Pumpellyite-, Sursassite-, and Epidote-Type Structures: Common Principles-Individual Features

Journal of MineralogicalPumpellyite and- ,Petrological sursassite-, and Sciences, epidote Volume-type structures 106, page 211─ 222, 2011 211

REVIEW

Pumpellyite-, sursassite-, and epidote-type structures: common principles-individual features

Mariko Nagashima

Graduate School of Science and Engineering, Yamaguchi University, Yoshida 1677-1, Yamaguchi 753-8512, Japan

Several hydrous silicates with similar chemical formulae and related crystal structures form under low-grade metamorphism and hydrothermal activity. Among them, epidote and pumpellyite are the most important minerals because of their common occurrence. Moreover, sursassite and iso-structural macfallite were also categorized as structurally related to pumpellyite. Their cation distributions and structural features are similar to those in epidote and pumpellyite. The following subjects are reviewed: 1) the topological relation of crystal structures among these minerals; 2) The cation distributions among the octahedral sites in epidote, pumpellyite, sursassite and macfallite and their effect on the structural variations; 3) the relationship between the oxidation state of transition elements and the hydrogen bonding systems in these hydrous silicates. Special attention was paid to 2+ − 3+ the variety of hydrogen-bond systems with the oxidation states of transition elements, Me + OH ↔ Me + O2−, commonly occurring in the pumpellyite, sursassite and macfallite structures. A model of the structural relationship among pumpellyite, sursassite, macfallite and epidote is proposed from a new stand point.

Keywords: Epidote, Pumpellyite, Sursassite, Macfallite, Hydrogen-bond, Topology

INTRODUCTION and M3. In case divalent (Ca2+) cations at A2 are substituted by trivalent cations of rare earth element (REE), M3 Under low-grade metamorphism and hydrothermal activ- is partly occupied by divalent cations (e.g., Mg2+, Fe2+). In ity, several hydrous silicates form having similar chemical pumpellyite, the larger octahedral X site is occupied by formulae, crystal structures and stability fields. Among both divalent and trivalent cations, such as Mg2+, Fe2+, them, epidote and pumpellyite are the important minerals Mn2+, Fe3+, Mn3+, Cr3+, V3+ and Al3+, whereas the smaller because of their common occurrence. Epidote-group min- Y is only occupied by trivalent cations. 2+ erals have been known to form at higher temperature than Sursassite, Mn2 Al3Si3O11(OH)3 (monoclinic P21/m, 3+ pumpellyite (Schiffman and Liou, 1980, 1983; Akasaka et Z = 2) and iso-structural macfallite, Ca2Mn3 Si3O11(OH)3 al., 2003). Crystal structures of epidote, A1A2M1M2M3 (monoclinic P21/m, Z = 2) are categorized as structurally Si3O12(OH) (monoclinic P21/m, Z = 2), and pumpellyite, related to pumpellyite (Mellini et al., 1984; Moore et al., W1W2XY2Si3O14−n(OH)n 3 < n < 4 (monoclinic A2/m, Z = 1985). Their formulae can be simplified as W1W2M- 2+ 4), are closely related. Both of them belong to the sorosil- 1M2M3Si3O14−n(OH)n, 3 < n < 4 (W = Mn and M = Al 3+ icate group having one additional isolated SiO4 tetrahe- in sursassite, and W = Ca and M = Mn in macfallite). In dron (Figs. 1a and 1b). There are two crystallographically both structures the W1 site is 7-coordinated, whereas the independent large cation sites, 9- and 10-coordinated (A1 W2 site is distorted 6-coordinated in sursassite and 7-co- and A2, respectively) in epidote and two 7-coordinated ordinated in macfallite. There are three independent octa- (W1 and W2) in pumpellyite. The epidote structure has hedral sites M1, M2 and M3. Their cation distributions multiple octahedral sites M1, M2 and M3. Octahedral and structural features are similar to those in pumpellyite sites in pumpellyite are named X and Y. If only Ca occu- and epidote (Fig. 1c). The stability field of sursassite and pies A1 and A2 in the epidote-type structure, trivalent macfallite is expected to overlap with the one of pumpel- cations, such as Al3+, Fe3+, Mn3+, and Cr3+, fill M1, M2 lyite. Pumpellyite intergrowth with sursassite was ob- doi:10.2465/jmps.110404 served by Mellini et al. (1984). M. Nagashima, [email protected] Corresponding au- In this paper following subjects are reviewed: 1) the thor topological relation of crystal structures among these min- 212 M. Nagashima

TOPOLOGICAL RELATIONS OF PUMPELLYITE, SURSASSITE, MACFALLITE AND EPIDOTE STRUCTURES

Compositional and structural similarities among pumpellyite, sursassite, macfallite and epidote have been interest- ed in and studied by several authors referred below. Mellini et al. (1984) found that sursassite from Mon- te Alpe, Italy, includes frequent (001) pumpellyite lamel- lae. They concluded that sursassite is related to pumpellyite by a (a + c)/2 shift of the pumpellyite structure (a and c = the unit-cell parameters of pumpellyite) (Figs. 2a and 2b). Moore et al. (1985) found that pumpellyite, sursassite, macfallite and other minerals, such as ruizite, orientite, lawsonite, ardennite, santafeite and bermanite, are based on the same fundamental building block (f.b.b.) (Fig. 3), a sheet

where the φ anion is not associated with a tetrahedron, □ represents a vacancy. For example, f.b.b. of pumpellyite

and sursassite can be described as Al2(OH)2(SiO4)2, and 3+ that of macfallite is Mn2 (OH)2(SiO4)2. Although the model by Moore et al (1985) clarified the structural relationship between pumpellyite and its related minerals, it does not provide a straight forward clue to understand the Figure 1. Crystal structures of epidote (a), pumpellyite (b) and sursas- structural relationship between pumpellyite and epidote. site (c) projected down [010] using VESTA (Momma and Izumi, Merlino (1990) re-defined the f.b.b. defined by Moore et

2008). al. (1985) as L1 layer. The part, which does not belong to

L1-layer, was defined as L0-layer. The L0-layer of

pumpellyite consists of XO6-octahedra and Si2O4-tetra- erals; 2) The cation distributions among the octahedral hedra (Fig. 4a), and that of sursassite M1O6-octahedra sites in epidote, pumpellyite, sursassite and macfallite and and Si3O4-tetrahedra (Fig. 4b). Topological relations be- their effect on the structural variations; 3) the relationship tween pumpellyite, sursassite (and iso-structural macfal- between the oxidation state of transition metal ions and lite), and ardennite are well illustrated in terms of trans- the hydrogen bonding systems in these hydrous silicates. formation of L0- and L1-layers (Merlino, 1990). Those In this review, a model of the topological relationship between sursassite and epidote can be also described us- among pumpellyite, sursassite, macfallite and epidote ing the modified L1-layer based on the concept of Merlino structures is proposed. Based on the new view of topolog- (1990) (Fig. 4c) (Makovicky, 1997; Ferraris et al., 2004). ic features among pumpellyite, sursassite, macfallite and However, Nagashima (2006) approached it from her epidote, their cation distributions at the octahedral sites standpoint and proposed a model of the structural rela- and hydrogen-bond systems are reviewed. Special atten- tionship between pumpellyite, sursassite (iso-structural tion was paid to the variety of hydrogen-bond systems macfallite) and epidote (Fig. 2). The starting structure in with the oxidation states of transition elements, M2+ + this model is the pumpellyite structure (Fig. 2a), because OH− ↔ M3+ + O2−, occurring in pumpellyite, sursassite the crystal structure of pumpellyite is the simplest. Three and macfallite structures. steps are assumed: (1) translation of the L0 layers = f.b.b. along the a-axis with a translation distance of (a + c)/2 (Fig. 2b), i.e., the layer of the YO6 octahedra; (2) translation of a layer consisting of W1, W2, X, Y and Z polyhedra along a direction inclined 25-30° from the c-axis by c/4 (~ 5Å along the dashed line in Pumpellyite-, sursassite-, and epidote-type structures 213

Figure 2. Model on the structural relationship between pumpellyite and epidote (Nagashima, 2006). The origin was shifted from empty channel (a) to M3 (b).

Figs. 2b and 2c); (3) conversion of the voids in pumpellyite by c/4 translation to distorted M3 octahedra of the epidote structure (Figs. 2d and 2e). These new M3 octahedra

are attached to adjacent sides of the YO6-octahedra in the pumpellyite structure. Y in pumpellyite corresponds to M1 in the epidote structure. In this topological relation (Fig. 2), W1 and W2 of the pumpellyite structure correspond to A1 and A2 of the epidote structure, respectively. X and Y (pumpellyite) correspond to M2 and M1 (epidote), respectively (Table 1). It is noted that the large X site in the pumpellyite structure is related to the smallest M2 site in the epidote structure. According to the above model, sursassite and macfallite structures can be recog- nized as intermediate ones between pumpellyite and epidote structures as shown in Figure 2b: M1 of sursassite and macfallite is corresponding site of X of pumpellyite and M2 of epidote, whereas M2 and M3 of sursassite and macfallite are comparable to Y of pumpellyite and M1 in epidote (Fig. 2b). Figure 3. Fundamental building block (f.b.b.) defined by Moore et al. (1985). 214 M. Nagashima

Figure 4. Topological relations among pumpellyite (a), sursassite (b) and epidote (c) structures based on Makovicky (1997) and Ferraris et al.

(2004). The crystal structures of pumpellyite and sursassite composed of two types of unit layers, L0 and L1, and that of epidote L0 and modi-

fied 1L layers.

Table 1. Topologically similar polyhedral in pumpellyite, sursas- ma et al., 2006) indicated that the Y site contains not only site, macfallite an epidote Al3+ but also Fe3+, even if there is sufficient Al3+ to fill the Y site. The X site is occupied by Fe3+ and Al as well as by Mg2+ and Fe2+. Partition coefficients of Fe3+ versus Al be- 3+ 3+ tween X and Y, defined as KD = [(Fe /Al)X/(Fe /Al)Y], calculated from the results by Nagashima et al. (2006) are 3+ 1.12-1.44, indicating that Fe cations prefer X rather than 3+ Y. However, julgoldite from Bombay, Ca2(Fe0.675Al0.275 3+ Mg0.05)Fe2 Si3O14(OH)3.5, with X and Y filled by trivalent cations and only Al3+ at X (Artioli et al., 2003) stimulated our interest for further studying the Fe-intracrystalline partitioning behavior in pumpellyite. * The site names of W1 and W2 were renamed in this study. They Ivanov et al. (1981) assumed that preferentially Cr3+, 3+ were originally Mn1 and Mn2 in sursassite, and Ca1 and Ca2 in rather than Al , occupies the Y site. Cr-rich pumpellyite macfallite. from Bisersk was named ‘shuiskite’. However, the structural studies of chromian pumpellyite from Sarany in the Russian Urals (Nagashima and Akasaka, 2007; Nagashi- CATION DISTRIBUTION AT OCTAHEDRAL ma et al., 2010b) and from Osayama, Okayama, Japan SITES AND ITS EFFECT ON PUMPELLYITE, (Hamada et al., 2010) revealed that Cr3+ distributes be-

SURSASSITE, MACFALLITE AND EPIDOTE tween X and Y with KD values of 3.56-4.39 (Sarany) and

STRUCTURES 1.66-4.54 (Osayama), where KD is defined as KD = [(Cr/

Al)X/(Cr/Al)Y]. These values are in agreement with strong Pumpellyite-group minerals preference of Cr3+ for the X site. Therefore, in general, the larger trivalent cations 3+ 3+ Based on the chemical compositions and the variation of such as Fe and Cr prefer the larger XO6-octahedra and - 3+ 3+ unit cell parameters of Ca2(Mg,Fe, Al)(Fe ,Al)2Si3O14− the smaller Al the smaller YO6-octahedra. However, the 3+ 3+ n(OH)n-pumpellyites, Passaglia and Gottardi (1973) as- reason why the partition coefficients of Cr versus Al sumed that the smaller trivalent cations prefer exclusively between X and Y are larger than those of Fe3+ versus Al3+ the Y site, and proposed to fill the Al-deficiency of the Y is not understood. Moreover, the partition coefficient of 3+ 3+ 3+ 3+ site with Fe . However, recent studies on the distribution V versus Al of 0.63 derived for poppiite, the V -dom- 2+ 3+ of Fe and Fe at X and Y using X-ray structural refine- inant analogue of pumpellyite (Brigatti et al., 2006), is ment and/or 57Fe Mössbauer analysis (Artioli and Geiger, below the values for Cr3+/Al3+ and Fe3+/Al3+. This is some- 1994; Akasaka et al., 1997; Artioli et al., 2003; Nagashi- how surprising because the ionic radius of V3+ is in be- Pumpellyite-, sursassite-, and epidote-type structures 215

Table 2. Angular and bond length distortions for the octahedral sites*

* 2 DI(oct) = 1/6Σ|Ri − Rav.|/Rav. (Ri: each bond length, Rav.: average distance for an octahedron) (Baur, 1974), and σθ(oct) = (θi:

O-M-O angle) (Robinson et al., 1971).

tween Cr3+ and Fe3+. substitution of trivalent transition metal ions, Fe3+ and 2+ 3+ 3+ The distribution of manganese in pumpellyite-Mn Cr , for Al at Y, and the increasing rate of mean Y-O 3+ (Kato et al., 1981) and okhotskite, the Mn -dominant an- distance (Å) against mean ionic radius (Å) at Y attains alogue of pumpellyite (Togari and Akasaka, 1987), is not 0.93 (Nagashima and Akasaka, 2007; Hamada et al., straight forward. Artioli et al. (1996) analyzed the distri- 2010) to 0.99 (Nagashima et al., 2006). On the other bution of Mn cations over X and Y by the Rietveld meth- hand, the XO6 octahedra are less flexible than the YO6 oc- od using synchrotron radiation, and concluded that Mn2+ tahedra and their volumes are not influenced by ionic sub- and Mn3+ are distributed between the X and Y site, re- stitution at X, because 1) they are surrounded by large spectively. On the other hand, in Akasaka et al. (1997), W1O7 and W2O7 large polyhedra, and 2) the four edges 3+ 57 Fe was partitioned between X and Y using Fe Möss- of XO6 are shared with W1O7 and W2O7 polyhedra. On 2+ 3+ bauer method; Mn and Mn were calculated based on the other hand, although the two edges of YO6 octahedra 2+ 3+ charge balance; Mn was assigned to X, and Mn to X are also shared with the W1O7 and W2O7 polyhedra, the 3+ and Y on the assumption that all Al occupies Y. The re- volume of YO6 varies with ionic substitutions at Y be- 3+ 3+ sult indicated that Mn prefers Y rather than X, and Mn cause the edges parallel to the c-axis are not shred with has a stronger preference for Y than Fe3+. any other coordination polyhedra (Nagashima et al., 2006; Recent studies on the site distribution of cations en- Nagashima and Akasaka, 2007; Nagashima et al., 2010b; abled us to understand the influence of the ionic substitu- Hamada et al., 2010). tion at X and Y on structural variations. The positive cor- The XO6 octahedra are geometrically more regular relation of the unit-cell parameters versus total Fe content than the YO6 octahedra (e.g., Yoshiasa and Matsumoto, at Y have been repeatedly confirmed for pumpellyite in 1985), and the bond-length and angular distortion param- pumpellyite-julgoldite series (Passaglia and Gottardi, eters defined by Robinson et al. (1971) of the YO6 octahe-

1973; Artioli and Geiger, 1994; Akasaka et al., 1997; Arti- dra are greater than those of the XO6 octahedra (Artioli oli et al., 2003; Nagashima et al., 2006). However, Artioli and Geiger, 1994). However, in terms of the distortion in- and Geiger (1994) pointed out that there is no volume de- dex DI(oct) of Baur (1974), the YO6 octahedra are less 2+ pendence upon Fe -content at the X site. This finding has distorted than the XO6 octahedra (Nagashima et al., been extended to the general observation (e.g., Nagashima 2006). The degree of distortion of the YO6 octahedra var- et al., 2006) that the volume of XO6 octahedra does not ies with ionic substitutions at Y: it decreases with increas- 3+ 3+ 3+ change with varying mean ionic radii at X. Mean Y-O ing substitution of Fe or Cr for Al approaching regu- distances and volumes of YO6 octahedra increase with lar shape (Nagashima et al., 2006; Nagashima and 216 M. Nagashima

Akasaka, 2007; Hamada et al., 2010). Similar results are Epidote-group minerals expected to be obtained for the substitution of Mn3+ and 3+ 3+ V for Al at Y. Epidote-group minerals are divided into three subgroups; (1) Members of clinozoisite subgroup are derived from

Sursassite and macfallite the mineral clinozoisite Ca2Al3Si3O12(OH) by homovalent substitutions only, (2) Members of the allanite subgroup The chemical compositions of sursassite and macfallite are REE-rich minerals typified by the eponymous mineral reported in previous studies implied that the octahedral M “allanite”, which may be derived from clinozoisite ho- sites were partly substituted by divalent cations, such as movalent substitutions and one coupled heterovalent sub- Mg2+ and Cu2+ (Heinrich, 1962; Cortesogno et al., 1979; stitution of the type A2REE3+ + M3M2+ → A2Ca2+ + M3M3+; Moore et al., 1979; Allmann, 1984; Mellini et al., 1984; (3) Members of the dollaseite subgroup are REE-rich Reinecke, 1986; Minakawa and Momoi, 1987; Basso et minerals typified by the eponymous mineral “dollaseite”, al., 1989; Miyajima et al., 1998; Hatert et al., 2008; Na- which may derived from clinozoisite by homovalent sub- gashima et al., 2008; Nagashima et al., 2009a). As result stitutions and two coupled heterovalent substitutions of of structural analyses by Allmann (1984) and Nagashima A2REE3+ + M3M2+ → A2Ca2+ + M3M3+ and M1M2+ + O4F− → et al. (2009a), this substitution only occurs at M1 which is M1M3+ + O4O2− (Armbruster et al., 2006). the largest octahedral in the sursassite structure. The M1 Among M1O6, M2O6 and M3O6 octahedra, the site is topologically similar to the X site in the pumpelly- M3O6 octahedra are the largest and most distorted, where- ite structure. Although X in pumpellyite is occupied by as M2O6 octahedra are the smallest and the least distorted. almost evenly by divalent and trivalent cations, the con- Al3+ at M3 and M1 is replaced by trivalent transition met- centration of divalent cations at M1 in sursassite amounts al ions, such as Fe3+, Mn3+ and Cr3+ (abbreviated to Me3+) generally 25-30% (Nagashima et al. 2009a) and rarely ca. (Deer et al., 1986; Liebscher and Franz, 2004; Nagashima 3+ 40% (0.34 Mg + 0.05 Cu in Sample EV84-30 7-3 by and Akasaka, 2004b). Me prefers M3 rather than M1. Reinecke, 1986). In case of macfallite, the amount of di- However, Me3+ tends to occupy M1 even if M3 is not ful- 3+ valent cation at M1 is ca. 10-25% at M1 (Moore et al., ly occupied by Me (i.e., Giuli et al., 1999; Langer et al., 1979; Basso et al., 1989; Miyajima et al., 1998; Nagashi- 2002; Nagashima and Akasaka, 2004a, b, 2010a; Na- ma et al., 2008), which is less than in sursassite.. gashima et al., 2007, 2009b). On the basis of published The relationship between cation occupancies and studies referred above, it is concluded that the preferences structural variation of sursassite was examined by Na- of Fe3+, Mn3+, and Cr3+ for M1 and M3 are Cr3+ > Mn3+ > gashima et al. (2009a). Their study reported that Ca2+ Fe3+ for M1 and Fe3+ > Mn3+ > Cr3+ for M3. On the other partly replaces Mn2+ at W1 (= Mn1 in their paper) and the hand, the substitution at M2 is limited (e.g., Giuli et al., substitution of (Mg + Cu + Mn) for Al3+ at M1 and M3. 1999; Nagashima and Akasaka, 2004), and M2 is general- W1 M1+M3 The unit-cell parameters with Ca + (Mg + Cu + ly occupied by only Al. Mn) (apfu) showed a positive correlation (see Fig. 3 in As result of systematic studies on the effects of Al ↔ Nagashima et al., 2009a). Nagashima et al. (2009a) con- Fe3+ and Al ↔ Cr3+ substitutions on the crystal structures cluded that the variation of unit-cell parameter was gov- of epidotes, the unit-cell parameters increase with in- 3+ 3+ erned by the cation distribution at octahedral sites, while creasing Fe or Cr contents. M3-Oi and M1-Oi dis- 2+ the effect of Mn ↔ Ca substitution on the cell-parame- tances versus mean ionic radius at each site show positive ters is insignificant. The substitution of (Mg + Cu + Mn) correlation (Giuli et al., 1999; Nagashima et al., 2007, 3+ for Al at M1 causes the increase of the M1-O1 and M1- 2009b). On the other hand, the unit-cell parameters of 3+ O5 distances, but does not affect M1-O7 distance, result- synthetic Ca2(Al,Mn )3Si3O12(OH)-piemontites having 3+ ing in anisotropic expansion of the M1O6 octahedra. Al-Mn substitution show non-linear variation with in- 3+ However, the increasing rate of mean M1-O distance (Å) creasing of Mn content, and M3O6 and M1O6 expand 3+ against the mean ionic radius (Å) at M1 is only 0.1 to 0.2. anisotropically due to Mn Jahn-Teller effect (Nagashima The site distortion of octahedra in sursassite and and Akasaka, 2004a, 2004b). The structural change with macfallite are shown in Table 2. It is noted that both an- Al ↔ Fe3+ ↔ Mn3+ substitution is influenced by both the 2 3+ gular distortion [σθ(oct) ] and bond-length distortion DI ionic radius effect derived from Al-Fe substitution and 3+ parameters of M1O6 octahedra for macfallite show larger the Jahn-Teller effect derived from Mn (Nagashima and values than those for sursassite. The reason is attributed to Akasaka, 2010a). Σ(Fe3+ + Mn3+) prefer M3 rather than 3+ 3+ 3+ 3+ the Mn Jahn-Teller effect, because Mn occupancy at M1, which is same preference as for Fe or Mn in bina- M1 in macfallite is higher than that in sursassite. ry series. Although Mn3+ rather than Fe3+ was expected to 3+ prefer most distorted M3 because of the Mn Jahn-Teller Pumpellyite-, sursassite-, and epidote-type structures 217 effect (e.g., Dollase, 1973), the neutron study for Sr-rich and M1 in sursassite and macfallite are occupied by both piemontite from St. Marcel, Italy by Ferraris et al. (1989) divalent and trivalent cations. In previous studies of indicated that Fe3+ rather than Mn3+ preferentially occu- pumpellyite, four hydrogen positions, H5, H7, H10 and pies M3. Nagashima and Akasaka (2010a) determined the H11 were directly determined (Brigatti et al., 2006; Naga site occupancies of Fe3+ and Mn3+ in synthetic epidote. As shima and Akasaka, 2007; Nagashima et al., 2010b), or the result of their study, Fe3+ rather than Mn3+ prefers M3. investigated using bond valence sum calculations (All- This result supports the study reported by Ferraris et al. mann and Donnay, 1971, 1973; Yoshiasa and Matsumoto, (1989). Fe3+ and Mn3+ ions partition among octahedral 1985; Nagashima et al., 2006; Hatert et al., 2007). The sites according to their individual site-preference. Thus, exact number of hydroxyl groups depends on the concen- 3+ the Mn Jahn-Teller effect influences cell parameters and tration of divalent and trivalent cations at X. It means that geometry of M3O6 octahedra rather than the site distribu- pumpellyite has four hydroxyl groups, if X is fully occu- tion of Mn3+ (Nagashima and Akasaka, 2004a, 2004b, pied by divalent cations but it has three hydroxyl groups 2010a). if only trivalent cations fill the X site. Yoshiasa and Mat- sumoto (1985) also suggested the following substitutional HYDROGEN BONDING SYSTEMS IN PUMPEL- mechanism: XMe2+ + H11H+ + O11O2− ↔ XMe3+ + O11O2−. LYITE, SURSASSITE, MACFALLITE AND EPI- Recently, Nagashima et al. (2010c) found one new DOTE hydrogen positions, H5’, in addition to the know hydrogen positions. In their study, the H5’ position has been Pumpellyite found in addition to the known hydrogen positions, H5, H7, H10 and H11. The relationship between donor and As mentioned above, X in pumpellyite-group minerals acceptor oxygens and their hydrogen bonds can be sum-

Figure 5. Hydrogen-bonding systems in pumpellyite (a), (b), sursassite (c) and macfallite (d). Pumpellyite has the two different systems, (a) 4OH and (b) 3OH (Nagashima et al., 2010c). The combined result of 3OH and 4OH systems in sursassite (c) and the 3OH system in macfallite (d) were determined by Nagashima et al. (2009a) and Nagashima et al. (2008), respectively. All structures projected down [010]. 218 M. Nagashima marized as follows: (1) O5-H5···O1/O5-H5···O5, (2) H10···O6, and (3) O11-H11···O2/O11-H11···O7 (Fig. O5-H5’ ···O10, (3) O7-H7···O3/O7-H7···O7, (4) O10- 5d). The hydrogen bond system is simpler than for iso- 2+ H10···O5, and (5) O11-H11···O7. The H5 and H5’ sites structural sursassite. The amount of Me at M1 in mac- are assumed to be half-occupied. In general, there are fallite is smaller than that in sursassite leading to prefer- four hydroxyl groups with divalent cations at X (Fig. 5a). ence of the 3OH system in macfallite. The H10 position, On the other hand, the 3OH system has O5-H5’, O7-H7 which could not be determined in sursassite, was success- and O11-H11 with trivalent cation at X (Fig. 5b). Due to fully determined in macfallite. Macfallite also has a silan- the substitution at X, half-occupied H11 (Yoshiasa and ol group as well as sursassite and pumpellyite. The Si3-

Matsumoto, 1985) was expected, but in contrast H10 was O10H10 distance is the longest in Si3O4 tetrahedron found to be partially occupied (Nagashima et al., 2010c). showing typical silanol property (Nyfeler and Armbruster, Thus, the substitution mechanism can be represented as 1998). XMe2+ + H10H+ + O10O2− ↔ XMe3+ + O10O2−. Moreover, one It should be noted that, in spite of structural similari- − OH group in pumpellyite is a silanol group Si-OH (Ny- ties between macfallite and pumpellyite, 3OH system in feler and Armbruster, 1998). In general, a Si-OH bond macfallite shows different feature from that in pumpelly- distance tends to be longer than the other Si-O distances, ite: oxygen at O7 of the M1O6 octahedra in macfallite but Si2-O10H10 in pumpellyite is the shortest in the does not form a hydroxyl group, while O11 of the XO6

Si2O4 tetrahedron. This shortened Si2-O10H10 can be octahedra in pumpellyite, which corresponds to O7 in derived from partial occupied H10 and O10 position, macfallite, hosts a hydroxyl group. which is not shared with the other polyhedra expect for

Si2O4 Epidote

Sursassite The hydrogen bonding system in epidote is different from that in pumpellyite, sursassite and macfallite. In general, OH positions in sursassite were reported by Mellini et al. epidote-group minerals have one hydroxyl group per for- (1984) based on bond valence calculation, while H sites mula unit. Ito et al. (1954) firstly proposed bonding of the were determined by Nagashima et al. (2009a). The oxy- proton to O10 of the M2 site with a hydrogen bond to O4 gens at O6, O7 and O11 form hydroxyl groups (Fig. 5c). of the adjacent M1 octahedral site based on crystal chemi- Five different hydrogen positions forming hydroxyl cal consideration. The H positions were suggested using groups were directly found in sursassite. The relationship difference Fourier synthesis (Dollase, 1968, 1969) and between donor and acceptor oxygens and their hydrogen bond-valence sum calculations (Gabe, 1973), and it was bonds can be summarized as follows: (1) O6-H6A···O10, directly determined by Nozik et al. (1979) using neutron (2) O6-H6B···O3/O6-H6B···O11, (3) O7-H7···O11/O7- diffraction. The donor and acceptor oxygens of the hydro- H7···O2, (4) O11-H11A···O2/O11-H11A···O6, and (5) gen bonds in epidote-group minerals are O10 and O4, re- O11-H11B···O2/O11-H11B···O7. Although no hydrogen spectively (Fig. 6). The length of hydrogen bond, O10- position could be found close to O10, the bond-valence sum for this position (1.37-1.43 valence unit) suggests that O10 not only acts as an acceptor but also as a donor of a hydrogen bond. The acceptor of H10 can be O6. The occupations of H6A and H6B, and H11A and H11B are considered to vary with the ratio of Me2+ and Me3+ at M1. All hitherto studied sursassite crystals seem to represent a solid-solution between members with 3 and 4 OH groups. Sursassite also has a silanol as well as pumpellyite. The

Si3-O10 distance is the shortest within the Si3O4 tetrahedron. The reason of this short Si-OH distance may be related to partial occupation of H10 similar to pumpellyite.

Macfallite

As the result of structural analysis (Nagashima et al.,

2008) the relationship between hydrogen donor and ac- Figure 6. Hydrogen-bonding system in epidote projected down ceptor is as follows: (1) O6-H6···O11, (2) O10- [010]. Pumpellyite-, sursassite-, and epidote-type structures 219

H···O4, and the O10-O4 distance increase with increasing Structural insensitivity of octahedra against the ionic sub- Fe content (Franz and Liebscher, 2004). Kvick et al. stitutions has been typically observed in pumpellyite (Na- (1988) pointed out that the hydrogen bond responses to gashima et al., 2006, 2007; Hamada et al., 2010). As the the electrostatic repulsions from cations at M3 and A2. results of the structural study of sursassite (Nagashima et 3+ The data for the Al-Fe series of epidote summarized by al., 2009), the mean M1-O distance increased with the in- Franz and Liebscher (2004) show that the angle of the crease of the mean ionic radius at M1, in contradict to the O10-H-O4 hydrogen bond varies from 156.4° to 174.3°. case of X site of pumpellyite. This is probably due to the In REE-bearing epidote-group minerals, the accep- enlargement of the adjacent W1 polyhedra caused by cat- tor oxygen at O4 is partly occupied by F− instead of O2− ion substitution of Mn2+ for Ca2+ in W1. The increasing M1 2+ O4 − M1 3+ based on the substitution scheme, Me + F ↔ Me rate of mean M1-O distance was very low, 0.1-0.2, and, O4 2− + O proposed by Armbruster et al. (2006). Especially, such low increasing rate of M1-O distance with the mean the O4 positions in dollaseite-subgroup minerals are pre- ionic radius is consistent with the inflexible property of dominantly occupied by F−. Moreover, significant amount ‘pumpellyite X site type’ octahedra. Unfortunately, the re- of Cl− has been rarely reported (e.g., Pan and Fleet, 1990). lation between ionic substitutions and volume change of − − Cl can be assigned to the O4 position by analogy with F M1O6 in macfallite has not been investigated well be- (Armbruster et al., 2006). cause of very narrow range of ionic substitution. Howev-

Anhydrous oxyallanite has been reported in labora- er, the M1O6 octahedra in macfallite are expected to have tory studies where it was produced by heating natural al- inflexible property as well as pumpellyite and sursassite. lanite (e.g., Dollase, 1973; Bonazzi and Menchetti, 1994). In epidotes of clinozoisite subgroup, the M2O6 octahe- The OH− in allanite is replaced by O2− because of oxida- dron is the smallest in volume among three octahedra. tion states of transition metal ions, such as Fe2+ to Fe3+ Because of its small volume and less flexibility, M2 is and Mn2+ to Mn3+. According to Bonazzi and Menchetti usually occupied by only Al3+. On the other hand, in alla- 2+ (1994), the loss of H compensating the oxidation of Fe nite subgroup, REE-analogue epidote-group minerals, 2+ 3+ and Mn is evident by lengthening of the donor (O10)- M2 is partly occupied by Fe . The reason of incorpora- acceptor (O4) distance. Recently, natural oxyallanite with tion of larger Fe3+ in M2 can be attributed to the enlarge- lengthened O10-O4 distance was found from a welded si- ment of the M2O6 octahedra which is governed by the licic volcanic rock was reported by Hoshino et al. (2010). large volume of the A2O10 polyhedra in which significant amounts of REE3+ substitute for Ca2+. This phenomenon

COMMON PRINCIPLES AND INDIVIDUAL FEA- is similar to that found in M1O6 in sursassite which is also TURES OF PUMPELLYITE, SURSASSITE AND influenced by the substitution at W1. MACFALLITE STRUCTURES The strong positive relation between cation substitutions at ‘pumpellyite Y site type’ sites and the structural Cation distribution at the octahedral sites changes is distinct not only in pumpellyite (increasing rate of the mean Y-O distances (Å) versus mean ionic ra- According to my model on the topologic relations of crys- dius (Å) is 0.9-1.0) but also in epidote. The increases of tal structures among pumpellyite, epidote, sursassite and M1-Oi and M3-Oi distances with increasing mean ionic macfallite, the octahedral sites can be categorized into radius at M1 and M3 in clinozoisite subgroup are evident ‘pumpellyite X site type’ and ‘pumpellyite Y site type’. (i.e., Giuli et al., 1999; Nagashima and Akasaka, 2004a, The former is less-flexible with the ionic substitutions at 2004b, 2010a; Nagashima et al., 2007, 2009b). In the the octahedral site because of its restricted environment published data for sursassite and macfallite, the ‘pumpel- where the edges of the octahedra are shared with adjacent lyite Y site type’ octahedra, M2 and M3, are almost filled 3+ coordination polyhedra. The M1O6 octahedra of sursassite with Al in sursassite and Mn in macfallite, and, thus, the and macfallite and M2O6 octahedra of epidote belong to variation rates of M2-O and M3-O distances with cation this category. The latter is more-flexible with the ionic substitutions are not known. Even so the flexible proper- substitution at the octahedral site. The M2O6 and M3O6 ties of M2O6 and M3O6 octahedra in macfallite with ionic octahedra of sursassite and macfallite, and the M1O6 and substitutions are inevitable, because their structural situa- M3O6 octahedra of epidote are grouped into this category. tions are characterized as ‘pumpellyite Y site type’ in my The ‘pumpellyite X site type’ octahedra of pumpel- model. lyite, sursassite and macfallite are occupied by both divalent and trivalent cations, such as Mg2+, Fe2+, Al3+, Fe3+, Hydrogen bonding systems and Mn3+, although Me2+/(Me2+ + Me3+) at M1 of sursassite and macfallite are lower than that at X of pumpellyite. The infrared spectra of pumpellyite, sursassite and mac- 220 M. Nagashima fallite are characterized by several sharp bands and one in sursassite, hydrogen atoms forming OH group are additional broad band around 3000 cm−1 (Hatert et al., H6B, H7, H10 and H11A positions. In this case, the hy- 2007; Reddy and Frost, 2007; Nagashima et al., 2008; drogen positions are very similar to those in pumpellyite, Nagashima et al., 2009a; Nagashima et al., 2010c). The but the total amount of hydroxyl group does not reach 4 associated OH···O distances can be calculated using the but 3.5 because H11A and H6B are very close to each correlation between the observed OH-band wave number other. These sites should be counted as half-occupied. and the O···O separation in Å as given by Libowitzky Hydrogen atoms at H11B do not play role in both 3OH (1999). The broad band correlates with a O···O distance and 4OH models mentioned above. It implies that there is of ca. 2.65 Å. This broad IR band was assigned to another type of hydrogen bonding system having H11B. SiOH···O in pumpellyite (O10···O5 = 2.73-2.74 Å) and sursassite (O6···O10 = 2.66-2.67 Å). On the other hand, ACKNOWLEDGMENTS in macfallite, it could not be assigned to Si-OH···O (O10···O6 = 2.85Å) but O6···O11 (2.63 Å), occurring in- I thank Prof. M. Akasaka and Prof. T. Armbruster for their side the narrow channel between M2O6 and M3O6 across constructive comments on this manuscript. I also thank the diagonal. Pumpellyite and sursassite also have similar Prof. A. Yoshiasa, associated editor, and an anonymous hydrogen bonds across the narrow channel. However, the referee for their constructive comments on this manu- broad band at ca. 3000 cm−1 observed also for pumpellyite script. and sursassite cannot be assigned to the cross-diagonal hydrogen bonds due to too long O-H···O distances REFERENCES (Nagashima et al., 2010c). Nagashima et al. (2010c) con- 3+ cluded that this difference is due to the Mn Jahn-Teller Akasaka, M., Kimura, Y., Omori, Y., Sakakibara, M., Shinno, I. 57 - distortion in the columns formed by the M2O6 and M3O6 and Togari, K. 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