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Two-Amphibole Assemblages in the System Actinolite-Hornblende Glaucophane 1

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Two-Amphibole Assemblages in the System Actinolite-Hornblende Glaucophane 1 THE AMERICAN MINERALOGIST, VOL. 54, JANUARY.FEBRUARY, 1969 TWO-AMPHIBOLE ASSEMBLAGES IN THE SYSTEM ACTINOLITE-HORNBLENDE_ GLAUCOPHANE 1 Connor.rs KrnrN, ln., Departmentof GeologicalSciences, H araar d U niv ersity, Carnbrid ge, M ass ac hus elt s 0 2 I 38 . Aesrnlcr This study gives electron probe analyses and optical descriptions for twenty-two amphibole pairs, which include, actinolite-hornblende, actinolite-glaucophane, hornblende- glaucophane, hornblende-hornblende, and several pairs which are intermediate in composi- tion between hornblende and glaucophane. One pair is quoted from the literature. fn actinolite-hornblende pairs the hornblende always has the lower Mg/(MgfFe) ratio. The one hornblende-hornblende pair shows a similar fractionation. In actinolite- glaucophane pairs glaucophane has lower Mg/(MgfFe) as well as lower Si/(Si+Al) ratios. rn four out of six hornblende-glaucophane pairs the glaucophane has a smaller Mg/(MgfFe) ratio than the hornblende, whereas in two pairs this is reversed. The Si/(Si+Al) fractionation between these two highly aluminous phases is very irregular. Four pairs, which consist of glaucophane and hornblende-glaucophane compositions, have Mg/(MgfFe) and Si/(Si-|AI) fractionation patterns very different from those observed in the other pairs. In a few of the 2-amphibole assemblages homogeneous, separate grains of the two amphiboles coexist in random intergrowth, but the majority of textures show complex zonation and patches of the two amphiboles in composite grains. The contact between the two amphiboles has been found to be optically and chemically sharp in every 2- amphibole assemblage, except one. These 2-amphibole assemblages are evidence for the probable existence of miscibility gaps in the system actinolite-hornblende-glaucophane at relatively low temperatures. The majority of the rocks in which these assemblages occur are part of the greenschist or epidote amphibolite facies, and the glaucophane schist facies, all of which were formed at relativelv low temoeratures. I*t*ooo"tro* Many studies have dealt with the extent of various chemical sub- stitutions in the clinoamphibolesof tremolite-actinolite ICaz(Mg,Fe)r- Sisorr(OH,F)21, hornblende [(Na,Ca)s-s(Mg,Fe2+)a(Al,Fe3+)SioAlzOzz- (OH,F)rl and glaucophane-riebeckiteINarM&Alr(Siooz)(OH,F)z to Na2Fe32+Fe2t+(Sirorr)(oH,F):]compositions. Some of these studies are basedupon collectionsof large numbers of chemicalanalyses for green, blue-greenand blue clinoamphiboles(Kunitz, 1930, Hallimond, 1943, and Sundius, 7946), and other studies are based upon the occurrence and analysisof two or more optically distinct Na-Ca clinoamphiboles in one assemblage(for references,see Table 1). In his study of the hornblende "family" Kunitz (1930) presents analyses for several amphiboles that are intermediate in composition betweentremolite-actinolite and glaucophane.Similar examplesof two "riebeckite-tremolite" compositions are given in Klein (1966). Such 1 Mineralogical Contribution No. 459, Harvard University. 212 TWO.A M P H I BOLE ASSEM B LAGES 213 Tarlr 1. Lrtnnatunn Rnroms ol Conxtsrrnc Allprrrtor,rs rN rur RewGB oF AcrrNolrrE-HopNnr,rNon-Sooruu AMprrrgoLB ColrlosrrroNs Coexisting actinolite and hornblende Reference, rock type and location Description Comfton (1958). IIafic greenstone(albite- Actinolite abruptly overgrown by green hornblende. Sharp actinolite-hornblende,epidote chlorite- boundary between the two amphiboles Electron probe leucoxenecalcite) Bidwell Bar region, Calif analysesare given in Table 2 (2-3) Compton's paper gives complete analyses for two amphiboles that were not in physical contact Dunham (pus conru,) Q\artz dioilte Colorless actinolite and green hornblende coexist in single (q .plagioclase-biotire- uartz-mi crocl i ne amphibole grains The margins betwe€n the 2 amphiboles hornblende-actinolite) Blodgett Forest are o{ten irregular but sharp Chemical analyses given in Pluton, El Dorado County, Calif Table 2 (2-4\ Miyas hiro (1958) Actinolite-epidote Blue-green hornblende occurs as rims around actinolite chlorite-plagioclase-hornblendeschist, with crystals, sometimes as patches within actinolite crystals mrnor amounts of quartz, calcite and biotite and sometimes as independent crystals In all cases the (Zone "B"). Gosaisyo-Takanuki district, boundary betrveen the 2 amphiboles is sharp Checks for Abukuma Plateau, Japan sharpnesswere made with tilted sections on the universaI stage ,Seilsaui (195 6) "Plagioclaseporphyrite, Blue-green hornblende with almost colorless or faintly consisting of plagioclase (An5-6), biotite, greenish kernels The two difierent parts of the amphibole hornblende, actinolite and epidote Tampere crystals are homoaxial Seitsaari gives complete analyses schist belt, Finland. for the dark and lighter colored amphibole fractions Shid,oand. Miyashi.ro (.1959).Schists similar Hornblende occurs in parallel grorvth with, as patches to those given above under Miyashiro, and within, and also as rims around the actinolite. The bound epidiorites (hornblende-chlorite-epidote- ary between the two amphiboles is ahvays sharp Sharp- oligoclaseschist) Irom Aberfeldy, Perthshire, ness of the contact was checked in several samples with Scotland tilted thin sections Electron probe analyses oJ one such occurrence are given in Table 2 (2 2) Wisman (1931) "Abnormal epidiodte, (a Describes hornblende of variable color in an epidiorite; uralitized gabbro consisting of labradorite, sometimes it shows a light green kernel with a blue-green augite, biotite, and 2 types of amphibole)_ rim The description suggests actinolite and hornblende Southwest Highlands, Scotland compositions. Coexisting hornblende (or actinolite) and Na amphibole, and zoned Na-amphiboles Banno (1958) Glaucophane schists, Omi Colorless actinolite and blue green glaucophane parts in district, Niigata, Japan. single crystals Usually the cores of the crystals are actin- olite, the rims glaucophane lIowever, the reverse is also observed Bloram (1960) Crossite anphibolite com- Hornblende mantled by crossite rims Complete hornblende posed of hornblende, crossite, epidote, al- and crossite analyses are given by Bloxam The Ca-content bite, sphene, apatite and garnet Girvan- of the crossite is probably high because of epidote and Ballantrae Complex,South Ayrshire, green hornblende contamination (Bloxam, personal Scotland. communication), BorS Q956) Glaucophane schists near Hornblende in cores of glaucophane crystals Borg inter- Healdsburg, Calif. prets these occurrences as glaucophane replacement ol hornblende Dudley (1967 ). Glaucophaneschists, Coexisting actinolite and glaucophane; as zones in single Tiburon Peninsula, Marin County, Calif. crystals and as separate crystals 2t4 CORNELIS KLEIN Taer,n 1. (Conhinued) Reference, rock type and location Description Ernst,et otr (pets-c,mn ) Glaucophaneschists Actinolite and glaucophane coexist in parts oI single from Tiburon Peninsula and Goat I\4ountain, crystals, and as separate crystals. Electron probe analyses Calif and the Shirataki district, Shikoku, are given for six such pairs, and conventional analyses for Japan one other pair Iuao (1939). Monzonite and dolerite Zoned crystals with hastingsite cores and arfvedsonite Barkevikite Nayosi district, Sakhalin, Japan rims with a transitional layer between them crystals rimmed by brorvn-greenhornblende The contact between the hornblende and the barkevikite appears to be sharp Izoasaki (1 960a) Magnesioriebeckite- Three layer zoning in amphiboles: a nearly colorless core (magnesioriebec- barroisite-Mg-rich glaucophane-garnet- (X4g rich glaucophane), colored mantle muscovitepiedmontite-aegidne albite- kite) and a rim of green amphibole A complete chemical quartz schists Eastern Shikoku, Japan analysis is given lor the magnesioriebeckite (barroisite is Iuasaki (1960b) "Quartz schistsnsimilar to Barroisite rims arcunC glaucophane cores those described above Eastern Shikoku, similar in composition to hornblende with a large Na of mag- Japan component, Deer eI al', 1963) Also description nesioriebeckite cores with barroisite rims Iwasaki (1963) Glaucophaneschists and Gives examples of many types of zonation: actinolite magnesioriebeckite-Mn-garnetpiedmontite (core)-glaucophane (rim); glaucophane (core)-actinolite stilpnomelane-quartz schists Kdtu Bizan (rim) ; glaucophane (core)- barroisite (rim) ; magnesiorie- (core) (rim); (core)- arear eastern Shikoku, Japan beckite barroisite magnesioriebeckite actinolite (rim); barroisite (core)- glaucophane (rim); barroisite (core)- magnesioriebeckite (rim) In all cases the boundaries between tbe 2 amphiboles were lound to be sharp Electron probe analyses of similar occurrences are siven in Tables 4 (4-3) and 5 (5-2, 5-3) in- Lee et al (1966). Glaucophane schists, Actinolite and glaucophane are present as distinct grain Cazadero area, Sonoma County, Calif dividual grains, but more olten a single amphibole may consist oJ both actinolite and glaucophane Complete conventional analyses for the 2 amphiboles are given Miyashiro and. Iwasoki (1957) Garnet- Detailed description of zoned magnesioriebeckiteof which The aegirine-amphibole-muscovite-quartz schist the core is colorless, the rim blue and pleochroic. with minor hematite and apatite Bizan, boundary between the rim and core is said to be sharp. A given for the blue rim material Shikoku, Japan complete analysis is (magnesioriebeckite)and the cote is stated to be a mag- nesioriebeckite witb less Fe, on the basis of optical properties blue cores and Van der Plas ud. Iliigi 11961).l|tretamoryhic
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