Crystal Chemistry of Mn2+-, Sr-Rich and REE-Bearing Piemontite from the Kamisugai Mine in the Sambagawa Metamorphic Belt, Shikoku, Japan
142 Journal of MineralogicalM. Nagashima, and T. Petrological Armbruster, Sciences,M. Akasaka Volume and T. 105, Minakawa page 142 ─ 150, 2010
Crystal chemistry of Mn2+-, Sr-rich and REE-bearing piemontite from the Kamisugai mine in the Sambagawa metamorphic belt, Shikoku, Japan
* * ** *** Mariko Nagashima , Thomas Armbruster , Masahide Akasaka and Tetsuo Minakawa
*Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, CH-3012, Bern, Switzerland **Department of Geoscience, Faculty of Science and Engineering, Shimane University, Matsue 690-8504, Japan ***Department of Earth Science, Faculty of Science, Ehime University, Matsuyama 790-5877, Japan
The crystal chemistry of piemontite from the Kamisugai mine in the Sambagawa metamorphic belt, Shikoku, Japan was studied using electron microprobe analysis (EMPA) and single-crystal X-ray diffraction methods. 2+ 3+ 3+ The chemical formula derived from EMPA is (Ca1.30Sr0.19La0.03Ce0.05Nd0.02Mn0.41)Σ2.00(Al1.70Mn0.77Fe0.48Mg0.01)Σ2.96
- - Si3.04O12.13(OH)0.87 (Z = 2). This piemontite is characterized by high Sr (2.2 6.2 SrO wt%), Mn (16.2 18.8
- Mn2O3 wt%) and REE (0.7 4.0 REE2O3 wt%) contents and it is mainly composed of clinozoisite Ca2Al3Si3O12
- 3+ 3+ 2+ 3+ (OH), piemontite (Sr) CaSrAl2Mn Si3O12(OH), epidote Ca2Al2Fe Si3O12(OH) and Mn CaAl2Mn Si3O12(OH) components. The crystal structure of piemontite [a = 8.8673(2), b = 5.6747(1), c = 10.1594(3) Å, β = 114.71(1)º,
space group P21/m] was refined using 1484 unique reflections toR 1 = 4.4%. The site occupancies at A1, A2, M1,
M2 and M3 are Ca0.59Mn0.41, Ca0.71Sr0.19La0.03Ce0.05Nd0.02, Al0.72Mn0.17Fe0.11, Al1.0, and Al0.11Mn0.55Fe0 .34, respec- M1 M3 tively. The distribution coefficient KD (= [(Mn + Fe)/Al] /[(Mn + Fe)/Al] ) is 0.048. Although it is known that 3+ 3+ high Sr contents at A2 seem to promote incorporation of Mn (+ Fe ) at M1, the KD value does not suggest a stronger transition metal preference at M1 than that of Ca end-member piemontite and epidote. High content of 2+ Mn at A1 modifies the arrangement of the coordinating O atoms ofA 1O9 polyhedra. Thus, the A1 coordination may be described as 6-coordinated.
Keywords: Piemontite, Sambagawa, Mn2+, Sr, REE, Crystal structure
INTRODUCTION sides along its length. Chains of octahedra run parallel to
- the b axis, linked by SiO4 and Si2O7 groups (Ito et al., 3+ Piemontite, simplified Ca2Al2Mn Si3O12(OH), is one of 1954; Dollase, 1968). This structural arrangement gives the common monoclinic epidote-group minerals in meta- rise to two types of highly coordinated sites: 9-coordinat- morphic rocks and manganiferous ore deposits subjected ed A1 and 10-coordinated A2 are mainly occupied by Ca. to low- to moderate-temperature metamorphism at very Generally, cations having large ionic radius such as Sr, high oxygen fugacity and low CO2 partial pressure. It is Pb, Ba and rare earth elements (REE) locate at A2, and also a late crystallization product in some acid and inter- cations smaller than Ca, such as Mn2+, may substitute at mediate lavas. A1. Trivalent cations, such as Al, Fe3+, Mn3+ and Cr3+, dis- Monoclinic epidote-group minerals with the struc- tribute among the octahedral M1 and M3 sites. In case of 2+ tural formula A1A2M1M2M3Si3O12(OH) belong to the a coupled substitution, according to the scheme Ca (A2) soro/neso-silicate class. Their structure is based on a + M3+(M3) ↔ REE3+(A2) + M2+(M3), divalent cations,
- 2+ 2+ chain of edge sharing M2 octahedra and a central chain such as Mg, Fe and Mn favor M3. The smallest M2O6 of M1 octahedra with M3 octahedra attached on alternate octahedron is generally occupied by Al only. The key cat- doi:10.2465/jmps.090709 ion-sites M3 and A1 determine the root name, and for the M. Nagashima, [email protected] Corresponding author dominant cation on A2 (other than Ca) the suffix designa- 2+ Crystal chemistry of Mn -, Sr-rich and REE-bearing piemontite 143 tion is used. This paper follows the recommended nomen- long, and are reddish purple in color. clature summarized by Armbruster et al. (2006). From this locality Ba-rich manganipiemontite-(Sr) The main components of most natural piemontites is reported by Fukushima et al. (2005). The systematic 3+ are Ca2Al3Si3O12(OH), Ca2Al2Mn Si3O12(OH), and Ca2 study of the manganese ore deposits in the Sambagawa
3+ - Al2Fe Si3O12(OH). Sr rich piemontite, such as piemon- metamorphic belt had done by Minakawa (1992). tite-(Sr) and manganipiemontite-(Sr) is also common. Ba-rich manganipiemontite-(Sr) (Ba-rich tweddillite in Chemical analysis (EMPA) the original paper) was also described by Fukushima et al. (2005) from the Kamisugai mine, Ehime, Japan, which is The chemical composition of piemontite was determined the same locality where our piemontite in the present using a JEOL JXA-8800M electron microprobe analyzer study comes from. at Shimane University. The abundances of Si, Ti, Al, Cr, 2+ Subject of this study is the Mn -, Sr-rich and REE- V, Fe, Mn, Mg, Ca, Sr, Ba, Na and K were measured us- bearing piemontite from the Kamisugai mine, Ehime, Ja- ing an accelerating voltage of 15 kV and a beam current 2+ pan. Although the composition of Mn -, Sr- and REE- of 20 nA, with a beam diameter of 1 µm. The following rich piemontite from Varenche, Italy (Bonazzi et al., standards were used: natural wollastonite (Si, Ca), syn-
2+- - 1996) has been reported, the formation of Mn , Sr and thetic rutile (Ti), synthetic spinel (Al, Mg), Cr2O3 (Cr),
- REE bearing piemontite is rather uncommon. Thus, we Ca3V2O8 (V), hematite (Fe), MnO (Mn), SrBaNb4O12 (Sr, investigated the influence of the unusual composition on Ba) and natural anorthoclase (Na, K). The rare earth ele- the structural variation and flexibility of this complex ments were analyzed at an accelerating voltage of 25 kV piemontite. with a beam current of 10 nA and a beam diameter of 20 µm after the method of Nishida et al. (1999) using syn-
EXPERIMENTAL METHODS thetic standards of LaP5O14 (La), CeP5O14 (Ce), PrP5O14
(Pr), NdP5O14 (Nd), SmP5O14 (Sm), EuP5O14 (Eu) and
- Samples GdP5O14 (Gd). The ZAF correction method was used for all elements. Analyses were collected for several piemon- Piemontite from a manganese deposit underwent meta- tite crystals in the same thin section, from which the crys- morphism of greenschist facies at Kamisugai in the Sam- tal used for structural analysis was extracted. Thus, all an- bagawa metamorphic belt, Ohzu, Ehime, Japan. The crys- alytical data comprise averages over several individual tal from this locality was used for structural analysis. This piemontite crystals of similar composition. piemontite is mainly associated with sursassite [average 2+ 3+ 3+ composition: (Mn1.70Ca0.30)Σ2.00(Mn0.42Al2.17Mg0.31Fe0.04V0.01 Single-crystal structure analysis
Ti 0.01Zn0.01)Σ2.97Si3.03O10.31-10.73(OH)3.27-3.69: Nagashima et al.
- - - - (2009)] and spessartine grossular andradite garnet (Spe85-94 X ray diffraction data for single crystals of piemontite
Gro5-11And0-9), crystallizing on the wall of a small cavity were collected using a Bruker SMART APEX II CCD dif- (Fig.1). The crystals form prisms, which are up to 50 µm fractometer of Bruker AXS K.K installed at the Universi- ty of Bern. The crystal (0.040 × 0.025 × 0.010 mm) was mounted on a glass fiber and intensity data were measured at room temperature using graphite-monochromatized MoKα radiation (λ = 0.71069 Å). Preliminary lattice pa- rameters and an orientation matrix were obtained from three sets of frames and refined during the integration process of the intensity data. Diffraction data were col- lected with ω scans at different φ settings (φ-ω scan) (Bruker, 1999). Data were processed using SAINT (Bruk- er, 1999). An empirical absorption correction using SAD- ABS (Sheldrick, 1996) was applied. The reflection statis- tics and systematic absences were consistent with space
groups P21 and P21/m. Subsequent attempts to solve the structure indicated that the observed average structure is
centrosymmetric and for this reason P21/m is the correct Figure 1. Backscattered electron (BSE) image of piemontite. Pm, space group. Structural refinement was performed using piemontite; Sur, sursassite; Sp, spessartine. SHELXL-97 (Sheldrick, 1997). Scattering factors for 144 M. Nagashima, T. Armbruster, M. Akasaka and T. Minakawa
Table 1. Composition of piemontite based on 9 point analyses Table 2. Experimental details of the single-crystal X-ray diffrac- tion analysis of piemontite
* Total Cr as Cr2O3, Fe as Fe2O3, V as V2O3 and Mn as Mn2O3. ** Total cations = 8. neutral atoms were employed. Positions of the hydrogen atoms of the hydroxyl groups were derived from differ- ence-Fourier syntheses. Subsequently, hydrogen positions 2 were refined at a fixed value of Uiso = 0.05 Å . The site occupancies of A1 and A2 were fixed at the average chemical composition. This became necessary in order to reduce correlations among site occupancies, scale factor, and displacement parameters. The site occupancy at M2 was fixed as 1.0 Al apfu, because the occupancy of Al at M2 refined at the preliminary stage indicated that M2 is fully occupied with Al within standard deviation. During the refinement, Mn and Fe atM 1 and M3 site were treated as Mn + Fe. The hydrogen position was refined with a bond distance constraint of O-H = 0.980(1) Å (Franks, 1973). * The function of the weighting scheme is w = 1/[σ2(Fo2) + (a·P)2 + 2 2 RESULTS b·P], where P = [Max(Fo ) + 2Fc ]/3, and the parameters a and b are chosen to minimize the differences in the variances for reflec- tions in different ranges of intensity and diffraction angle. Chemical composition of piemontite
Piemontite in this study is characterized by high Sr ments Mn at A1 must be divalent. Although all Mn at M3
- - (2.2 6.2 SrO wt%), Mn (16.2 18.8 Mn2O3 wt%) and REE was calculated as trivalent, it could also be partially diva-
- 2+ (0.7 4.0 REE2O3 wt%) contents. The average chemical lent based on the coupled-substitution scheme, Ca (A2) composition of piemontite is given in Table 1. The corre- + M3+(M3) ↔ REE3+(A2) + M2+(M3). As derived from the sponding chemical formula is (Ca1.30Sr0.19La0.03Ce0.05Nd0.02 chemical formula, ~ 1/3 of total Mn is divalent. This pie- 2+ 3+ 3+ Mn0.41)Σ2.00(Al1.70Mn0.77Fe0.48Mg0.01)Σ2.96Si3.04O12.13(OH)0.87, montite is mainly composed of clinozoisite Ca2Al3Si3O12
- 3+ where the total number of cations, except H, was normal- (OH), piemontite (Sr) CaSrAl2Mn Si3O12(OH), epidote 3+ 2+ 3+ ized to 8 and the amount of OH was calculated using Ca2Al2Fe Si3O12(OH) and Mn CaAl2Mn Si3O12(OH) com charge balance considerations. Because of size require- ‑ponents. The crystal used for the X-ray single-crystal re- 2+ Crystal chemistry of Mn -, Sr-rich and REE-bearing piemontite 145
Table 3. Atomic positions and equivalent displacement parameters crystal structure of piemontite is shown in Figure 2a. 2 (Å ) of piemontite The determined site occupancy is listed in Table 6. The site occupancies at A1 and A2 were fixed at the result of chemical analysis, and those at M1 and M3 calculated by following procedure: (1) Elements with less than 0.01 atoms per formula unit (apfu) were omitted, (2) Mn and Fe were treated as Mn during the refinement, and (3) Mn and Fe were prorated using the chemical composition. M at M1 and M3 in Table 6 consists of Mn : Fe = 0.77 (= to- tal Mn ― Mn at A1) : 0.48 (= total Fe). On the basis of chemical data and occupancy refine-
ment, the crystallochemical formula becomes; (Ca0.59 A1 A2 M1 Mn0.41) (Ca0.71Sr0.19La0.03Ce0.05Nd0.02) (Al0.72Mn0.17Fe0.11) M2 M3 (Al1.0) (Al0.11Mn0.55Fe0.34) Si3O12(OH). Total Fe and Mn are 0.45 and 1.13 (= 0.41Mn2+ at A1 + 0.72Mn3+ at M1 and M3) apfu, respectively, and thus, consistent within one standard deviation with the average chemical compo- sition. However, the highest (1.84 eÅ3) and the lowest ( ―1.04 eÅ3) difference Fourier peaks were observed at positions close to A2 and A1, respectively. This may im- * Multiplicity ply that the average chemical composition slightly deviat- ** Wyckoff letter ed from that of the structurally analyzed crystal. finement is from the same hand specimen as the material DISCUSSION analyzed by EMPA. Crystallization of piemontite Crystal structure solution and refinements Piemontite of the Kamisugai mine occurs as (1) main Crystallographic data and refinement parameters are sum- constituent of the host piemontite schist, (2) constituent of marized in Table 2. The refined atomic positions and the banded ores formed at early stage, and (3) late stage anisotropic displacement parameters are listed in Tables 3 vein mineralization (Fukushima et al., 2005). Piemontite and 4. Interatomic distances, selected angles and distor- of occurrence (3) is enriched in Sr, Ba and Mn. Veinlets tions of octahedral sites are presented in Table 5. The containing this piemontite intrude into ores consisting of
Table 4. Anisotropic displacement parameters (Å2) of piemontite 146 M. Nagashima, T. Armbruster, M. Akasaka and T. Minakawa
Table 5. Selected interatomic distances (Å), angles (°), volume of polyhedra (Å3), and distortion param- eters for the octahedral sites*
* DI(oct) = 1/6Σ|Ri − Rav.|/Rav. (Ri, each bond length; Rav., average distance for an octahedron) (Baur, 1974). 6 2 - - <λoct> = Σ (li − l0) /6 (li, each bond length; l0, center to vertex distance for an octahedron with Oh sym- i =1 metry, whose volume is equal to that of a distorted octahedron with bond lengths li) (Robinson et al., 1971). 12 2 2 - - σθ(oct) = Σ (θi − 90°) /11 (θi, O M O angle) (Robinson et al., 1971). i =1
braunite, hollandite and ordinary piemontite. Ba-rich The occurrence suggests that this piemontite was formed manganipiemontite-(Sr) occurs in late stage albite vein- from a vapor-phase or under hydrothermal conditions 2+ lets (Fukushima et al., 2005). The Mn -, Sr-rich and during retrograde metamorphism. The Ba concentration REE-bearing piemontite, studied here, occurs in a small in manganipiemontite-(Sr) reported by Fukushima et al. druse, indicating the latest stage of crystallization (Fig. 1). (2005) is considered to have been supplied by the break- 2+ Crystal chemistry of Mn -, Sr-rich and REE-bearing piemontite 147
As a result, Ca at A2 was partly replaced by Sr and REE. Sr accumulation at the latest stage in the development of Sambagawa metamorphic roc ks has also been described in previous studies (e.g., Minakawa, 1992; Nagashima et al., 2006). Sakai et al. (1984) suggested that detrital alla- nite is the major source of REE for REE-rich epidote in the Sambagawa pelitic schist.
Structural variation due to cation substitution at the A and M sites
Bonazzi et al. (1996) reported that high content of Mn2+ at A1 modifies the arrangement of the linked O atoms of
A1O9 polyhedra. The arrangement of the A1O9 and A2O10 polyhedra are shown in Figure 2b. The O3 and O7 atoms are shared between A1 and A2. Despite O3 and O1 (2nd to 5th neighbor oxygen atoms) are getting closer to A1 with increasing Mn2+ content at A1, O6 (7th neighbor) and O9 (8th and 9th neighbors) are shifted away from A1 (Fig. 3). In this case the A1 site can be appropriately described as 6-coordinated. There is no correlation between A1-O5, 2+ A1-O7 distances and the Mn concentration at A1. Be-
cause O5 and O7 are common to both the M1O6 octahe-
dron and the A2O10 polyhedron, the exact positions of the 2+ Figure 2. Crystal structure of Mn -, Sr-rich and REE-bearing O sites may be influenced by the substitution at M1 and piemontite. (a) Projected onto (010). (b) The A1 and A2 polyhe- A2, respectively. - dra in the unit cell using the program VESTA (Momma and Izu- Piemontite in this study contains also 0.19 Sr and mi, 2008). 0.10 REE at A2, leading to a
- M1 M3 ble, which may be for lack of a Ba rich source. Main [(Mn + Fe)/Al] /[(Mn + Fe)/Al] is 0.048. The KD value minerals in our host specimen are sursassite, spessartine- is similar to that of synthetic piemontite (0.063-0.080: grossular-andradite garnet along with small amounts of Langer et al., 2002; 0.038-0.063: Nagashima and Akasa- calcite, quartz and rhodochrosite. Piemontite might be ka, 2004) and epidote (0.033-0.054: Giuli et al., 1999). It formed in a vapor phase associated with the partial break is known that Sr-rich piemontite has a higher solubility of down of sursassite due to a hydrothermal fluid having transition elements at the octahedral sites than common
2+ - high fO2. The source of Mn in piemontite may be sursas- Ca end member piemontite; for example, manganip- 2+ site. After Mn -rich piemontite crystallization, Sr and iemontite-(Sr) (original name: tweddillite) from the Kala- REE were subsequently supplied by hydrothermal fluids. hari manganese field, South Africa (0.90 Sr, 1.58 Mn3+ + 148 M. Nagashima, T. Armbruster, M. Akasaka and T. Minakawa
Figure 4. Variation of the A2-Oi distances as a function of Sr con- 2+ tent at the A2 site. The circles represent Mn -, Sr-rich and REE- bearing piemontite (this study), the squares with cross piemontite (Dollase, 1969), the diamonds strontian piemontite (Catti et al., 1988), the squares strontian piemontites (Catti et al., 1989), the inverted triangle strontian piemontite (Ferraris et al., 1989), the triangles piemontite-(Sr) (Bonazzi et al., 1990), and the hexa- gons manganipiemontite-(Sr) (Armbruster et al., 2002).
- - the KD value even for synthetic Ca2Al3Si3O12(OH) 3+ Ca2Mn3 Si3O12(OH) piemontite (Anastasiou and Langer, 2+ Figure 3. Variation of the A1-Oi distances as a function of Mn 1977; Langer et al., 2002; Nagashima and Akasaka, 2004) 2+ content at the A1 site. The circles represent Mn -, Sr-rich and is only poorly understood. - REE bearing piemontite (this study), the squares with cross Armbruster et al. (2002) pointed out that the mean piemontite (Dollase 1969), the triangles piemontite-(Sr) (SRPM size of M1 is governed by Mn3+ + Fe3+ occupancy in this from Bonazzi et al., 1990), the diamonds REE-bearing piemon- tite (Bonazzi et al., 1992), the squares piemontite and androsite- site, confirmed also in this study. The relationship be- (La) (Bonazzi et al., 1996), and the inverted triangle Pb- and tween mean ionic-radius of M1 and
2+ - Mn at A1 has a bearing on the KD value, and (2) The Thus, their argument is not regarded in this study.
- - - KD value may be temperature and/or pressure dependent. A large Si1 O9 Si2 bridging angle (156.6°) in man- However, the temperature and/or pressure dependence of ganipiemontite-(Sr) was reported by Armbruster et al. 2+ Crystal chemistry of Mn -, Sr-rich and REE-bearing piemontite 149
(2002). Bermanec et al. (1994) suggested that REE-rich lin, N. and Medenbach, O. (2002) Manganvesuvianite and 3+- and REE-poor epidote-group minerals can roughly be tweddllite, two new Mn silicate minerals from the Kalahari manganese fields, South Africa. Mineralogical Magazine, 66, distinguished using the bond-length distortion DI(oct) 137-150. - - (Baur, 1974) for M3 versus the Si1 O9 Si2 angle. They Armbruster, T., Bonazzi, P., Akasaka, M., Bermanec, V., Chopin, also found a strong negative linear relationship between C., Heuss-Assbischler, S., Liebscher, A., Menchetti, S., Pan, the Si1-O9-Si2 angle and the M3-O8 distance. The Y. and Pasero, M. (2006) Recommended nomenclature of ep- - M3-O8 distance decreases with increasing bridging angle. idote group minerals. European Journal of Mineralogy, 18, 551-567. The Si1-O9-Si2 angle in this study (147.4°) plots in the Baur, H. (1974) The geometry of polyhedral distortions. Predic- - - - field of REE rich epidote group minerals. The M3 O8 tive relationships for the phosphate group. Acta Crystallo- distance (1.892 Å) is shorter than the expected value (~ graphica, B30, 1195-1215. 1.93 Å) from the regression line (Bermanec et al., 1994). Bermanec, V., Armbruster, T., Oberhänsli, R. and Zebec, V. (1994) - - Similar Si1-O9-Si2 angles have been found for REE- Crystal chemistry of Pb and REE rich piemontite from Ne- zilovo, Macedonia. Schweizerische Mineralogische und bearing piemontite (148.2-148.7°: Bonazzi et al., 1992) Petrographische Mitteilungen, 74, 321-328. - - and Pb and REE rich piemontite (148.8°: Bermanec et Bonazzi, P., Menchetti, S. and Palenzona, A. (1990) Stronti- al., 1994). All of them are characterized by significant opiemontite, a new member of the epidote group from Val 2+ Mn in A1, and also have shorter M3-O8 distances than Graveglia, Liguria, Italy. European Journal of Mineralogy, 2, predicted by the regression line of Bermanec et al. (1994). 519-523. The possible reasons are as follows; (1) possibly the Bonazzi, P., Garbarino, C. and Menchetti, S. (1992) Crystal chem- istry of piemontites: REE-bearing piemontite from Monte structurally investigated crystal had higher REE content Brugiana, Alpi Apuane, Italy. European Journal of Mineralo- than indicated by chemical analysis of material from the gy, 4, 23-33. same specimen, and (2) Mn2+ at A1 may have special Bonazzi, P., Menchetti, S. and Reinecke, T. (1996) Solid solution bearing on M3-O8 distances and Si1-O9-Si2 angles. It is between piemontite and androsite-(La), a new mineral of the epidote group from Andros Island, Greece. American Miner- also well known that the Si1-O9-Si2 bridging angle de- alogist, 81, 735-742. creases with increasing Fe3+ and Mn3+ content in octahe- Bruker (1999) SMART and SAINT-Plus. Versions 6.01. Bruker dral sites for Ca end-member epidote and piemontite (i.e. AXS Inc., Madison, Wisconsin, USA. Giuli et al., 1999; Langer et al., 2002; Nagashima and Burns, R.G. and Strens R.G.J. (1967) Structural interpretation of Akasaka, 2004). Thus, the substitution on both the A and polarized absorption spectra of the Al-Fe-Mn-Cr epidotes. M sites affects the variation of the bridging angle. Mineralogical Magazine, 36, 204-226. Catti, M., Ferraris, G. and Ivaldi G. (1988) Thermal behavior of Piemontite investigated in this study has a complex the crystal structure of strontian piemontite. American Miner- chemical composition. Thus, an interpretation of structur- alogist, 73, 1370-1376. al variations due to cation substitution is not straightfor- Catti, M., Ferraris, G. and Ivaldi G. (1989) On the crystal chemis- ward. The observed structural flexibility is governed by try of strontian piemontite with some remarks on the nomen- several, sometimes correlated, parameters. For better un- clature of the epidote group. Neues Jahrbuch für Mineralogie. Monatshefte, H.8, 357-366. derstanding it is necessary to investigate synthetic epi- Dollase, W.A. (1968) Refinement and comparison of the structures dote-group minerals having simple compositions accom- of zoisite and clinozoisite. American Mineralogist, 53, panied by studies on natural epidote-group minerals in 1882-1898. order to analyze the effect of ionic substitution on the Dollase, W.A. (1969) Crystal structure and cation ordering of variation of the crystal structure. piemontite. American Mineralogist, 54, 710-717. Enami, M. and Banno, Y. (2001) Partitioning of Sr between coex- ACKNOWLEDGMENTS isting minerals of the hollandite- and piemontite-groups in quartz-rich schist from the Sanbagawa metamorphic belt, Ja- We thank Prof. A. Yoshiasa, associate editor, and two pan. American Mineralogist, 86, 205-214. Ferraris, G., Ivaldi, G., Fuess, H. and Gregson, D. 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