AmericanMineralogist, Volume 64, pages 1027-1035, 1979 Titanium, fluorine, and hydroxyl in the hrrmiteminerals Paul H. RBns Department of Geological Sciences Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061 Abstract In the humite homologousseries, nMrSiO 4' M F,TL,(F,OH)2-2,O2,(where n : I for nor- bergite, 2 for chondrodite, 3 for humite, and 4 for clinohumite, M : Mg >> Fe > Mn > Ca,Zn,Ni and 0 < x < l), x appearsnever to exceed-0.5 Ti atomsper formula unit, because local electrostaticcharge imbalances at the 3-coordinated(F,OH,O) anion and the 4-coordi- nated oxygen atoms increasevery rapidly as Tia* substitutesfor trP*, even with the con- comitant substitutionof 3-coordinated02- for (F,OH)'-. Both electrostaticand geometricar- gumentssuggest that Ti ordersinto the M(F,OH)O "layer" of the humite structures,t.e., into the M(3)OIV(F,OH,O)|I octahedron,which is the smallestof all octahedrain all the humite minerals(cl Fujino and Tak€uchi, 1978). Refractive indices, density, unit-cell dimensions, and volume are dramatically affected by the substitution of OH for F, and this has been studied as chemistry varies from 1.0 < (F,oH)'ill[(F,oH)"' + o''] = 0.0and 0.0< silv/2(F,oH,o) < l,/8 from sellaiteMg(F'oH), (n : 0) through the humitesto forsterite(r : o1. The volume per anion increasesmuch more rapidly as OH substitutesfor F in theseminerals than would be expectedon the basisof ob- serveddifferences in individual M-(F,OH) distances.As Yamamoto (1977)noted, the effec- tive radius of OHIII would be -0.06A larger than that of Fttt based on individual bond lengths.However, if the grand mean M-(F,OH,O) distanceminus the effectiveradius of the M cationis used,OHIII would have to be -0.15A larger, and if the efective anion radius cal- culated from the observedvolume per anion is used,values -0.10A larger are expected.Pro- ton-proton repulsions,which increasedramatically both within a given mineral series(say F- to OH-chondrodite)and as n goesfrom 4 to 0, expandthe unoccupiedpolyhedra for the hcp anion array very much more than the occupiedoctahedra; physical parametersare accord- ingly affected. Introduction crystal structuresof no less than three titanian cli- nohumites(Ti-CD and one titanian chondrodite(Ti- The formula for the homologous seriesof minerals Ch) have been examined. Least-squaressite refine- collectively called "humites" is nMrSiOo' Mr-,Ti, ments have produced somewhat conflicting results, (F,OH)r-r,Or, where n : I for norbergite, 2 for leaving us with two questionsregarding titanium: (l) chondrodite, 3 for humite, 4 for clinohumite, M is Why doesx appearnever to exceed0.5 Ti atoms per Mg >> Fe > Mn,Ca,Zn,Ni, and 0 < x < l. A seriesof formula unit in natural specimens?(2) Is Ti really or- Mn-isotypes for n : 2 (alleghanyite), 3 (mangan- deredinto the M3 octahedralsite, and if so,why? Fu- humite), and 4 (sonolite) also exists in nature, and jino and Tak6uchi (1978) implied answersto these these exhibit a substantial range of solid solution questions with vague referencesto Pauling's elec- with Mg; most are OH-rich. trostatic valencerule. One purposeof this paper is to Titanium-rich clinohumites and chondrodite (x > addressthat matter more specifically. 0.25) have recently attracted attention as possible Another crystal-chemicalproblem has to do with mineralogical sites for water in the earth's upper the nature of the OH e F substitutionin all the hu- mantle (McGetchin et al., l97O; Merrill et al., 1972; mites.The recentsynthesis of OH-Ch and OH-CI by Aoki er al.,1976;Mitchell, 1978),and as a resultthe Yamamoto and Akimoto (1974, 1977),the determi- 0nn3-{0/.X/79/09 l0- 1027$02.00 1028 RIBBE: HUMITE MINERALS nation of the structure of OH-Ch by Yamamoto panciesin Ti-Cl Robinson et al. (1973ab) took dif- (1977),and the synthesisof many F,OH intermediate ferent approachesthan Fujino and Tak6uchi (1978); compounds by Dutry (1977) have provided a data the former report disorderedTi, the latter ordered. basefrom which the systematicsof the OH ? F sub- Kocman and Rucklidge (1973)-perhaps most stitution has beendeduced. wisely-simply assignedTi to M3 and refined Mgl Fe distributions within the constraintsof their chem- Titanium in natural humites ical analysis. Fujino and Tak6uchi's (1978) referenceto Paul- Titanium content ing's electrostaticvalence principle holds the key to Table I lists, anong others, the compositions of both the restrictedTi substitutionand Ti distribution the most Ti-rich humite minerals analyzed to date. among the nonequivalentoctahedral sites in humites. Note that x does not exceed 0.5 Ti atoms per formula Let us consider the hypothetical homologues, unit in any of the humites. Norbergite (No; n: l) is nMg,SiOo'Mg,-"Ti,(F,OH)2_2.O,.with 0 S x S l. rare in nature and apparently is relatively Ti-free. Note that one half of the octahedral sites are filled Ctnohumite (Cl; n :4) commonly is Ti-rich (Jones with cations in the proportion (x)Ti + (2n + l - et al.,1969, Fig. 4), but only one titanian chondrodite x)Mel. (Ch; n: 2) has ever been reported (Aoki er al., 1976), Charge imbalance with Ti disordered.With Ti dis- and it is epitaxially associated with a Ti-Cl which ordered there are only two types of anions as far as contains the same number of Ti atoms per formula local chargebalance is concerned,regardless of min- unit as Ti-Ch. Notably absent from this intergrowth eral species.The averagecharge on all M cations is is the intermediate phase, humite (Hu; n : 3). the same:(zl : alx/(2n + l)l + 2l(2n+ | - x)/(2n + l)1. The oxygensare 4-coordinatedto lSi + 3M and Titanium distribution are all overbondedby [ + 3(zl/61- 2.00esu. The The structures of both Ti-Ch and Ti-Cl have been (F,OH,O) anion is bonded to 3M atoms and is al- determined by Fujino and Tak6uchi (1978) who ways underbondedby an amount 3(zl/6 - (l + x) found by least-squares site refinement that most, if esu, where x : Or,,/(F + OH + Or,). Figure la in- not all, of the Ti is ordered in the M(OH)O layers, dicatesthe imbalancesas a function of x for each of i.e., in the M3 octahedral sites. Of course there are the four humitesin which Ti is disordered. problems associated with determining unambi- Charge imbalance with Ti ordered. If all the tita- guously the cation distribution among the octahedral nium is ordered into M3, there are collectively sites when three species of atom are present, namely among the humite homologuesthree types of 4- : -26) Me 0 l2), Fe * minor Mn,Ni (f and Ti 6f : coordinatedoxygens, Oo, O,, Or, the subscriptsrefer- 22) (J is the atomic scattering factor at snfl/)r: 0.0). ring to the number of M3 atoms to which they are Some basically unsatisfying assumptions must be bonded, plus one (F,OH,O) anion. From the multi- made to obtain convergence: in refining site occu- plicities of M sitesin the humites (Table 2) it can be Table l. Compositional data for humite minerals whose structures have been refined z Mineral (space group) r Ms Fe Mn Nt ca sio4 ri F oH* ori ,#.trTtl]r, I NorbergiEe (Pbm) xx O.42 (36.2) 0.oil 1.99 <.01 <.01 <.01 1 0.01 1.81 0,17 0.02 Jones et aL. (L969) 2 chond,rodite (p21'/b) 9.36 (22.L) ' o.424 3.99 0.57 0.0r 0.01 o.42 0 1.15 0.85 Fujino & Tak6uchi (1978) o.o3 (22.1) <.0r 4.95 0.03 0.01 0.0r 2 <,oL L.27 0.73 <.0r cibbs e, aL. (I91O) 3 Homite (Pbm) 3.25 (15.9) 0. 201 6,49 0.22 0.09 <.01 3 0.20 0.79 0.81 0.40 Joles et aL. (1969)*** 0.10 (15.9) 0.01 6.60 0.35 0.05 <,01 3 0.01 1.06 0.93 0.01 Ribbe & Gibbs (1971) 4 Cllno}rtnite (P2r,/b) 5.59 (L2.4) 0.468 7.33 1.04 0.05 <.01 4 o.47 0 1.06 0.94 Robinson et aL. (1973) " 3.\4 (r2.4) 0.26 7.34 1.36 0.08 4 o,26 0.40 1.08 0.52 KocDao & Rucklidge (1973) 5.O1 (r2.4) o.429 7.44 r.09 0.02 0.02 4 o.43 0 1.14 0.86 Fujino 6 Tak6uchi (1978) o.22 (r2.4) o.o2 8.42 0,s0 0,06 < .01 0.02 1 . 04 0. 93 0. 03 Robinson et aL. (L973) * Calculated by difference: OH = 2 - 1f + Orr), where OT. = O2x. ** Space group convention of Jones (1969), ***Most Ti-rich hunite analysis reported; the structure of this specinea has not been refined. RIBBE: HUMITE MINERALS Ti DISORDEREDIN M Ti ORDEREDINTO M3 dered humites; the proportions of O0, O,, O, and ) o5 (F,OH,O) anionsper formula unit are listed in Table o o2 2 for eachof the structuretypes. C .9 It is clear from Figure I that the more Ti, the c o o vo greater the local charge imbalances, regardlessof o 0 whether Ti is ordered or disordered in any of the o c 0 homologues.Quite likely these imbalancesplace a o o - practical upper limit on Ti substitution. Comparing E o.5 o the two graphs,it is also evident that chargebalance o o is somewhatimproved when Ti is ordered into M3, E o -t.o exceptfor norbergite in which all anions are bonded o.o o.5 r.o r.o o.o o.5 Thus may argue that Ti prefers x = Ti per formulounil x = Ti per formulounil to two M3 sites.