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Rhodesite from Zeilberg, Unterfranken, Germany (Z), from Trinity County, California, USA (TC), and from San Venanzo, Rieti, Italy (SV), is orthorhombic with Space group Pmam. Crystallographic data, given in the order (Z), (TC), (SV), are: «0 = 23.416(5)/23.444(5)/23.428(20) Ä, bo = 6.555(l)/6.553(l)/6.557(8) Ä, CQ = 7.050(1 )/7.055(l)/7.064(8) Ä, Z = 2, £), = 2.268(1 )/2.264(l)/2.261 (4) g • cm"^ calculated for HKCa2Si80i9 • 5H2O. The crystal structure was refmed to /?(unweighted) = 0.044/0.079/0.044 and Ä(weighted) = 0.036/ 0.055/0.045 using 939/898/536 non-equivalent reflections. The structure contains loop-branched dreier double layers of corner-sharing [Si04] tetrahedra. Channel-like pores running parallel to [010] and [001] between the two sublayers of a double layer are occupied by potassium ions and water molecules. Adjacent double layers are held together by rather strong hydrogen bonds (ca. 2.60 Ä) and by two sets of calcium ions which are octahedrally coordinated by six terminal oxygen atoms from the Silicate layers, and by four terminal oxygen atoms plus two water molecules, respectively. Correspondence author: Prof. F. Liebau. 2 6 K.-F. Hesse. F. Liebau and S. Merlino Although no sodium has been found during refinement of the site occupation Factors it is concluded from comparison with the closely related crystal structures of delhayelite, hydrodelhayelite, macdonaldite and monteregianite that stoichiometric rhodesite is the sodium-free endmember of a solid Solution H K , _ , N a ^ + 2,Ca2-y{/Ä,3,2x}[Si80i9] • (6 z) H2O. Conditions for the formation of double-layer silicates of the rhodesite family in nature and for their synthesis are discussed. 1. I n t roduc t ion Rhodesite has first been described by Mountain (1957) as an alkali calcium Silicate hydrate from the Bultfontein Mine, Kimberley, South Africa. Gard and Taylor (1957) published crystallographic data and deduced from a chemical analysis made by Chalmers that the composition is "approximately (Ca, Na i , K2)i6Si3208o • 22H2O" with "atomic cell-contents calculated" for Ca, Na and K ca. 4.3, 2.7 and 2.0, respectively. Sheppard and Gude (1969) discovered rhodesite in a deposit of magadiite, NaSi70i3(0H)3 • SHzO, near Redding, Trinity County, California, USA, and gave its composition as Ca4.23MSo.27Nao.60K. i.95Sii 7.11O40 • IO.94H2O. From a third locality, Zeilberg, Unterfranken, Germany, rhodesite crystals of good quality have been described (Jakob, 1976). Gard and Taylor (1957) deduced from lattice constants, zeolitic water content and compositional similarity that rhodesite is probably structurally related to the fibrous zeolites such as thomsonite, NaCa2[(Al5Si5)02o] • 6H2O. In 1968 Cannillo, Rossi and Ungaretti, after solving the crystal structure of macdonaldite, H2Ca4Ba[Si80i9]2 • IO.4H2O, pointed out that rhodesite as well as delhayelite, (Na3Ca)Ca4K7[(AlSi7)0,9]2Cl2F4, the structure of which the same authors published in 1970, probably have Silicate double layers which are similar to those in delhayelite. Ragimov, Chiragov, Mamedov, and Dorfman (1980) described the crystal structure of hydrodelhayehte, H2KCa2[(AlSi7)0,9] • 6H2O, to have essentually the same double layers as delhayelite. Eventually, Ghose, Sen Gupta and Campana (1987) determined the structure of monteregianite, Na4K2Y2[Sii6038] • IOH2O, and assigned this mineral to the same family of double-layer silicates. This paper deals with structure determinations of rhodesite which have been carried out independently on Single crystals from Zeilberg and Trinity County (Hesse) and from San Venanzo (Merlino) and reports on optica! data. The relationships between the crystal structures of rhodesite, delhayelite, hydrodelhayelite, macdonaldite, and monteregianite are discussed and conclusions drawn about the chemical composition of rhodesite (Liebau). A short description of the rhodesite structure has already been given by Hesse (1987). Crystal structure of rhodesile 27 Table 1. Optical data and density of rhodesite from Zeilberg, Unterfranken. Parameter Value Orientation 1.5035(10) parallel [010] 1.508(2) parallel [100] < 1.5180(10) parallel [001] (needle axis) 2 V^ (calc) 68. r Optic sign ( + ) 0.0080 0.0075 0.0070 /)e.p[g cm-^] 2.27 Rhodesite from Zeilberg, Trinity County and San Venanzo are indicated as Z, TC and SV respectively, corresponding values are given in the order z r̂c/sv.

zeitschrift für kristallographie - crystalline materials

Crystal structure of rhodesite, HK 1-x Na x+2y Ca 2-y {lB,3,2 2  ∞ }[Si 8 O 19 ]·(6-z)H 2 O, from three localities and its relation to other silicates with dreier double layers