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عدد المساهمات : 108
تاريخ التسجيل : 20/03/2010
العمر : 32
|موضوع: INOSILICATES 23rd نوفمبر 2010, 11:18 pm|| |
InosilicatesInosilicates are chains of linked silica tetrahedra sharing oxygen(s). There are single chain inosilicates called pyroxene (SiO3), and double chains called amphibole (Si4O11)(OH)2. The pyroxenes for which you are responsible are diopside, augite, and orthopyroxene; the amphiboles are tremolite-actinolite and hornblende. In lab you will notice that augite and hornblende are very similar in their physical properties. Both are monoclinic, both are black and vitreous, they have similar hardness and specific gravity. The repeat distance in chains along the c-axis is about the same distance in each mineral; the distance along the a-axis is similar, however, along the b-axis, the repeat distance in the amphibole is twice that of the pyroxene. Each mineral has two cleavages; however, cleavage in augite is at about 90º whereas, in hornblende, it is about 60/120º.
Pyroxene in general tends to crystallize in stout prisms whereas amphibole crystallizes in elongated crystals that are usually acicular. Pyroxenes form at higher temperature than amphibole in igneous melts. Also amphiboles occur in lower grade metamorphic rocks than pyroxene.
Pyroxene: has the general formula XYZ2O6, where X= Na+, Ca+2, Mn+2, Fe+2, Li+; Y= Mn+2, Fe+2, Mg+2, Fe+3, Al+3, Cr+3, Ti +4. In general the X cation is larger than the Y. Z= Si+4 or Al+3. One way to remember the pyroxenes that you need to know, is to use the pyroxene quadrilateral as defined below.
Enstatite-Hypersthene (Mg,Fe)SiO3 is orthorhombic and forms short stubby crystals. There is solid solution between the Mg and Fe end members, however the Fe:Mg ratio is rarely > 1:1. The maximum amount of CaO that can fit in the structure is 1.5 wt%. The Ca ion is a large ion and doesn’t fit into the orthorhombic structure. It is usually recognized by its brown color, cleavage, and unusual bronzy-iridescent luster.
Occurrence: Orthopyroxene is found in gabbro, basalt, and ultramafic rocks. It is usually associated with augite, olivine, and plagioclase (Ca rich).
Diopside-Hedenbergite: Is monoclinic and occurs in prismatic crystals, and has imperfect cleavage at 87º and 93º. It is green in color
Occurrence: Diopside-Hedenbergite occurs in metamorphic rocks. It is formed during the following reaction:
Dolomite + Quartz = Diopside and CO2 ↑
It is associated with tremolite- garnet and titanite.
Augite: is monoclinic and is the most common pyroxene. It is commonly found in igneous and metamorphic rocks. It is black in color and is distinguished from massive hornblende by its cleavage.
Amphibole: Its general formula i:
where W= Na+, K+
X=Ca+2, Na+, Mn+2, Fe+2, Mg+2, Li+
Y=Mn+2, Fe+2, Mg+2, Fe+3, Al+3, Ti+4
Amphiboles can be either monoclinic or orthorhombic but the minerals that we learn are monoclinic. They are tremolite-actinolite and hornblende.
Tremolite-Actinolite: Ca2Mg5Si8O22(OH)2 to Ca2(Fe,Mg)5Si8O22(OH)2 tends to be prismatic, bladed and radiating fibrous. It has perfect cleavage at 60/120º, a vitreous to silky luster and is white to green to green-black in color. The color is dependent on the amount of iron in the structure. If it is white, it is tremolite. If it is green in color, it is actinolite. It is distinguished from pyroxene by its cleavage and from hornblende by its lighter color.
Occurrence: Tremolite occurs in metamorphosed dolomite and limestone. It results from the following reaction:
dolomite + quartz + water = tremolite +calcite + CO2↑
5CaMg(CO3)2 + 8SiO2 +H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2↑
At higher temperatures, diopside is made instead of tremolite.
Actinolite is a characteristic mineral of greenschist facies mafic rocks, that it, it occurs in low grade metamorphosed basalt.
Hornblende: If Al is substituted into the Si (Z) tetrahedral site, Ca and Na are added into the W site and we get: (Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(0H)2. It is monoclinic, prismatic, with perfect amphibole cleavage. It is dark green to black in color.
Occurrence: It is a common mineral occurring in igneous rocks and in metamorphosed mafic rocks.