Strontiopiemontite, a new member of the epidote group, from Val Graveglia, Liguria, Italy

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1 Eur. J. Mineral. 99,,59-5 Strontiopiemontite, a new member of the epidote group, from Val Graveglia, Liguria, Italy PAOLA BONAZZI*, SILVIO MENCHETTI* and ANDREA PALENZONA** * C.N.R. Dipartimento di Scienze della Terra delγuniversità, Via La Pira,-5 Firenze, Italy ** Istituto di Chimica Fisica dell'università, Corso Europa 6,-6 Genova, Italy Abstract: Strontiopiemontite, a new member of the epidote group with simplified formula CaSr(Al, Mn, Fe) - SbOnOlOH), occurs as small deep red prismatic crystals in the manganese ore deposit of Val Graveglia, Liguria, Italy. Physical properties, including the powder pattern, are similar to those of piemontite. Crystals are often twinned {}. Crystal structure refinements performed on two separate crystals clearly show that the entry of Sr into the structure mainly occurs on the A() site (max. 7%) and leads to a significant increase of the A() polyhedron as well as the c sin/? value. Key-words: Strontiopiemontite, epidote group, structure refinement, chemical analysis. Introduction Piemontite from Val Graveglia, Italy, was first reported by Cortesogno et al. (979) in a paper dealing with manganese minerals from Liguria. The "piemontite" from the same occurrence described in the present study, however, should be considered as a new member of the epidote group. In fact the structural site A() is mainly occupied by Sr, which is a different chemical component from the Ca occurring in piemontite. The new mineral and the name have been approved by the New Minerals and Mineral Names IMA commission. The type material is preserved in the Museo di Mineralogia delγuniversità di Firenze. Occurrence, appearance and physical properties The manganese mineralization at Val Graveglia is associated to the "Mt. Alpe Cherts" formation, overlying the ophiolitic rocks of Northern Apennines. The complex tectonic and metamorphic history of the ligurian-piedmont ocean produced a complex mineral association which, according to Cortesogno & Venturelli (978), can be ascribed to low-temperature metamorphic conditions (prehnite-pumpellyite facies). In addition to braunite, a wide variety of uncommon mineral species are present (Cortesogno et al. 979). The strontiopiemontite from Val Graveglia was collected in the mining area of Molinello and Cassagna. Two samples, and, were selected for the present study: the standard properties refer to. The mineral appears generally as small prismatic crystals, elongated parallel to [] (up to.5 mm), in veinlets about mm thick, which cut black manganese ore deposit made up mainly of quartz and braunite. The veinlets also contain calcite, rhodonite, rhodochrosite, and ganophyllite. Physical properties are very similar to those of piemontite (colour deep red, streak purple-brown, lustre vitreous and transparent). Strontiopiemontite shows a perfect {} cleavage and hardness 6. The density value measured by heavy liquids ranges from.65() to.7() g/cm, whereas the calculated value is.7 g/cm. Strontiopiemontite is biaxial (+) and strongly pleochroic with X yellow-orange, Y violet and Z reddish violet. As regards the optical orientation, Y = /?, while, because of experimental difficulties, the Z:c angle and the refraction indices were not 95-/9/-59$.5 (< ' 99 E. Schweizerbart'sche Verlagsbuchhandlung, D-7 Stuttgart

2 5 P. Bonazzi et al measured. The value of the refraction index calculated from the Gladstone-Dale relationship, is.76, using the empirical formula given below as well as the measured density and the constants given by Mandarino (976). This calculated refraction index matches the value of n (.76) derived from the data published by Cortesogno et al (979) for the piemontite from Val Graveglia. Using this n value, the l-(kp/kc) parameter is.5, so the compatibility (Mandarino, 98) is good. Chemical composition Strontiopiemontite was analysed using an ARL- SEMQ electron microprobe with an operating voltage of 5 kv: analyses were carried out on several crystals. Results are given in Table. Table. Chemical analyses of strontiopiemontite from Val Graveglia. Oxide wt % range probe standard Crystallography The indexed powder pattern is given in Table. It was obtained with a Guinier camera using CuKα radiation and Si as internal standard. Indexing was performed on the basis of the single crystal data and comparing the observed intensities with those calculated from the refined structure. The pattern is very similar to that of piemontite. The unit cell parameters, determined from powder data by s of a least-squares method, are: a = 8.86(), b = 5.68(), c =.9() Å, ß=.7(). Intensity data were collected on two crystals, coming from and respectively. Both crystals are tiny (:. X.6. mm ; :.6 X.6 X. mm ) and the latter was found to be twinned with () as twin plane. Fig. reports the reflection population due to the whole twinned crystal, drawn in the a*c* reciprocal lattice plane. The intensity data can be sorted into two categories; the first contains the well-sep- Table. X-Ray powder pattern of strontiopiemontite from Val Graveglia; d-spacings in Å units. CaO SrO MgO AIO Fe O Mn O Ti Si MnO HO total.69.5 < <..7.87*.7** clinopyroxene synth. anorthite ilmenite spessartine clinopyroxene spessartine amphibole spessartine * MnO/Mn O ratio was assumed on the basis of charge balance and site occupancy refinement. ** H O wt % was calculated to give one H per formula unit. Manganese has been partitioned between MnO and Mn O on the basis of the the site occupancy refinement as well as charge balance criteria for the octahedra and the A polyhedra. H O was calculated to give one H per formula unit. With these assumptions the empirical formula (based on O = ) can be written: (Ca 79 Mn i ) (Sro.6sCao.i) (Al,. 8 Mno. 86 Fe.)- Si OnO(OH). The ideal formula for the Sr end-member is: CaSr (Al, Mn +, Fe + ) Si OnO(OH). l/lo ^meas dcalc Unit cell parameters: a '- 8.86() c-.9() h k 5 b= 5.68() =.7()

3 Strontiopiemontite: a new mineral from Val Graveglia, Italy 5 arated reflections (/ =, 5, 9, ) and the second groups more or less overlapping reflections. From the intensities of non-superimposed reflections, the relative s of the two members of the twin were computed; the resulting A/B ratio is about.9. Many uncertainties, however, occur in determining the intensities of the partially overlapped reflections. Intensity data were corrected for absorption by s of the semiempirical Table. Crystal data and experimental details. Fig.. An a*c* layer of the reciprocal lattice of a twinned strontiopiemontite crystal. The open circled lattice belongs to the A member and the black-dotted lattice to the B member of the twin. Crystal system space group Cell parameters Apparatus Wavelength theta-range ( ) Scan mode Scan width ( ) Scan speed ( /s) No. of independent reflections No. of reflections with Fo > na (Fo) R(%) a = 8.89() b = 5.67() c =.() ß =.6() V =65. () Enraf Nonius CAD MoKα (.769 Å) - ω n = 8 5. Monoclinic P,/m a = 8.87() b = 5.68() c =.9() ß =.88() V =66.7 () Enraf Nonius CAD MoKα (.769 Å) -8 ω n = 8.5 Table. Atomic positional parameters and equivalent isotropic temperature factors (Å ) with their standard deviations. X y z u eq. X y z u eq. A(l).76( ) A().599( ) Si(l).7( 5) Si().685( 5) Si().88( 5) M(l).( ) M().( ) M().95( ) O(l).5( 8) ().( 9) ().7957( 9) ().569() (5).86() (6).67() (7).566() (8).57() (9).68(7) ().89().75( ).75( ).75( ).5( ).75( ).( ).( ).5( ).995().976().5().5( ).75( ).75( ).75( ).5( ).5( ).5( ).559( ).8( ).( 5).77( ).5( 5).( ).5( ).89( ).( 7).9( 8).( 8).7().5().77().7().().(6).().8().6( 7).75().69().59().85().69().65( 8).(5).6(6).7(5).().78().97().6().().9().6() A(l).766( ) A().595( ) Si(l).( 5) Si().689( 5) Si().8( 5) M(l).( ) M().( ) M().9( ) O(l).( 8) ().( 7) ().795( 8) ().58() (5).7() (6).65() (7).55() (8).5() (9).69() ().8() H.5(5).75( ).75( ).75( ).5( ).75( ).( ).( ).5( ).99().978().55().5( ).75( ).75( ).75( ).5( ).5( ).5( ).5( ).559( ).( ).( ).78( ).56( ).( ).5( ).98( ).5( 7).5( 6).( 7).().6().().7().8().().9().5(5).().5( 8).8().97().99().().().97( 8).8(5).97().9().7(5).9(7).6(8).95().5(8).9().(7).5(5) Eur. J. Mineral.

4 5 P. Bonazzi et al. method of North et al (968). Structure refinement performed using the SHELX program (Sheldrick, 976) led to a final R values of.5% () and 5.% () for all the observed reflections. Other details of the experimental work are given in Table. Atom parameters are given in Table, while Table 5 shows the interatomic distances. Crystal chemical formulae obtained from structure refinements of and are: (Ca a97 Sr a ) (Sr α59 Ca α ) (Alo.7Mo.a) Al (M. 98 Al. Ü )Si OiH (Ca α7s Mn α ) (Sr α7 Ca a7 ) (A. 78 M.) A (M. 98 Al. Ü )Si OiH where M = Mn + + Fe +. Table 5. Selected interatomic distances (Å) and s of coordination polyhedra (Å ). A() -(7) (, vi) (, vii) -(, viii) -() -(. ix) (, i) -(.) -(8, i) -(8).7().68().6 (8).6 (8).75 (8).75 (8).7 (9).7 ().9 ().9 ().().69 (9).599 (7).599 (7).7 (7).7 (7).7 (8).7 ().7 ().7 () A(D -(7) -(, i) -(, ii) -(, iii) (, iv) -(5,v) -(6. v) -(9, i) - (9).7(). (9). (9).55 (8).55 (8).59().857 (9).9 (5).9 (5).68(). (8). (8).65 (8).65 (8).59().86 (8). (). () ? M() -() -(, xiv) -(. XV) -(. xvi) (6. xii) - (6, xiv.).86 (9).86 (9).865 (6).865 (6).98 (9).98 (9).866 (9).866 (6).85 (6).85 (6).95 (9).95 (9) M(l) -() -(, xi) -(, xii) -O(l.xiii) -(5, xii) -(5, xi).866 (7).866 (7).97 (7).97 (7).975 (8).975 (8).87 (6).87 (6).95 (7).95 (7).979 (8).979 (8) Si(l) -(7) -(9.x) -() -(, ix).57().59(5).68 (8).68 (8) l.567( 9).67().6 (8).6 (8) M() -(8) -() -(, xii) -(, ix) -(, xii) -(, ix).855().9(). (8). (8).7 (8).7 (8).88().899 (9).8 (7).8 (7).7 (8).7 (8) Si() -() -(, ix) -(6) -(5).66 (8).66 (8).68(5).667().6 (7).6 (7).6().65 (9) Si() -(8) -() -(, ii) -(9).68(5).68 (8).68 (8).67(5).67().69 (7).69 (7).65() symmetry key: none x,y,z; i) x, l+y,z; ii) x, / y,z; iii) x, /+y, z; iv) x, y, z; v) l+x,y,z; vi) x, /+y, z; vii) -x, -/+y, -z; viii) -x, -y, -z; ix) x, /-y,z; x) -x, /+y* -z; xi) - x, -/+y, -z; xii) x, l+y,z; xiii) x, y, z; xiv) x, /+y, z; xv) l+x,y,z; xvi) x, y, z.

5 Strontiopiemontite: a new mineral from Val Graveglia, Italy 5 The structural arrangement is quite similar to that of monoclinic epidotes and the octahedral M O distances well agree with the proposed cation distribution in M(l), M() and M (). However the high content of Sr leads to some interesting comparisons with samples having little or no Sr (Dollase, 969; Catti et al, 989; Bonazzi & Menchetti, 989). The substitution Sr ±* Ca which takes place in the -coordinated A() site, causes an increase in the A()-O distance and the polyhedral. This lengthening is not isotropic, affecting mainly the shorter distances, in particular A() O(7). The A(l) site is not affected by significant Sr ** Ca substitution. However, little geometrical variation is induced by the entry of Sr into the adjacent A() site. In fact, with increasing Sr, O(7) goes away from A() and, to save the charge balance, approaches A(l). Therefore, the A(l) O(7) distance decreases with increasing Sr and, since O(7) moves along the c D \ i Sr *(A) \ a.p.f.u. Fig.. Linear relation between the z/c coordinate of (7) and the Sr content in A(). The best straight line fitting is y = x, with r = (filled squares = this work; star = Dollase, 969; empty squares = Catti eta/., 989). i N sin/? direction, its z/c coordinate is well correlated to the Sr content in A(), as shown in Fig.. The shortening of the A( ) O(7) distance is greater in the crystal from because of the entry of Mn + into the A () site. It is clear that the unit cell expands with the entry of Sr, mainly because of an increase of the c sin ß value. Acknowledgement: The authors are grateful to Prof. G. Vezzalini (Università di Modena, Italy) for her help in the probe analysis. References Bonazzi, P. & Menchetti, S. (989): Contribution to the crystal chemistry of piemontites: the A-polyhedra. Godisniak Iugoslavenkog Centra za Kristalografiju, (suppl.\58. Catti, M., Ferraris, G., Ivaldi, G. (989): On the crystal chemistry of strontian piemontites with some remarks on the nomenclature of the epidote group. NeuesJahrb. Miner. Mh., 989, Cortesogno, L., Lucchetti, G, Penco, A. M. (979): Le mineralizzazioni a manganese nei diaspri delle ofioliti liguri: mineralogia e genesi. Rend. SIMP, 5, Cortesogno, L. & Venturelli, G (978): Metamorphic evolution of the Ophiolite sequences and associated sediments in the Northern Apennines-Voltri Group, Italy. I.Ü.G.S. Sci. Rep., 8, 5-6. Dollase, W. A. (969): Crystal structure and cation ordering of piemontite. Amer. Mineral., 5, Mandarino, J. A. (976): The Gladstone-Dale relationship: Part I. Derivation of new constants. Can. Mineral,,98-5. (98): The Gladstone-Dale relationship: Part IV. The compatibility concept and its application. Can. Mineral., 9,-5. North, A.C.T., Phillips, D.C., Mathews, F. S. (968): A semiempirical method of absorption correction. Acta Cryst., A, Sheldrick, G M. (976): SHELX-Program for crystal structure determination. Univ. of Cambridge, England. Received 9 November 989 A ccepted March 99

The crystal structure of BiOCI

The crystal structure of BiOCI Zeitschrift fur Kristallographie 205, 35-40 (1993) COby R. Oldenbourg Verlag, Munchen 1993-0044-2968/93 $ 3.00+0.00 The crystal structure of BiO K. G. Keramidas, G. P. Voutsas, and P. 1. Rentzeperis Applied