Refinement of Hydrogen Positions in Natural Chondrodite by Powder Neutron Diffraction: Implications for the Stability of Humite Minerals

Mineralogical Maga~ine. June 2002. Vol. 66(3). pp. 44/-449

Refinement of hydrogen positions in natural chondrodite by powder neutron diffraction: implications for the stability of humite minerals

A. .I. BERRyl AND M. JAMES1 1 Rcscarch School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia 1 Neutron Scattering Group, Building 58, Australian Nuclear Scicncc and Technology Organisation, PMB I, Menai NSW 2234, Australia

ABSTRACT

Thc structure of a natural sample of chondrodite (Mg4.xqFeoo7Si1.o40sF l54(OH)OA6) was refined using powder neutron diffraction data and the Rietveld techniquc 2; a = 4. 7204( I) A; h = o 0 (P2/h; Z = 7.8252(2) A; rx = I09.11( I); V = 1 10.2360(3) A; c = " 357.26(2) A-). Hydrogen was found to occupy the H I site. The significance of hydrogen at this site is discussed in terms of hydrogcn-bond stabilization of humite structures containing varying amounts of OH, F and Ti. Arguments are proposed as to why the F and Ti contents of natural humites usually result in only onc H pcr formula unit when there is no crystal-chemical reason why fully hydrated samples should not be favourcd.

KEYWORDS:powdcr ncutron diffraction, chondrodite, Rietveld refinement, humite minerals.

Introduction Humite minerals usually occur in metamor- phoscd limestones and dolomites. However, THE humitc group minerals have the general titanian clinohumite occurs in mctamorphosed formula nMg2Si04.Mgl_Ji,(F,OHh 2x02x mantle rocks (TrommsdortT and Evans, 1980) and where n = I (norbergite), 2 (chondrodite), 3 both titanian clinohumite and titanian chondrodite (humite) or 4 (clinohumite) (Jones et al., 1969). havc bccn found in kimberlites (McGetchin et al., The structures of the whole scries are, in general, 1970; Aoki et al., 1976). Expcrimcntal studics well known and that of chondrodite has been indicate that end-member Ti-, OH- and F-bearing determincd previously by both single-crystal X-ray clinohumite (togcthcr with OH-chondrodite) and neutron diffraction (Gibbs et al., 1970; would all be stable at the pressure (P) and Yamamoto, 1977; Friedrich et al., 2001). The tcmpcrature (7) of the upper mantle and may be simplistic nMg2Si04.Mg(F,OHh representation of some importance for watcr storage in this conveniently conveys the compositional variation region (Yamamoto and Akimoto, 1977; Burnlcy but is misleading in relation to the structure (Ribbe and Navrotsky, 1996; Wunder, 1998; Ulmer and et al., 1968). The hydrated layer is better describcd Trommsdorff, 1999; Pawley, 2000). Also impor- as Mg(F,OH)O, where Mg corresponds to M3, tant is the way in which water is incorporatcd in leading to an expression of the chondrodite the structure (as a hydrated, Si-deficient layer formula as Mg]Si206.2Mg(F,OH)O. Substitutions within an olivine-like unit) which may provide a of F and Ti02 can then be written in the form model for the water defect in nominally Mg]Si206.Mg2F, (OH)l_,02 and Mg3Si206. anhydrous olivine (Kitamura et al., 1987). Mg2-xTiAOHh_2,02+2x' The initial humite structure determinations identified a centrosymmetric pair of hydrogen positions arising from a single hydrogen site (H I ) E-mail: [email protected] (Fujino and Takeuchi, 1978). These positions are * 001: 10.1180/0026461026630040 separated by only -I Aand, if fully occupied, this

2002 The Mineralogical Societl' A. J. BERRY AND M. JAMES

short H...H distance would dcstabilize the structure (Fig la). Seemingly to avoid this interaction, natural samples are characterized by the substitution of F for OH and Ti02 for Mg(OHh (Fig. Ih,c) at levels which usually a (but not always) reduce the amount of H to < I per formula unit (p.fu.) and the occupancy of this H site to <0.5 (Jones et al., 1969). The synthesis of OH-clinohumite and OH-chondrodite (here- after, synthctic materials are referred to by the prefix "OH-") were explained by the need for high pressures to ovcrcome the H...H repulsions (Ribbe, 1979; Camara, 1997). In reality there are two possible H sites with the OH vector pointing in approximately opposite directions; H I located near a centre of symmetry within the Mg(OH)O b layer and H2 in a cavity of the olivine-like block (Fig. la) (Berry and James, 2001). This means that OH-rich humites can occur without a destabilizing H...H interaction due to occupancy of both H I and H2. These two positions were first identified in X-ray Fourier difference maps of OH-chondrodite by Yamamoto (1977) and latcr confirmcd for hydroxylclinohumite by Fcrraris et al. (2000). They have also been suggested for a range of humite minerals by ah initiu calculations which indicatcd that H2 was in fact the favoured site (Abbott et al., 1989). Despite these results, c the significance of the two-site model has been ignored and the misconception about the H...H repulsion persisted (Fujino and Takeuchi, 1978; Williams, 1992; Camara, 1997; Lin et al., 1999, 2000). Only recently have neutron diffraction data been used to unambiguously detennine the H FIG. I. Substitutions in the hydrated layer of the humite positions in OH-clinohumite and OH-chondrodite minerals. (a) OH end-member showing H I and H2 and establish that there is no crystal-chemical positions; only half of the H sites are occupied to reason why these structures should not occur in prevent a short H...H distance arising from occupancy of nature (Berry and James, 2001; Lager et al., the adjacent HI sites shown; (h) substitution of F for 2001). This coincides with the discovery of 05-H and occupancy ofthc HI site; and (c) substitution natural hydroxylclinohumite (Ferraris et al., of Ti052 for Mg(OHh; Ti on 1/4 of the M3 sites 2000). removes completely the need for both HI and H2 sitcs. Without a structural impediment to the natural occurrence of OH-rich humites it is now not clear why the substitutions in natural samples are to Camara, 1997). Due to the possibility of both HI such an extent as to give the deceptive appearance and H2 sites, it is unclear which site is occupied of occurring so as to stabi lize the structure. The when ForTi substitution results in less than one critical factors are the favourable site in the H p.f.u. Although these maps indicate the humites for Ti and F relative to other phases, and presence of H] it is possible that the region the potential role of hydrogen bonding. In the corresponding to H2 was not closely examined. A original structure detenninations of the substi- recent single-crystal neutron study of F-bearing tuted humites, the hydrogen positions were either chondrodite, F-bearing titanian clinohumite, and not identified or only approximately located by F-free titanian hydroxylclinohumite, each X-ray difference maps (Gibbs et al., 1970; containing <1 H p.f.u. found that in all cases H Yamamoto, 1977; Fujino and Takeuchi, ]978; occupies the HI site (Friedrich et al., 2001). Here

442 HYDROGEN IN CHONDRODITE

we report the structure of a further sample of with a contribution of only Tiooo7 to the chemical F-bearing ehondrodite. Based on these results we form ul a. then discuss the compositions of natural humite The initial parameters used in the refll1ement minerals. were those determined by Gibbs et al. (1970) for Mg295Feoo5Si20(,.Mg2Fu(OH)o702. Iron was fixed on the MI site in accordance with the Experimental findings of Gibbs et al. (1970) and Friedrich et al. Chondrodite fl'om Orange County, New York, (200 I). The H positions Were added after an USA (Australian Museum 03142) was hand- initial refinement of the structure according to the picked after dissolution of a calcite matrix in HC!. model of Yamamoto (1977). H I and H2 positions The chemical composition of the sample was with a maximum occupancy of 0.5 were tried both determined using a CAMECA Camebax electron individually and together for a range of starting microprobe. Approximately 5 g of powdered coordinates. All refinements involving the H2 material were loaded into a vanadium sample position were unstable while those involving HI can with a 5 mm diameter. Powder neutron invariably converged to a constant and physically diffraction measurements were made on the High reasonable H site. The crystallographic data Resolution Powder Diffractometer (HRPO) using obtained from the full refinement are given in thermal neutrons (I, = I.SS42 A) from the HIFAR Table I. while the atomic coordinates and nuclear reactor at the Australian Nuclear Science isotropic thermal parameters are given in and Technology Organisation (ANSTO). Data Table 2. Selected bond-lengths are given in were collected using a bank of 24 3He detectors Table 3 and distances and angles associated with over the range 0 < 20 < 153 steps, in 0.05 . the I I I site arc given in Table 4. The results are Structural refinements were carried out by the reported in the unconventional space group P21/h Rietveld method (Rietveld. 1969) using the (unique axis a) to maintain consistency with LHPM program (Howard and Hunter, 1997), previous studies which have used this form to with Voigt peak shapes and refined backgrounds. emphasize the structural relationship with forsterite (PIJ/lI7l). The standard space groups P2/c (for chondrodite and clinohumite) and Results PI7lCI1(for both lorsterite and the orthorhombic The chemical composition was calculated on the humites) similarly assign the same lattice basis of seven cations as Mg4s9Feo07Si2040sF 154 parameter label to structurally related crystal- (OH)04h or Mgos9Feol17Si2040(,.Mg2F 1.54 lographic directions. The observed, calculated and (OH )04h02' The sample is effectively Ti fl'ee difference neutron di nJ'action profi les arc shown

T\BLE I. Crystallographic data for chondrodite.

Formula Mg4.K

-.C2.:lri(obs) "i(calc)I/Lr,(obs). where 1', is the (gross) intensity ~lt the /th /I" step of the profile. R!) ~ IA-(obs) I,' where 1/\ is the intcnsily aSSi~jH..'d 10 Ihl' [h Brag~ !\.,nl'cti\~.!1

443 A. J. BERRY AND M. JAMES

TABLE2. Fractional atomic coordinates and isotropic thermal parameters 2 (Biso) (,4. x ]00) for chondroditc with esd's in parentheses. MgI atoms occupy the special 2d (1/2,0, I/2) site, while all the other atoms occupy genera] 4e (x,y,.:::)sites.

Atom x v .::: Biso

MgI/Fe" 0.5 0 0.5 0.7(] ) Mg2 0.0]25(]2) 0.1711(4) 0.3052(7) 0.7(1) Mg3 0.4979(] I) 0.8845(5) 0.0781 (7) 0.6(1) Si 0.0784( 11) O.]459(6) 0.7027(9) 1.0( I) 01 O. 7805( I 0) -0.000 I(5) 0.2926( I0) 1.1(I) 02 0.7254(10) 0.2417(8) 0.1263(8) 0.7(] ) 03 0.2286( I]) 0.1692(8) 0.5263(9) 1.2( I) 04 0.2657(9) 0.8527(6) 0.2920( II ) 0.8(1 ) 05/Fb 0.2726(] I) 0.0561(8) 0.1007(]0) 0.9(1 ) H]C 0.082(4) 0.023(2) 0.028(3) 3.0

a Occupancy fixed at ana]ytically determined values: Mgl ~ 0.93, Fe = 0.07. b Occupancy fixed at analytically determined values: I" = 0.77, 05 = 0.23. C lIydrogen occupancy constrained to cqual that of 05.

in Fig. 2 and a representation of the structure I A (H] ...H I 0.94(2) A). The potential for a given in Fig. 3. destabilizing H] ...H I interaction is avoided by the substitution of I" for OH. For a given H I position, the nearest neighbour H] site must be unoccupied Discussion and the associated 05 position occupied by I" Hydrogen was found to occupy the H] site and (Fig. Ih). The maximum H I occupancy is 0.5 ] two centrosymmetric HI positions occur within which corresponds to I OH and I" p.f.u. For higher leve]s of H the H2 site is occupied (Berry TABLE 3. Selected bond-lengths and hydrogen and James, 200]; Lager et al. 200 I). The H] bond distances (A) with esd's in parentheses. environment is consistent with that determined by Friedrich et al. (2001) for a more OH-rich Atom]-Atom2 Bond-]ength (A) chondrodite (Mg2 7Feo ]Si206.Mg2F1 02 (OH)09702)' The 05/1" -05/1" distance of Mgl/Fe-O] x 2 2.094(6) 2.939 A in this compound is shorter than that of Mg1/Fe-03 x 2 2.110(6) 3.040 A found here, in accordance with the Mg1/Fe-04 x 2 2.131(8) assertion that vacant H positions result in anion- Mg2-01 2.041 (7) anion repulsion which increases this distance. Mg2-02 2.231(8) Mg2-03 2.014(7) TABLE4. Selected bond distances (A) and angles C) Mg2-03 2.190(8) for the H] cavity with esd's in parentheses. Mg2-04 2.165(7) Mg2-OS/F 2.054(8) 05-HI ].06(2) Mg3-01 2.166(8) HI...F 1.98(2) Mg3-02 1.996(8) 05-Hl...F 174(1 ) Mg3-02 2.085(8) HI...OI 2.53(2)/2.58(2) ] Mg3-04 2.1 ]3(9) 05-HI...O] 03( I )/98( I) Mg3-OS/F 2.01 ](8) HI...02 2.71 (2)/2.73(2) Mg3-05/F 2.015(9) 05-HI...02 107(])/93(1 ) Si-O] 1.646(8) 05/F...05/F 3.040(11 ) Si-02 1.609(9) 05/1"...01 2.923(8)/2.951(11) Si-03 ].637(9) 05/1"...02 2.984(9)/3.174(9) Si-04 1.625(7) 05/F...04 2.936(] 0)

444 HYDROGEN IN CHONDRODITE

3000

2000

(/) C o:J o 1000

o I 1111111 1111111111111111111 111111I1111111111111111111111111111111111111111UIIIIIIIIIII 1111111111111111111111111111111111111111111111111111111111111111111111

20 40 60 80 100 120 140

Flc;. 2. The observed (+), calculated and difference (solid lines) powder neutron diffraction profilcs for ehondrodite.

Flc;. 3. The structure of chondrodite showing the Mg-O ti-amework and Si04 tetrahedra. Fluorine is disordered on the 05 site. All HI positions generated by symmetry are shown although from the chemical composition only around 1/4 are occupied. Two adjacent III positions arc never occupied simultaneously. For a given H I position (which points either up or down) the 05 site associated with the adjacent HI (directly above or below the proximal HI in the figure) is occupied by F.

445 A. J. BERRY AND M. JAMES

The determination of the H site in substituted maintains hydrogen bonding across the layer and humites is important for understanding the stabilizes the structure. The most F-rich clinohu- stability, composition and extent of F/Ti substitu- mite reported, has F/(F+OH) equal to 0.71 tion. The H2 site is located within a forsterite like (Satish-Kumar and Niimi, 1998). The F content block and no real stabilization of the structure is at which one humite converts to another is a possible from hydrogen bonding. In contrast, the balance between the destabilization caused by H I site allows hydrogen bonding across the increasing the F/(F+OH) ratio of a single layer eleavage plane and stabilizes the substitution of and the potential instability of F-poor additional F and Ti02. Cleavage in the humites occurs along layers. For example, if the F/(F+OH) ratio is 0.6 the substituted layer and reflects the structural and the number of layers was doubled without the integrity (or intrinsic stability) of the layer addition of further F, the resultant F/(F+OH) ratio binding the forsterite-like units together. would be 0.3 and the stability of the material Maximum strength will be achieved when a restricted to a limited p, T and Xco. range due to hydrogen bond occurs between every 05-05/F the instability of the OH component (Rice, pair (for chondrodite). This is the case for the OH 1980a.b). Fluorine stabilizes the clinohumite + end-members and F substitution at up to half the calcite stability field to more COrrich fluids. 05 sites. Indeed the strongest interlayer bonding High clinohumite F contents can also be avoided might be expected for a F/(F+OH) ratio of 0.5 by a discontinuous reaction forming chondrodite since F is a stronger H bond acceptor than O. The and forsterite (Rice, 1980b). The effect of structure is weakened by F/(F+OH) values greater balancing the extent of fluorination in humite than 0.5 due to anion-anion repulsion, and the transformations is that the amount of F relative to substitution of Ti which is associated with a OH increases from clinohumite to norbergite charge imbalance (discussed below). (Jones et al., 1969). Natural chondrodites have F and/or Ti partition preferentially into the F/(F+OH) ratios of up to -0.8 while for higher F humites (Rice, 1980a). F appears to be favoured activities norbergite is formed, all known samples over Ti and it has been suggested that the amount of which have ratios >0.8. of F controls the level of Ti that can enter the For a given geological environment, the stable structure (Jones et aI., 1969). Indeed such is the humite mineral and its F content represents a favourability of clinohumite for accommodating F balance between the intrinsic stability of the that this phase may be much more important as a structure and the relative stability referenced to reservoir of mantle F than of H20 (Engi and the mineral assemblage and fluid composition. Lindsley, 1980). In the absence of F and Ti, the This is also apparent from the temperature of the pure OH end-member humites can be formed elinohumite breakdown reaction, which for a within the appropriate stability field. given pressure increases in the order OH < Ti- Hydroxylclinohumite has been found in nature bearing < Ti-saturated < Ti, F-bearing < F (Ulmer (Ferraris et al., 2000) while OH-elinohumite, and Trommsdorff, 1999; Pawley, 2000; Wunder, OH-humite and OH-chondrodite have all been 1998). This does not reflect the intrinsic stability synthesized (Pawley, 2000; Wunder, 1998; of each structure but rather the favourable site Wunder et al., 1995; Burnley and Navrotsky, available for F and Ti in the humites relative to 1996; Yamamoto and Akimoto, 1977). OH- other phases in the Mg(Fe)0-SiOrTiOrH20- norbergite is currently unknown. MgF2 system (and hence the preferential parti- The importance of hydrogen bonding from the tioning). The F site in the humites stabilizes the F H I site is apparent from the extent of F end-member relative to the other analogues, even substitution. Despite the ability of clinohumite without any hydrogen bonding contribution, due to readily accommodate F, the F/(F+OH) ratio in to the high energy of both F substitution into other natural samples is nearly always -0.5 (from -0.45 lattices and of the breakdown product MgF2/HF. to -0.6; Jones et aI., 1969). This is intrinsically Indeed the stability of the substituted humites the most stable F content and also a lower limit, correlates negatively with the stability of the since at the P, T and Xco, at which these minerals breakdown phases, Mg(OHh/H20, Ti02 and form in skarns, the hydroxyl end-member is MgF2/HF. unstable, or metastable, relative to other phases In Ti-substituted chondrodite, Ti is ordered (Rice, 1980a,b; Wunder, 1998). If more F is onto the M3 site (Fujino and Takeuchi, 1978) available, additional layers are formed rather than with a maximum occupancy of 0.25. This level fully substituting the existing layers. This results in only I H p. f.u. (Fig. I c). Theoretically

446 HYDROGEN IN CHONDRODITE an occupancy of 0.5 is possible but the Conclusions substitution is associated with a local charge imbalance which will destabilize the structure There is no crystal chemical reason why humites (Ribbe, 1979). The sharp limit at 0.25 may be with > I H p.f.u. should be unstable since explained by the stabilizing effect of H bonding occupation of an H2 site, of similar energy to which can only occur for occupation of the HI HI, prevents the occurrence of a short H 1...H I site. This bond is particularly strong since the distance that was thought to destabilize the receptor 05 coordinated to Ti is highly under- structure. Accordingly, hydroxylclinohumite and bonded carrying enhanced negative charge other OH-rich humites have been found in nature. density (Ribbe, 1979). The strength of this In general, however, the amount of H is :<:;I p.f.u. bond is reflected in the short 05-05 distance due to the presence of F and Ti in the structure. and straight 05 -H 1...05 angle reported by The humites act as a sink for these elements and Friedrich et al. (200 I). At occupancies up to the substitutions arc stabilized by hydrogen 0.25, every 05 with a charge imbalance can be bonding from the H 1 site. In the case of F, the stabilized by a hydrogen bond. Above 0.25 this extent of substitution tends to reduce H to does not occur and the structure is unstable. :<:;I p.f.u. due to the instability of the OH end- Accordingly we would not expect a composition member at the conditions typical of the skarns saturated with Ti to contain any F since where most of these compounds form. There is stabilization would not be possible. Indeed a also a tendency not to fully fluorinate layers due negative correlation exists between Ti and F in to F...F repulsion and the involvement of HI in a the humites (Jones et al., 1969; Evans and strong O-H...F hydrogen bond. For Ti the Trommsdorft~ 1983; Dymek et al., 1988) with substitution is associated with a charge imbalance the maximum amount of F p.f.u. for a given Ti that is stabilized by hydrogen bonding. The p.f.u., x, being 1-2x. For any mixed substituted maximum Ti content for which this stabilization compound the amount of OH p.f.u. must be at can still occur is that which reduces H to I p.f.u. least double that of Ti. This relationship is The extent of these substitutions is thus a balance consistent with all known natural samples (see between accommodating as much F and Ti as for example Gaspar, 1992). Fully fluorinated possible while still maintaining the integrity of the compounds arc of course possible since structure through hydrogen bonding from the HI hydrogen bonding is not necessary for stabilizing position. Confirmation that H occupies H I rather the substitution. than H2 in substituted humites is thus funda- Finally, it is interesting to note the relative ease mental to our understanding of the chemistry of with which OH-clinohumite and OH-chondrodite these minerals. (Pawley, 2000; Wunder, 1998; Burnley and Navrotsky, 1996; Yamamoto and Akimoto, Acknowledgements 1977) can be synthesized and the rarity of OH- humite (Wunder et al., 1995), despite the over- We thank R. Pogson for providing the sample of lapping stability fields. The reaction products are ehondrodite, the Australian Institute of Nuclear dependent upon the stoichiometry of the starting Science and Engineering for access to the HRPD oxides, the water activity, the effect of silica (Grant No 01/007), N.G. Ware for electron solubility in the fluid, and the reaction conditions. microprobe analyses, and J. Hermann for helpful A TEM study of products from the attempted discussions. AJ.B. also thanks the Australian synthesis of OH-clinohumite (Weiss, 1997) found Research Council for the award of a Fellowship. intergrowths of forsterite, clinohumite and chondrodite. Isolated hydrous layers were also References observed and olivine blocks of the form (Mg2Si04)".Mg(OHh within a humite layer Abbott, R.N., Burnham, C.W. and Post, J.E. (1989) always have even n. The rarity of humite may lIydrogen in humite-group minerals: Structure- be explained if clinohumite first forms with energy calculations. American Mineralogist, 74, subsequent layers being inserted between the 1300-1306. clinohumite n = 4 units to give n = 2 chondrodite. Aoki, K., Fujino, K. and Akaogi, M. (1976) Transformation from an n = 4 to n = 3 structure Titanochondrodite and titanoclinohumite derived would involve a rearrangement of the existing ti'om the upper mantle in the Buell Park kimberlite, network. Arizona, USA. Contributions to Mineralogy and

447 A. J. BERRY AND M. JAMES

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