MINERALS/Amphiboles 505 Al2 ẵSi6 Al2 O22 OH; Fị2 The replacement of Mg by Fe raises the refractive indices as does the substitution of Mg,Si by Al,Al; Mg-rich anthophyllites are optically negative, whereas the more Fe-rich anthophyllites and the gedrites are optically positive In general, these orthorhombic amphiboles are virtually unknown in igneous rocks, but occur in a great variety of metamorphic and metasomatic rocks The cummingtonite–grunerite series, ðMg; Fe; Mnị ẵSi8 O22 OHị2 , are monoclinic and show characteristic multiple twinning on (100); cummingtonite is optically positive, but the more iron-rich grunerite is optically negative Cummingtonite occurs in amphibolites derived by the regional metamorphism of basic igneous rocks and in hybrid rocks of intermediate composition The more iron-rich (and sometimes manganese-rich) grunerites are typical of the banded iron formations of regional metamorphism, where they form a characteristic magnetite–grunerite–quartz association In the calcic amphiboles, the tremoliteferro-actinolite series, Ca2 Mg; Fe2ỵ Þ5 ½Si8 O22 ðOH; FÞ2 , are colourless to yellow and green, and are essentially metamorphic minerals occurring in both contact and regionally metamorphosed rocks In the metamorphism of siliceous dolomites, tremolite forms early on by reaction between dolomite and quartz: 5CaMgCO3 ị2 ỵ 8SiO2 ỵ H2 O ! dolomite Ca2 Mg5 Si8 Oc rocks tremolite quartz 22 OHị2 ỵ 3CaCO3 ỵ 7CO2 calcite Both tremolite and actinolite are characteristic minerals in low-grade regionally metamorphosed ultrabasic rocks, and actinolite is a common mineral in the greenschist facies Hornblende is used to describe a specific calcic amphibole, Ca2 Mg; Fe2ỵ ị4 Al; Fe3ỵ ịẵSi7 AlO22 OHị2 , but the continuous chemical variations towards pargasite (see above) and tschermakite, Ca2 Mg; Fe2ỵ ị3 Fe3ỵ Alị2 ẵSi6 Al2 O22 OHị2 , as well as to Al-poor tremolite–ferro-actinolite tend to be called hornblendes in petrographic descriptions, when more appropriate names would pargasitic-, tschermakitic- or ferro-actinolitic-hornblende This is illustrated in Figure 3, showing the typical chemical variations in analysed calcic amphiboles The ‘hornblendes’ are typically pleochroic from pale green to yellowbrown or brown-green and have extinction angles in the range 12–34 (as compared with 45 for the clinopyroxenes) The hornblendes are typical minerals of intermediate plutonic rocks, and occur as products of primary crystallization of igneous rocks Figure The chemical variations of calcic amphiboles ex pressed as the numbers of (Na + K) atoms in A sites and Si atoms per formula unit Tr tremolite, Hb hornblende, Ed edenite, Pa pargasite, Ts tschermakite The more densely stippled areas show the more commonly occurring compositions ranging from granites (see Igneous Rocks: Granite) and syenites to gabbros and ultrabasic rocks They are neither as common or as abundant in volcanic rocks (due to the loss of volatiles from typical basalts and trachytes); they do, however, occur in a variety of andesites and dacites Hornblende is also a dominant constituent in many regionally metamorphosed rocks, from the greenschist to the lower parts of the granulite facies Kaersutite, NaCa2(Mg,Fe2ỵ)4Ti[Si6Al2O22](O, OH, F)2, is brown to reddish brown under the microscope and is characterized chemically by its high Ti content (TiO2 5–10 wt.%, equivalent to 0.5–1.0 atom pfu) It is a typical constituent of alkali volcanic rocks, occurring as phenocrysts in trachybasalts, trachyandesites, trachytes, and alkali rhyolites Among the sodic amphiboles, glaucophane, in the glaucophaneriebeckite series, Na2 Mg; Fe2ỵ ị3 Al; Fe3ỵ ị2 ẵSi8 O22 OH; Fị2 , is the iron-poor endmember and characteristically shows blue–violet pleochroism in thin section, and may give a blue shade to its rocks, forming typical blueschists as the result of regional metamorphism of basaltic rocks Riebeckite, the iron-rich equivalent, is an even deeper blue to almost black, and occurs in peralkaline granites, syenites, microgranites and in acid volcanic rocks The eckermannite-arfvedsonite series, NaNa2 ðMg; Fe2ỵ ; Mnị4 Al; Fe3ỵ ịẵSi8 O22 OH; Fị2 , is even richer in sodium and ranges compositionally between both the Mg and Fe2ỵ and the Al and Fe3ỵ