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 miner- als because of their common occurrence. Moreover, sursassite and iso-structural macfallite were also catego- rized 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 rela- tionship 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 substi- tuted 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 pumpel- lyite, 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 pumpelly- ite 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, sursas- site, macfallite and other minerals, such as ruizite, orien- tite, 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 mod- el by Moore et al (1985) clarified the structural relation- ship 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 0L -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 pumpelly- ite by c/4 translation to distorted M3 octahedra of the epi- dote 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 corre- spond to A1 and A2 of the epidote structure, respectively. X and Y (pumpellyite) correspond to M2 and M1 (epi- dote), 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 epi- dote 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.