American Mineralogist, Volume 74, pages 1097-1 105, 1989 Cation distribution in the octahedral sites of hornblendes KuNuxr M.Lxruo, Klrsurosrrr Tovrru. Department Geology and Mineralogy, Kyoto University, Kyoto 606, Japan Ansrn-Lcr The structure refinements of five calcic amphiboles with t4rAl > 0.5 have been carried out in order to characterizethe cation distributions in the octahedral sites in amphiboles formed at diferent temperatures.The location and atomic fractions of Al and Fe3+in the octahedral M(l), M(2), and M(3) sites were determined together with other constituent cations on the basis of the site refinement and the relations between mean bond length and mean ionic radius. The studied specimensare two metamorphic pargasites,a volcanic pargasite,a volcanic magnesio-hornblende,and a hastingsitefrom a skarn. In the metamorphic pargasitesand the hastingsite,Al, Fe3+,and Ti occupy only the M(2) site, and Mg prefers the M(2) site to the M(l) and M(3) sites (Mg-Fe'z+partitioning; K${t>t"rtzr: 0.30 between the M(l) and M(2) sites). On the other hand, the volcanic pargasite and hornblende indicate a more disordered cation distribution among the octahedral sites than that from metamorphic rocks and skarn. INrnooucrrou tetrahedral sites in calcic amphiboles was demonstrated by Robinson et al. (1973) and Hawthorne and Grundy Calcic amphibole is an important rock-forming min- (r977). eral and occurs in extremely wide varieties of metamor- Finding the ordering of trivalent cations at the smaller phic, plutonic, and volcanic rocks. A chemicaldisconti- M(2) site in Bolivian crocidolite (magnesio-riebeckite) nuity in the calcic amphiboles exists betweenAl-rich and (Whittaker, 1949) and in glaucophane(Papike and Clark, Al-poor phases(e.g., Shido and Miyashiro, 1959; Ya- 1968) helped in understanding the crystal chemistry of maguchiet al., 1983).The Al-rich phasewith I4rAl> 0.5 the octahedralsites in hornblende;the smallerAl, Fe3*, is termed hornblende, subsumingthe appropriate names and Ti cations are confined to the M(2) site in horn- for amphiboledefined by Leake(1978), such as pargasite, blende,because it is usuallymuch smallerthan the M(l) tschermakite, and actinolitic hornblende. and M(3) sites.Robinson et al. (1973)demonstrated that The basic structural unit in calcic amphibole is a dou- the mean bond lengths of the octahedral siteswere a lin- ble chain of TOo tetrahedra extending parallel to the c ear function of the mean ionic radius of the constituent axis (Warren, 1929, 1930).The octahedralcation strips cations. This relationship was extended to the grand oc- are sandwichedbetween the double chains. There are two tahedralsite, which combinesthe M(l) sitewith the M(2) distinct tetrahedralsites, T(l) and T(2), which contain Si and M(3) sites,and to individual M(l), M(2), and M(3) and Al. The octahedral sites are subdivided into three sitesof the C2/mamphlbolesbyHawthorne (1978, 1981, crystallographicallynonequivalent sites, M( I ), M(2), and 1983). M(3), which accommodatevarious cations such as Mg, In the metamorphic cummingtonites, the Ca-poor am- Fe2+,Mn, Fe3*,Al, and Ti. The M(4) site accommodates phiboles with spacegroup C2/m or P2r/m, the Mg-Fe'z+ Ca, Na, Mg, and Fe2+.A large A site may be vacant or distributionbetween the M(4) site,and the M(l, 2, 3) site may containNa and K calions. was discussedby Hafner and Ghose(197l). The temper- Using data obtained from modern techniques,the ge- ature dependencyof the Mg-Fez+ distribution between ometry and chemistry of the cation sites in hornblendes the M(4) site and the M(1, 2, 3) site was suggestedand have been discussedin detail (Papikeet al., 1969;Kita- discussed by Mueller (1962) and Ghose and Weidner mura et al., 1975; Robinson et al., 1973; Hawthorne and (1972). The kinetics of the Mg-Fe2+exchange reaction Grundy, 1973;Hawthorne et al., 1980;Ungaretti et al., betweenthe M(4) site and the M(1, 2, 3) site in an an- l98l). The stereochemistryof the tetrahedral double thophyllite were discussedby Seifert and Virgo (1975). chains and the octahedral strips inthe C2/m amphibole On the contrary, the chemical complexity of horn- wassummarizedby Hawthorne (198 I, 1983). blende has obscured the temperature dependencyof its Papikeet al. (1969) observedthe preferenceofAl for cation distribution. In particular, the site preferenceofAl the T(l) siterelative to the T(2) sitein hornblendesbased (or other trivalent cations) in the octahedraat high tem- on the (T(IFO) and (T(2)-O) lengths.The correlation peraturehas not been studied. This study was undertaken betweenthe A1 content and the mean bond lensth of the to characterizethe distribution of cations among the oc- 0003-004x/89/09l 0-l 097$02.00 r097 1098 MAKINO AND TOMITA: OCTAHEDRAL CATION DISTRIBUTION IN HORNBLENDES Taere 1, Chemicalcomoosition of the hornblendes mass, emplaced in Sanbagawaschists, in Ehime Prefec- ture, Japan.The associatedminerals are hypersthene,au- t-P gite, and plagioclase.The pargasite crystallized in the sio, 41.50 41.03 38.82 41.07 47.87 granulitefacies 'C and 5-10 kbar, as estimatedby Alros 14.40 17.34 9.07 17.94 7.52 [750 Tio, 1.66 0.42 0.58 0.18 1.35 Yokoyama (1980)l and sufferedSanbagawa metamor- FerO3 1.96 5.57 3.59 phism [epidote amphibolite facies; about 600 "C and 8- FeO 12.60 5.04 25.87 7.79 12.74 13 kbar, estimatedby Yokoyama (1980) and Takasu MnO o.12 0.0 0.25 0.14 0.43 as Mgo 11.88 14.39 1.91 13.37 15.30 (1984)1.The pargasiteis slightly poorer in Mg and Al CaO 10.65 12.71 11.79 12.05 10.95 than the Einstddingen sample describedbelow. Naro 3.22 1.21 2.79 1.83 1.29 K.o 0.32 3.19 0.92 0.31 0.44 HrO* 1.15 2.31 1.46 Einstiidingen pargasite (E-P) H.o- o.27 0.57 The pargasiteoccurs in a skarn intercalatedwith garnet cl 0.65 0.20 -Cl=O 0.15 0.05 biotite gneissesin an islet of Einstiidingen, Liitzow Holm Total 96.35 98.94 100.30 10030 97.89 Bay, EastAntarctica (Matsubaraand Motoyoshi, 1985). Cations on ihe basis of 23 oxygens The studied specimen was supplied by Dr. Matsubara. 5l 6.19 5.99 6.31 5.92 6.90 The skarn consistsof aluminous diopside, potassianpar- AI 1.81 2.01 1.69 2.08 1.10 gasite, and phlogopite. The pargasite is the product of > 8.00 8.00 8.00 8.00 8.00 t4l granulite-faciesmetamorphism. The metamorphic tem- AI 0.72 0.97 0.05 0.96 0.17 perature pressurewere estimated to be about 800- Ti 0.19 0.05 0.07 0.02 0.14 and Fe3* 0.05- 0.22 0.68 0.39 0.43' 850'C and 8-10 kbar on the basisofthe clinopyroxene- Fe2* 1.52 0.62 3.52 0.94 1.11 orthopyroxenegeothermometer (Wood and Banno, 1973; Mn o.02 0.0 0.03 0.03 0.05 geobarometer Mg 2.64 3.13 0.47 2.87 3.29 Wells, 1977)and the garnet-orthopyroxene > t6l 5.14 4.99 4.82 5.21 5.19 (Harley and Green, 1982)by Matsubaraand Motoyoshi 1.70 1.99 2.05 1.86 1.69 (1985). The pargasiteshows a high K content and Mg/ Na 093 0.35 0.88 0.51 0.36 (Mg + Fe'?*).It containshigh t6tAl(0.97 pfu). K 0.06 0.59 0.19 0.06 0.08 Note; l-P, lratsu paragasiteanalyzed by electron microprobe.E-P, Ein- Obira hastingsite (O-H) stodingen pargasite; combinationby electron microprobe and wet-chem- The hastingsitewas from a skarn in the Obira mine, ical analyses(Matsubara and Motoyoshi,1985). O-H, Obira hastingsite analyzedby wet-chemicalmethods (Matsumoto and Miyashita, 1 960). P-P, Oita Prefecture,Japan. Chemistry and occurrenceof the Parau pargasite analyzedby wet-chemicalmethods (Tomita, 1965). K-H, hastingsite were described in detail by Matsumoto and Kawanabehornblende analyzed by electron microprobe. (1960).The hastingsiteis fibrous. Associated 'The amountof Fe3*oer formulaunit in amohibolewas derivedfrom Miyashita the total occupancy of Na and K in the A site determined by refinement minerals are datolite and stilpnomelane, which crystal- (afterHawthorne et al., 1980). lized later than other skarn minerals (garnet,wollastonite, hedenbergite,actinolite). Judging from the assemblageof skarn and ore-forming minerals, the hastingsite crystal- lized during the hydrothermal stage(Matsumoto and Mi- tahedral sites in hornblendesformed at different temper- yashita, 1960).The hastingsitehas a high Fe2+/(Mg * atures. Fe'*) and contains little Al in the octahedral sites. SpncrtunNs EXAMINED Parau pargasite(P-P) The following five specimenswere examined. Chemi- This pargasitewas found in andesitic agglomeratebe- cal compositions are given in Table 1, along with the longing to the Babeldaob agglomeratein Gapson, Parau methods of analysesfor each specimen. Island (Tayama, 1939).The Babeldaobagglomerate in- tercalateslimestone, shale,sandstone, and tufl suggesting Iratsu pargasite(I-P) submarine deposition. The andesitic agglomeratecon- Iratsu pargasiteis one of the constituent minerals of tains pargasite,clinopyroxene, and plagioclase.The crys- the basic granulite from the Iratsu epidote-amphibolite tallization temperature of the pargasitewas estimated to TABLE2. Crystaldata for the hornblendes,space group C2lm E.P o-H a (A) 9.805(3) 9.900(2) 9.967(4) 9.834(4) 9.829(8) b (A) 17.96(1) 17.95(2) 18.269(2) 18.01(3) 18.06(1) c (A) 5.302(1) s.311(2) 5.347(1) 5.2e7(2) 5.304(1) pf) 104.93(2) 105.42(2) 104.97(2) 105.04(2) 104.70(2) v(A') 902.1(8) 910.5(9) 940.05(9) 906.0(4) 910.2(8) Size(mm) 0.15x0.10x0.25 0.15x0.10x0,30 0.15x0.10x0.50 0.30x 0.20x 0.50 0.20x 0.10x 0.30 F (weighted) 0.037 0.049 0.057 0.058 0.076 No.of F 2234 2967 1344 2701 2234 MAKINO AND TOMITA: OCTAHEDRAL CATION DISTRIBUTION IN HORNBLENDES 1099 'C, be about 900 on the basis of the phaserelations in were correctedby the semiempirical method of North et tonalite (Wyllie, 1977).