Structure of Synthetic Liimg,Cu)Cuz[Siz06]Z: a Unique Chain Silicate Related to Pyroxene

American Mineralogist, Volume 82, pages 143-148, 1997 Structure of synthetic LiiMg,Cu)Cuz[Siz06]z: A unique chain silicate related to pyroxene HIROYUKI HORIUCHI,1 AKIHIRO SAITO,I TOSHINAGA TACHI,2 AND HIROSHI NAGASAWA2 'Mineralogical Institute, Faculty of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan 'Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171, Japan ABSTRACT A unique Cu-bearing chain silicate, Li2(Mg,Cu)Cuz[Si206b was synthesized, and the structure was determined by single-cry~tal X-ray diffraction techniques. The structure was found to be triclinic, spacoe group PI, with unit-cell parameters a = 5.7068(7), b = 7.4784(9), c = 5.2193(3) A, ex = 99.911(8), J3 = 97.436(8), 'Y = 84.52(1)°, and Z = 1. The arrangement of zweier single chains, [Siz06], differs significantly from chain arrange- ments in the pyroxene and pyroxenoid structures, and the "I-beam" description of the pyroxene structure is not applicable. The structure may be classified as a new derivative type of the pyroxene structure, with an "oblique I-beam". Cu atoms are coordinated by four atoms in a square-planar arrangement with 1.94-2.00 A for Cu-O and two atoms with °longer Cu-O distances of 2.41-2.92 A, consistent with the crystal-field stabilization° ~f the d9. electroni~ structure of Cu2+. The square-planar Cu04 units form a [CunOZn+2] nbbon with n = 3 ill the structure, which is also found in Cu-bearing chain silicates such as shattuckite and plancheite with n > 3. Mg is octahedrally coordinated by atoms, but the configuration is affected by the partial replacement by Cu. ° INTRODUCTION Examples of Cu-bearing silicates with single-chain Co- and Ni-bearing silicates have been studied mainly structures are also quite rare. The mineral shattuckite and for the purpose of simulating the structural behavior of synthetic CuMg[Siz06] and Na2Cu3[Si40,z] are the only ferromagnesian silicate minerals under the high-temper- known examples. CuMg[Si206] was reported to be anal- ature and high-pressure conditions prevailing in the ogous to the clinopyroxene structure with space group Earth's interior. However, the crystal-chemical behavior P2/c, with an ordered arrangement of Mg and Cu as of Cu-bearing silicates is different, possibly because of determined by the semiquantitative comparison of ob- the electronic configuration of Cu2+. Most of these natu- served and calculated X-ray powder diffraction intensities rally occurring minerals are hydrous or hydrated silicates (Breuer et al. 1986). However, the detailed structure is and some are chain silicates. The following are examples unknown. The configuration of the [Siz06] chain of shat- of Cu-bearing silicate minerals that have chain structures: tuckite is similar to that of pyroxene with straight chains, shattuckite, Cus[Siz06MOH)2 (Mrose and Vlisidis 1966; but the cation arrangement is different from that of py- Kawahara 1976; Evans and Mrose 1966, 1977); plan- roxene. The structure of plancheite is a derivative struc- cheite, Cug[Si4011]z(OH)..xHzO (Evans and Mrose 1966, ture of shattuckite but with amphibole-type double chains 1977); and liebauite, Ca6CulO[Si,gOs2](Zoller et al. 1992). ~nstead of single chains. The [Si206] unit of Na,Cu3[Si40,z] Liebauite is one of the few anhydrous Cu-bearing min- is a single chain; however, it is different from that of erals. The layer silicate cuprorivaite, CaCu[Si40,o] (Mazzi pyroxene but similar to that of haradaite (Takeuchi and and Pabst 1962), is another example of a natural anhy- Joswig 1967). drous Cu-bearing phase. On the other hand, most syn- Thus, Cu-bearing pyroxene or pyroxene-derivative thetic Cu-bearing chain silicates are anhydrous. Typical structures are very rare, even though the ionic radius of Cu2+ is similar to that of other transition metal ions. In examples include Na,Cu3[Si40,zJ (Kawamura and Kawa- hara 1976), Na4Cu,[Sig02o] (Kawamura and Kawahara the present paper, a new Cu-bearing silicate structure with 1977), CuMg[Si206] (Breuer et al. 1986), and straight single chains is described and the crystal-chem- CaBa3Cu[Si6017] (Angel et al. 1990). However, synthetic ical behavior of Cu2+ in the structure is discussed. Cu-bearing chain silicates related to pyroxene are rare in EXPERIMENTAL METHOD comparison with silicates containing Co, Ni, or Zn (e.g., Morimoto et al. 1970, 1974, 1975). Nevertheless, knowl- Sample preparation and chemical analysis edge of the phase stabilities and Cu configurations of Cu- The crystalline phase of Liz(Mg,Cu)Cu,[Siz06]z was -bearing silicate structures is important for understanding synthesized in an oxide-flux mixture, which was primar- the mineralogy and geochemistry of transition metals. ily prepared for the synthesis of Cu-bearing pyroxene. 0003-004X/97/0 102-0 143$05.00 143 -~._-- 144 HORIUCHI ET AL.: STRUCTURE OF A SYNTHETIC CHAIN SILICATE TABLE 1. Results of chemical and structure analyses TABLE2. Crystallographic data and experimental conditions for structum analysis Chemical analysis Result of structure analysis Compo- nents wt% (expt.) mole ratio wt% (calc.) mole ratio Chemical formula Li...(Mg,Cu)Cu2[Si20.]2' Z ~ 1 Space group P1 Si02 50.6(4) 4 50.63 4 Calculated density 3.580 g/cm' MgO 8.4(3) 0.99 7.39(3) 0.871(3) Size of specimen 0.10 x 0.20 x 0.37 mm' CuO 33.7(4) 2.01 35.69 2.129 X-ray source MoKo: 40kV, 25mA (fine focus) Lip 7.3(7) 1.16 6.3(1) 0.98(2) Monochromator Pyroly1ic graphite Total 100 100 Apparatus AFC5S/Rigaku Scan mode, width, and 26 w, 6,26 ~ 1.4°(tan 6) + 0.65°, 6.0°/ Note: The weight percent values were normalized to a total of 100% speed min o;f 0), and the mole ratio values of Si02 were constrained to 4. The results 01 Obs. refl. 4526 (Fob. 0° < 26 100° (hemi- structure analysis were obtained by the refinement of site occupancies sphere) "" (see text) of Cu, M, and Li sites. Other sites were assumed to be fully o;f 0), Indep. retl. 4221 (Fob. Rw = 0.041, R = 0.057 ~ 0.041, R ~ occupied by each single atom. Refl. used 3573 (Fob. 2> 5"FOO'), Rw 0.047 The molar ratio of oxides was MgO:CuO:Si02 = 1:1:2, ic positions were found for cations and six for atoms. and that of the flux was Li20:Mo03:V20s = 42.8:51.2: ° 6.0. The percentage of MgO-CuO-Si02 mixture was They were initially assigned as Mg, Cu, Si I, and Si2 for cations and as 01-06 for the atoms. The value of about 20% of the total weight of the sample. The mixture ° was heated to 850°C for 3 h in a platinum crucible. The l/u2pob, was used as a weight for each reflection. The temperature was gradually decreased from 850 to 650°C weighted residual Rw = 0.068 was obtained after the over 8 d, after which the mixture was slowly cooled to least-squares refinement of the atomic positions and an- room temperature by turning off the electricity. All pro- isotropic displacement factors. The site occupancies were cesses were performed in air. In the high-temperature re- assumed to be 100% for the above atoms. action, Lip, Mo03, and V20S serve as the flux, although Further difference-Fourier syntheses using the above Li is incorporated into the final product. Transparent refined atomic coordinates revealed positive residual greenish blue crystals (1-2 mm) were obtained by dis- peaks at the position (M) in which Mg was assigned, and solving surrounding materials in warm dilute HCl. at another new position (P). No other significant residual The chemical composition of the products was ana- peaks were observed. From the chemical composition, the lyzed by electron probe microanalysis for Mg, Cu, and M position was modeled as a statistical distribution of Si, and by ICP for Li. The results are shown in Table 1 Mg and Cu with a total occupancy of unity. The value of together with the chemical composition obtained from 0.871(3) was obtained for the occupancy factor of Mg in structure analysis, which is discussed later. On the basis the M site by least-squares refinement. As a result, the of the chemical analysis, the idealized chemical formula statistical distribution of M is 0.87Mg + 0.13Cu. The P is assumed to be Li2MgCu2Si40'2' site was assigned to Li on the basis of the chemical composition and the electron density on the Fourier and dif- Structure determination and refinement ference-Fourier maps. The occupancy value of the Li site The crystal-structure determination was performed us- refined to 0.98(2). Therefore, the Li site was assumed to ing single-crystal X-ray diffraction techniques. The crys- be fully occupied within experimental error. Finally, dif- tals are often twinned, so the single-crystal specimen for ference-Fourier syntheses using all atoms, including the the structure determination was obtained by careful sep- statistical distribution of Mg and Cu for M, were calcu- aration from the twinned crystal. Experimental conditions lated and no residual peaks were observed. Although the for X-ray diffraction data collection and reliability factors space group was initially assumed to be PI, the structure for the final results of the refinement are given in Table model finally refined to the space group PI. This result 2. The structure analysis was performed using 3573 in- is consistent with the crystal structure and morphology. dependent reflections with Fob, 2:: 5upob,' An absorption According to the classification by Liebau (1985), the correction was made by using a calibration curve ob- actual structure formula was concluded to be Lip tained by the tV scan method.

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