American Mineralooist Vol. 57, pp. 1404-1412 (1972\ SIGNIFICANCE OF RIEBECKITE AND FERROHASTINGSITE IN MICROPERTHITE GRANITES Peur, C. Lvous, Boston Uniuers'ity,College ol Basic Studies,Boston, Massachusetts02215 Arsrnecn Riebeckite and ferrohastingsite occur separately in closely related hypersolvus granites of eastern Massachusetts. Mineralogical, chemical, and phase equilibria data suggest that, the Quincy riebeckite granite and the Peabody ferrohastingsite granite crystallized at similar temperatures, most, likely 6b0-750"C. The chemical composition of the parent magma and oxygen fugacity played important rolbs in determining the composition of the amphibole. Parent composition favored a calcium-iron-aluminum amphibole, ferrohastingsite, NaCazFer+rFd+AlzSiOzs(OH):, in the Peabody, and a soda-iron amphibole, riebeckite, NazFeF+sFd+zSi8O22(OH)2, in the Quincy. Higher oxygen fugacities favored the ferric-rich amphibole, in the Quincy; lower oxygen fugacities favored the ferric-poor arrphibole, ferrohastingsite, in the Peabody. Mineral relations in the Quincy suggest,an increase in oxygen fugacity with decrease in temperature. Two new chemical analyses, one of the Quincy granite riebeckite and one of the Peabody granite ferrohastingsite, are reported. INrnooucrrox Riebeckite (or arfvedsonite)and ferrohastingsiteare found in one- feldspar granites of Nigeria, New Hampshire, and eastern Massa- chusetts.The riebeckiteand ferrohastingsitegranites typically occur in separateplutons. In New Harnpshireand Massachusetts,a small amountof riebeckitegranite is assoeiatedwith ferrohastingsitegranite in the same pluton. The Quincy and Peabodygranites of easternMassachusetts will be usedto addressthe problem of the spatial independenceof riebeckite and ferrohastingsite.New chemicaldata are presentedfor the Quincy riebeckiteand the Peabodyferrohastingsite. This paper will attempt to explain the field relations of these amphibolesin light of their chemistry, mineralogiial and whole-rock chemical data, and phase equilibriastudies. The spatial relationsof the alkali graniteplutons of easternMassa- chusettsare shown in Figure 1. The Quincyl and RattlesnakeHill are riebeckitegranites, and the Peabodyand Cape Ann are ferrohast- ingsitegranites. 'Investigators should clearly state whether they are referring to the Quincy, Peabody, or Cape Ann granites, because using Quincy granite for a.ll three has causedconfusion in the literature (see Toulmin. 1964). t404 BIEBECKITE IN GRANITES 1405 cRANtrEt MASS. = QUINCY GRANITE --' -T-'-'-'-l RATTLESNA iirt"^D,uli..r enanrre l'lrlrl ot02030 MILES 30 71 30 Frc. 1. Location of the granite plutons. Mrxnner,ocrcAr,DATA Modal data for the Quincy and Peabodygranites are summarized in Table 1. Pyrochlorein the Peabodyand fluorite in the Quincy sug- gest that mineralizerswere presentduring the crystallizationof the magmas.The similar youngergranites of Nigeria (Borley, 1963) con- tain, in addition, cryolite and topaz, again indicating the presenceof mineralizers,particularly fluorine. The alkali amphibolesof Nigeria (Borley, 1963) and of Massachusettsand Rhode Island (Lyons, un- publisheddata) also contain considerableamounts of fluorine sub- stituting for OH. The absenceof aegirineand aenigmatitefrom the Peabodyand fayalite from the Quincy are noteworthy.Fayalite has not been reported from the Peabody,although it does occur in the closely-relatedCape Ann granite and is commonto the ferrohasting- site granitesof New Hampshireand Nigeria. The Peabodyand Quincy graniteshave counterpartsin New Hamp- shire and Nigeria; all three areashave ferrohastingsiteand riebeckite (or arfvedsonite)granites with strikingly similar mineralogicalcom- positions. Tbpically one granite is a riebeckite- or arfvedsonite- aegirinegranite and the other is a ferrohastingsite-fayalitegranite. 1406 PAUL C. LYONS labL 1 ATD CMMICAL DAII X@AL DATA CI{EMICAL DATI qriacyr !ruod8 lrcrthitr* 62.7 6t.z sig Qu{tt 29.o 25.o zr% .2o .2o (.2o) .2o .w trbrrobrstlng!ltc o.0 6.lr ^\% Lt.57 l-t.l? 12.09 11.61 12.98 12 1A 13.08 Rirbcckitc 5.i tr rcr\ 2.25 t.?I z.9L 2.2g .81 .2lt <2 l'.Blrfu. 1.8 0.o .Fto .93 J,.26 1.55 1.25 2.85 2.81 DE(,rcrc Tr .2 lfgo .0] .0L .08 .O5 .02 .11 Blotlt. .1 .J lho tr .O5 t! .oZ .o8 .10 .09 lregrctit .2 .7 Cao .l+b .L9 .31 .Ll I.O)r f ln HcDtltc tr o.0 Ns?o 1.21 lL03 lJ.6 lr.lo L.19 3.99 1.09 fhcnitc tr tr K2o \.62 L.58 U.53 b.5L t 6. 5.ar <in Su].phLd!s tr tr rto* .I9 .3b .lr1 .3L .2O .20 Ztrc6 tr .2 II2O- .OL .1O O.o .OIr !.d. .o8 .o8 SphoE tr tr lzg tr tr in tr n.d. NF .Vts .[Usitr o.o tr Tiq .n e2 J8 Jo .Jlt .to oJ| Ca1cltG tr 0.0 rotaT 99.76 99.87 [email protected] 1OO.1Btoo.13 99.56 99.e8 Apatit Auiglatit 0.o 13taoplyUlt6 tr ? Eluorit o.o lFochhe Ily31lt. S*Lcite tr tr 12 anrlylcri Dind conrtltunts ao apprdinatc (Iftrren, F- "1 19ll; Iyoror upub.). constitucnts cc apqcinatc (Touluin, 1951r; /lt!s;:i ": 3 T'""*r_*nor ryons, upub.). of 3 airalyscr f|l&cn, l9U). 'rba of 2 enelyaaa (Tdlrjr, 1951r). *CoroLsts of deocl.ir ad rlbttci iacludcr mircr aEout8 sln of s3pcatc albitc. placct, prrtly katophorit . The ferrohastingsitegranites of the NorthwestAdirondacks (Budding- ton, 1948)contain similar mineralsto the comparablegranites of New Hampshire (Billings, 1956) such as microperthite,minor plagioclase, quartz, biotite, and apatite. The New Hampshire ferrohastingsite graniteshave, in addition,traces of fayalite. NIEBECKITE IN GNANITES .,.Reer!rrnMrsrnY The chemicalcompg6itions of the Quincy and Peabodygranites are given in Table 1. Tle Quincy is higher in silica and soda,but lower in alumina,potash, and lime than is the Peabody.There is no signifi- cant differencebetween the total iron contentsof the two granites; the major differenceis in the oxidation state of the iron: the Quincy is ferric-rich and the Peabodyis feric-poor. Obviously,the oxidation state of the iron partly conditionedthe nature of the amphibolewhich crystallized from the magmas.This will be discussedlater in the paper. The ferric-rich nature of the Quincy is consistentwith its mineral- ogy,riebeckite and aegirine.The three chemicalanalyses of the Quincy granite consistentlyshow ferric ion dominant over ferrous ion. The ferric/ferrous ion ratio of the riebeckiie in the Quincy pegmatite (War- ren and Palache,1911) is nearly identical to that of the riebeckiteof the Quincy granite proper,which suggeststhat similar oxygenfugaci- ties prevailedduring their crystallization. The higher silica content of the Quincy is reflected by more qtartz in its mode. The higher alumina and potash contentsof the Peabodyare shown by a greater modal amount of perthite and a more aluminous amphibole. The amphibole compositions (Table 2) indicate the dif- ferencesin lime and sodalthe Peabodyhas a calcium amphiboleand the Quincy has a soda amphibole.The marked distinction in the ferric and ferrous iron contents is clearly revealedby the amphiboles;the Quincy has riebeckite,Na2Fe2*rFet*rSirOrr(OH)r; the Peabodyhas ferrohast- *nFea*AlrSi6Or, ingsite,NaCarFe' (OH)r. AMpHrsoLpCnniursrny A summaryof the chemicalcompositions of the two amphibolesis givenin Table 2. The chemicalcomposition of the Peabodyamphibole is very similar to the compositionsof the ferrohastingsitesof Northern Nigeria (Borley and Frost, 1963) and the hastingsitesof the Adiron- dack hornblendegranites (Buddingtonand Leonard, 1953). The atomic proportions of the various constituentsin the two amphi- boles are shown in columns (7) and (8) of Table 2. The formula of the Peabody ferrohastingsiteis: Na6."sK6.raCa1.76,Fe'*r.urMgo.a7Mns.13 Lis.s3, Fe8*.."eTio.23A-10.06,Alr.5ssi6.41, (OH)o.elFo.ra. The data for the Quincy amphiboleindicates an arfvedsoniticriebeckite, or, if you prefer, a riebeckitic arfvedsonite.The formula is: Nar.eeKr.rrCao.or,Fe'*r.ru Mgo.orMno.ro, Fet*r.url,io.rnTio.o., Alo.4nTio.lESi".nr,(OI[)r.rrFo.ur. In contrastingthe chemicalcompositions of the amphibolesof the Quincy and Peabodygranites, note that the major differencesindicated 1408 PAUL C. LYONS Teblc 2 1IIPIIIBOIEDATA _* (1) si02 L8.7lr L{3Jer,{3ls ',J};. s[2e J.5J' s,. {li" J:h [ao3 2.3L 2.36 7.29 9.08 9.7h 8.51 rlrE J$9 i..t9 Tioz 1.L8 t.53 .67 2.88 3.10 1.89 .llE .ooo .05 rcloj 13.22 t2.9L 5.75 6.59 7.06 6.11 Tr' .t5? .oo rbo .19.95 N.92 27.78 23.09 2\.8? 26.30 Ti .028 .23 l[nO .7b .76 .97 .77 .83 .90 rb(rrr) 1.532 .79 Itgo .29 .30 1.90 1.83 a.96 a,93 tu(rr) 2,7t5 3.62 r4eo .35 .38 u.d. .0lr .olr .olr E .o?o .ll? cao 3,6I 3.76 9.5t+ 9.oJ 9.73 9.61+ !r| .101 .13 Fzo 6.Q1 S.ge 2.86 1.99 2,L5 2,5L Ii .2w .03 (zo Lu l.2z .99 1,2o 1.29 1.11r ce .63b 7,70 F f lt 1.16 .o5 .tp .4> .4> Na 1.820 .8o B2O* a.96 1.61r t.ot .56 .6 .83 r. .2W .zl+ HaO- .27 .28 .88 .# .e5 oE 1.728 .91 Total 100.81 10O.2lr100.00 9.38 99.38 100.1r r .578 .21+ -0:s .W ,IL9 .oz .18 .19 .t9 iff,T-Te-$' ffi 9-i'F WE (l) Rietrcldte, QuiacEr grettc, rcr ana\rsis; Tailashi A8arl, analyst. til LT::Fl,"i !*fry_s.r*b, sorrectedfor lrpur$ies (:.ol quartr, z.Blt qejfltrc). fErrohaltlngsitc, leabody granltc, Toulntn (1!6[); conEcted for jtmftlcJ. granitc, *a"lp:9+"S"11c, tcaboff. w analyslsi Tadashi A!ai, Enalyst . -rcrrohstllgritcr_leabody Eranite, cotrected for irFrities @.4 qr,z, .31 bLobf:@). ltee of (3) and (5). El.etogkil?, Qdncjr_grantie (2)i_I{o:_gf ioB on basis of ZIr (O,OHrF) rbltohastingsltc. lcaboqr granitc (6); ilo. ot lons onlasis'oi-2-t-to,OBrF). t,hc_pnaclslon of tlr ttata for thc uaJor cmstittrcnts of thc tro n6r mlyscs pcfcsnt. is l.-2 Ths ir€cislo! b tlrc flZO+ concentratl,onr ic pocr snd nay be ix ^high as 509 dcvlatioa fron.