Spinel group and haematite group

Spinel group and haematite group

• 1.
  Chemical formula, structure,diagnostic properties and occurrence of Group Spinel and Hematite Group
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  Spinels generally referto metal oxides having general formulaAB2X4 Where A=a divalentcationlike Mg,Cr, Mn, Fe, Co, Ni, Cu, Zn,Cd, Sn B = trivalent like Al, Ga, In,Ti,V, Cr,Mn, Fe, Co, Ni X= ,O, S, Se .,etc. The spinel that is most popularly known is MgAl2O4 Spinel, (Mg,Fe)(Al,Cr)2O4, is common in PERIDOTITE in the uppermost Earths Mantle, between approximately 20 km to approximately 120 km, possibly to lower depths depending on the chromium content. At significantly shallower depths, above the Moho, calcic PLAGIOCLASE is the more stable aluminous mineral in peridotite while GARNET is the stable phase deeper in the mantle below the spinel stability region. Spinel, (Mg,Fe)Al2O4, is a common mineral in the CA-AL-RICH INCLUSIONS (CAIs) in some CHONDRITIC METEORITES . Spinel
• 3.
  Crystal StructureofSpinels Spinelstructures areusuallycubic close-packed oxides with one octahedral and two tetrahedral sitesper formula unit. B3+ions occupy half the octahedral holes, while A2+ions occupyone-eighth of the tetrahedralholes. Spinel is a common high-T mineral occurring in contact metamorphosed limestones and metamorphic argillaceous rocks poor in SiO2. Occurs also as accessory mineral in many dark igneous rocks. In contact metamorphosed rocks it is associated with phlogopite, Pyrrhotite, chondrodite and graphite. Found frequently as rolled pebbles in stream sands. The ruby spinel found in this way often associated with the gem corundum.
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  Due to greaterrefractive index, spinels are generally used as Gemstones. These spinel gemstones are considered very rare and precious. Their crystalline structure isidentical to natural spinel, So surprisingly, they can mimic the blues of aquamarine, zircon or blue topaz, and blue sapphire. 398.72- carat Catherine the Great’s ruby Black Prince’s Ruby TimurRuby Samarin Ruby • The fourth largestSpinel. • Itisa 500-carat spinel gemstone.
• 5.
  • Hematite, alsospelled as haematite, is the mineral form of iron(III) oxide (Fe2O3), one of several iron oxides. It is the oldest known iron oxide mineral and is widespread in rocks and soils[ •Hematite is colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. • Hematite is harder than pure iron, but much more brittle. Maghemite is a hematite- and magnetite-related oxide mineral.
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  Structure  Hematite, Fe2O3(iron = orange; oxygen = red). A hexagonal (rhombohedral) crystal system constructed of iron atoms surrounded by six oxygen atoms not at the corners of a regular octahedron.
• 7.
  Diagnostic Properties  Crystalsystem: TrigonalCrystal classHexagonal scalenohedral (3m)  Color: Metallic gray, dull to bright red  Crystal habit: Tabular to thick crystals; micaceous or platy, commonly in rosettes; radiating fibrous, reniform, botryoidal or stalactitic masses, columnar; earthy, granular, oolitic  Twinning: Penetration and lamellar  Cleavage:None, may show partings on  Fracture: Uneven to sub-conchoidal  Tenacity: Brittle  Mohs scale: hardness5.5–6.5  Luster: Metallic to splendent  Streak: Bright red to dark red  Diaphaneity:Opaque  Specific gravity: 5.26  Optical properties: Uniaxial (-)
• 8.
  Occurrence and uses Hematite is one of the most abundant minerals on Earth's surface and in the shallow crust. It is an iron oxide with a chemical composition of Fe2O3.  It is a common rock-forming mineral found in sedimentary,metamorphic, and igneous rocks at locations throughout the world.  Hematite is a minor gem material used to produce cabochons, beads, small sculptures, tumbled stones, and other items.  The material used to manufacture these products is a silver-colored hematite with a solid, uniform texture. The bright silver color of hematite and its "weighty feel" make it a very popular tumbled stone
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