Search

The ganophyllite-group minerals, with general formula [(K, Na, Ca). sub.x][Mn.sub.6]([Si.sub.9]Al)[O.sub.24][(OH).sub.4]*n[H.sub.2]O (ganophyllite = K, eggletonite = Na, tamaite = Ca), are complex modulated layer silicates that contain 2:1 trioctahedral layers with [Mn.sup.2+][-O.sub.6] octahedra. Pervasive superstructures have frustrated the numerous attempts at solution of the atomic arrangement of these modulated layer silicates. An orthorhombic dimorph of tamaite has been discovered and its atomic arrangement solved, elucidating the elusive structure of the ganophyllite-group minerals. The dimorph was discovered in the Val Graveglia mining district in the northern Apennines, approximately 35 km east of Genoa, Italy. The phase crystallizes in space group Pnma, a = 16.8146(6), b = 25.2036(9), c = 13.3866(5) [Angstrom]. The weak reflections from the 3x 'superstructure' along a, long observed but never successfully measured in ganophyllite-group minerals, were obtained using a CCD detector and subsequently the atomic arrangement was solved and refined (R = 0.079). The structural modulation in ganophyllite-group minerals results from the misfit between the sheets of [Mn.sup.2+] [O.sub.6] octahedra and silicate tetrahedra. The atomic arrangement consists of corrugated T-O-T layers, with inverted tetrahedra in the tetrahedral sheets connecting adjacent layers along b. The inverted tetrahedra exist as four-member rings, and incorporate Al, with a maximum Al occupancy of [Si.sub.2.00][Al.sub.2.00]. Charge balance for the substituent Al is maintained by adding cations (Ca, K, Na) or [6] Al sufficient to balance the charge lost by the Al [left and right arrow] Si substitution in the interlayer tetrahedra. Zeolitic [H.sub.2]O molecules also exist in the interlayer channels. Contrary to earlier speculation, the 5.6 [Angstrom] 'subcell' (along a in the orthorhombic dimorph) observed in ganophyllite-group phases is not a true subcell, in that it does not have approximately equivalent atoms at (x, y, z), [~(x + 1/3), ~y, ~z], and [~(x + 2/3), ~y, ~z) for all atoms. Although the majority of the cation scattering (all Mn + 2/3 of the Si atoms) exist in a supercell-subcell relationship, as manifested in very strong h = 3n and very weak (heretofore immeasurable) h [not equal to] 3n reflections, the lack of such a relationship for all atoms prohibited a successful solution based on previous assumptions of a subcell-supercell relationship.

The new mineral species rüdlingerite, ideally Mn2+2V5+As5+O7·2H2O, occurs in the Fianel mine, in Val Ferrera, Grisons, Switzerland, a small Alpine metamorphic Mn deposit. It is associated with ansermetite and Fe oxyhydroxide in thin fractures in Triassic dolomitic marbles. Rüdlingerite was also found in specimens recovered from the dump of the Valletta mine, Canosio, Cuneo, Piedmont, Italy, where it occurs together with massive braccoite and several other As- and V-rich phases in richly mineralized veins crossing the quartz-hematite ore. The new mineral displays at both localities yellow to orange, flattened elongated prismatic, euhedral crystals measuring up to 300 μm in length. Electron-microprobe analysis of rüdlingerite from Fianel gave (in wt%): MnO 36.84, FeO 0.06, As2O5, 25.32, V2O5 28.05, SiO2 0.13, H2Ocalc 9.51, total 99.91. On the basis of 9 O anions per formula unit, the chemical formula of rüdlingerite is Mn1.97(V5+1.17 As0.83Si0.01)Σ2.01O7·2H2O. The main diffraction lines are [dobs in Å (Iobs) hkl]: 3.048 (100) 022, 5.34 (80) 120, 2.730 (60) 231, 2.206 (60) 16-1, 7.28 (50) 020, 2.344 (50) 250, 6.88 (40) 110, and 2.452 (40) 320. Study of the crystal structure showcases a monoclinic unit cell, space group P21/n, with a = 7.8289(2) Å, b = 14.5673(4) Å, c = 6.7011(2) Å, β = 93.773(2)°, V = 762.58(4) Å3, Z = 4. The crystal structure has been solved and refined to R1 = 0.041 on the basis of 3784 reflections with Fo > 4σ(F). It shows Mn2+ hosted in chains of octahedra that are subparallel to [-101] and bound together by pairs of tetrahedra hosted by V5+ and As5+, building up a framework. Additional linkage is provided by hydrogen-bonding through H2O coordinating Mn2+ at the octahedra. One tetrahedrally coordinated site is dominated by V5+, T(1)(V0.88As0.12), corresponding to an observed site scattering of 24.20 electrons per site (eps), whereas the second site is strongly dominated by As5+,T(2)(As0.74V0.26), with, accordingly, a higher observed site scattering of 30.40 eps. The new mineral has been approved by the IMA-CNMNC and named for Gottfried Rüdlinger (born 1919), a pioneer in the 1960–1980s, in the search and study of the small minerals from the Alpine manganese mineral deposits of Grisons. [ABSTRACT FROM AUTHOR]

Human-megafauna interaction in the Americas has great scientific and ethical interest because of its implications on Pleistocene extinction. The Arroyo del Vizcaíno site near Sauce, Uruguay has already yielded over 1000 bones belonging to at least 27 individuals, mostly of the giant sloth Lestodon. The assemblage shows some taphonomic features suggestive of human presence, such as a mortality profile dominated by prime adults and little evidence of major fluvial transport. In addition, several bones present deep, asymmetrical, microstriated, sharp and shouldered marks similar to those produced by human stone tools. A few possible lithic elements have also been collected, one of which has the shape of a scraper and micropolish consistent with usage on dry hide. However, the radiocarbon age of the site is unexpectedly old (between 27 and 30 thousand years ago), and thus may be important for understanding the timing of the peopling of America.