibole groups are the addition of O- and OH-groups in the amphiboles, and the groups' different silicate structures. Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica.

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