AMPHIBOLE GROUP MINERALS

AMPHIBOLE GROUP MINERALS  The amphiboles are a group of hydroxylated chain silicates with some substitution of F and Cl for (OH).  They include both orthorhombic and monoclinic members.  The amphiboles contain essential (OH) group in the structureand the Si:O ratio is 4:11, not 1:3 as in pyroxenes.  The general formula for all members of the amphibole group can be written as ( W,X,Y) 7-8 ( Z4 O10)2 (OH)2., In which the symbols W,X,Y,Z indicate elements having similar ionic radii and being capable of replacing each other in the structure. W stands for large cations Ca and Na sometimes K. X for smaller cations Mg and Fe2+ sometimes Mn  Y for Fe3+,Ti, and Al in six-coordination  And Z for Si and Al in four coordination The main physical and optical difference between the amphiboles and pyroxenes. Amphiboles:  Crystals elongate prisms, often acicular or bladed.  Basal sections are six sided and ‘diamond’ shaped  Two prismatic cleavages meet at 124 degree in the basal sections  Usually pleochroic  Maximum extinction angle (measured to cleavage) on an (010) section= ~20, biaxial negative(rarely positive) with large 2V (>70)  Exsolution lamallae very rare or not present  Twins common, with no re-entrant angle. Pyroxenes:  Crystals squat prisms  Basal sections are eight-sided and square shaped  Two prismatic cleavages meet at 88 degree in the basal section  Non-pleochroic except for the Na-Fe3+bearing varieties.  Maximum extinction angle (measured to cleavage) on an (010) section= ~48, biaxial positive(rarely negative) with large 2V (50-60)  Exsolution lamallae common in cpx and opx from basic igneous rocks.  Twins rare with a re-entrant angle Monoclinic amphiboles  Cummingtonite series: cummingtonite (Mg, Fe)7 Si8 O22 (OH)2 Grunerite Fe7 Si8 O22 (OH)2  Actinolite series: Tremolite Ca2 Mg5Si8 O22 (OH)2 actinolite Ca2 (Mg,Fe)5Si8 O22 (OH)2 Hornblende (K,Na) 0-1 (Ca, Na, Fe, Mg)2 (Mg, Fe, Al)5 (Si, Al)8 O22 (OH)2 Na- amphiboles: Glaucophane Na2 Mg3 Al 2Si8 O22 (OH)2 Riebeckite Na2 ( Fe,Mg)3 (Fe,Al) 2Si8 O22 (OH)2  Orthorhombic amphiboles Anthophyllite (Mg,Fe)7 Si 8 O22 (OH)2 Gederite (Mg,Fe, Al)7 (Al, Si) 8 O22 (OH)2  Calcic amphiboles are monoclinic  Ca- free amphiboles are orthorhombic  The name hornblende is used by petrologists to refer to the common black amphiboles found in many igneous and metamorphic rocks. Anthophyllite (Mg,Fe)7 Si 8 O22 (OH)2  Name from the Latin anthophyllum, meaning ‘clove leaf’ referringt to this mineral’s colour.  In hand specimen it is characterized by it its clove- brown colour, usual prismatic habit, and prismatic cleavages.  Anthophyllite is usually Mg- rich; iron rich compositions yield cummingtonite. Al and Na may be present in anthophyllite; if Al content is great enough, the amphibole is called gederite.  Anthophyllite is found in low- grad Mg- rich metamorphic rocks where it may be associated with cordierite.  Amosite is asbestiform variety of anthophyllite. cummingtonite (Mg, Fe)7 Si8 O22 (OH)2  Named after Cummington, Massachusetts, its type locality.  Forms prismatic, fibrous crystals,aggregates of radiating fibers or blades are common.  Occurs in mafic or marly medium grade metamorphic rocks.  Amosite is asbestiform amphibole similar to Fe- rich comingtonite.  Commingtonite is closely related to the other ambhiboles and is polymorphic with the members of the anthophyllite series. Grunerite Fe7 Si8 O22 (OH)2  Named after L.E Gruner, a 19 th century mineralogist, who first analyzed grunerite.  Typically forms fibrous, bladed or columnar crystals often radiating.  Found with Fe- rich minerals such as hematite, magnetite, hedenbergite, fayalite or garnet in metamorphosed iron rich sediments. Tremolite Ca2 Mg5Si8 O22 (OH)2  Named after Val Tremola, Switzerland,. Where it was first found.  Tremolite is typically prismatic  Mg rich endmember of the calcic amphibole series.  Complete solid solution between tremolite and Fe- actinolite.  Intermediate compositions are simply termed as actinolite.  Tremolite Is one of the first mineral to form when impure carbonate are metamorphosed. [...]... needles- either radiating or in parallelel aggregates, or columnar massees Actinolite is characteristic of medium- grade metamorphosed mafic rocks It is one of the minerals that gives greenschists their characteristic colour Associated minerals typically include albite, epidote, chlorite and quartz Hornblende (K,Na) 0-1 (Ca, Na, Fe, Mg)2 (Mg, Fe, Al)5 (Si, Al)8 O22 (OH)2  Name from the German horn... glaucophane  Compositions intermediate between glaucophane and riebeckite are called Crossite  Glaucophane is a high pressure metamorphic mineral characteristic of blue schist facies  Other blue schist minerals include jadeite, lawsonite and aragonite . AMPHIBOLE GROUP MINERALS  The amphiboles are a group of hydroxylated chain silicates with some substitution of F and Cl. members.  The amphiboles contain essential (OH) group in the structureand the Si:O ratio is 4:11, not 1:3 as in pyroxenes.  The general formula for all members of the amphibole group can be. Si) 8 O22 (OH)2  Calcic amphiboles are monoclinic  Ca- free amphiboles are orthorhombic  The name hornblende is used by petrologists to refer to the common black amphiboles found in many