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Like pyroxenes, amphiboles are classified as inosilicate (chain silicate) minerals. However, the pyroxene structure is built around single chains of silica tetrahedra while amphiboles are built around double chains of silica tetrahedra. In other words, as with almost all silicate minerals, each silicon ion is surrounded by four oxygen ions.
Pyroxenes were so named due to their presence in volcanic lavas, where they are sometimes found as crystals embedded in volcanic glass; it was assumed they were impurities in the glass, hence the name meaning "fire stranger". However, they are simply early-forming minerals that crystallized before the lava erupted.
Finally, the amphiboles are usually hydrated, that is, they have a hydroxyl group ([OH] −), although it can be replaced by a fluoride, a chloride, or an oxide ion. [122] Because of the variable chemistry, there are over 80 species of amphibole, although variations, as in the pyroxenes, most commonly involve mixtures of Ca 2+, Fe 2+ and Mg 2 ...
Pyroxenes differ in their cleavage planes, which intersect at 87° and 93°. [8] Hornblende is an inosilicate (chain silicate) mineral, built around double chains of silica tetrahedra. 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]
Purely pyroxene-bearing volcanic rocks are rare, restricted to spinifex-textured sills, lava tubes and thick flows in the Archaean greenstone belts.Here, the pyroxenite lavas are created by in-situ crystallisation and accumulation of pyroxene at the base of a lava flow, creating the distinctive spinifex texture, but also occasionally mesocumulate and orthocumulate segregations.
Hornblende is an amphibole, a group of minerals resembling pyroxenes but with a double chain structure incorporating water. Hornblende itself has a highly variable composition, ranging from tschermakite ( Ca 2 (Mg,Fe) 3 Al 2 Si 6 Al 2 O 22 (OH) 2 ) to pargasite ( NaCa 2 (Mg,Fe) 4 AlSi 6 Al 2 O 22 (OH) 2 ) with many other variations in ...
Cross and his coinvestigators later clarified that micas and aluminium amphiboles belonged to a separate category of alferric minerals. They then introduced the term mafic for ferromagnesian minerals of all types, in preference to the term femag coined by A. Johannsen in 1911, whose sound they disliked.
Single-chain inosilicates: pyroxenes; Double-chain inosilicates: amphiboles; Other inosilicates: Mineral structures with a tetrahedral unit, framework silicates. Cyclosilicates (subclass 9.C): Phyllosilicates (subclass 9.E): Tridimensional silicate frameworks: tectosilicates (subclass 9.F) Silica family (class 4, family DA): dioxosilicate [SiO 2]