Gabrielle Donnay

A New Allotropic Form of Carbon from the Ries Crater

A new allotropic form of carbon occurs in shock-fused graphite gneisses in the Ries Crater, Bavaria. The assemblage in which it occurs consists of hexagonal graphite, rutile, pseudobrookite, magnetite, nickeliferous pyrrhotite, and baddeleyite. Electron-probe analyses indicate that the new phase is pure carbon. It is opaque and much more strongly reflecting than hexagonal graphite. Measurement of x-ray diffraction powder patterns leads to cell dimensions a = 8.948 � 0.009, c = 14.078 � 0.017 angstroms, with a primitive hexagonal lattice.

X-ray Diffraction Studies of Echinoderm Plates.

X-ray diffraction studies confirm that, with few exceptions, each skeletal element of echtinoderms is a single crystal of magnesium-rich calcite and that a relation exists between the shape of the element and the crystallographic a- and c-axes. The exceptions incluide the teeth of echinoids, and the calcareous ring as well as the anal teeth of holothurians. The tubercles of an echinoid plate begin their growth as parts of the single crystal of the plate; under the mechanical action of the spines that are attached to them, they become partly polycrystalline, as shown by scanning electron microscopy and by x-ray powder diffraction. The interface between inorganic crystalline and organic amorphous matter in the skeletal element appears to be the first example reported in nature of a periodic mninimal suirface.

Magnetic susceptibility and triangular exchange coupling in the tourmaline mineral group

Abstract Magnetic susceptibilities of three iron-rich tourmaline crystals from Mexquitic (Mexico), Pierpont (New York), and Madagascar with different and known chemical compositions have been studied from 8° to 300°K. The iron atoms in the tourmaline crystal structure, space group R3m , a∼15·9, c∼7·2 , are situated at the three corners of an equilateral triangle and are close enough for magnetic exchange interaction. For buergerite, the Mexquitic sample, the susceptibility data lead to an exchange constant J/k of 7·5°K. Although the amount of aluminum would be sufficient to fill point position 18(c) exactly, the magnetic data are consistent with some substitution of ferric iron for aluminum, as previously determined from X-ray and neutron diffraction studies. Some aluminum thus replaces iron in position 9(b). Exchange constants were also estimated for the other two magnesium-iron specimens, of which the madagascar sample is aluminum deficient. The results agree with the evidence from optical spectra that there is considerable deviation from octahedral symmetry in the oxygen coordination polyhedra about the 9(b) and 18(c) point positions.

Crystal data on chlorophyll a

Abstract By using the hexagonal cell: a = 8.75 8 , c = 42.0 7 A., it is possible to index the eight sharp lines of the powder pattern of crystalline chlorophyll a. The cell volume, in view of the observed density of 1.079 g./cc., requires two molecules per cell. Chlorophyll is pyroelectric and optically active in solution, so that only space groups isomorphic with holoaxial point groups need be considered. In the hexagonal and orthorhombic systems molecular disorder must be assumed in any structural hypothesis. In the monoclinic system the only possible space group is P2 with cell dimensions: a = c = 8.75 8 , b = 42.0 7 A., β = 120 °0′. Packing considerations make this symmetry unlikely. The remaining possible space group is the triclinic P1 in which the two molecules are not symmetrically related to each other. The only prediction that can be made concerning their packing rests on the presence of reflection 1.10.0—the planes of the chlorin rings should be parallel and should make an angle of about 29 ° with (010).

The crystal structure of sonoraite, Fe3+Te4+O3(OH)·H2O

Sonoraite, FeTeO3(OH)·H2O, is monoclinic,P 21/c, witha=10.984(2),b=10.268(2),c=7.917(2) A, β=108.49(2)°. For 8 formula units per cell the calculated density is 4.179(2) g/cm3; the observed value is 3.95(1) g/cm3. The Supper-Pace automated diffractometer was used to collect 1884 independent reflections which were corrected for absorption. The structure was determined by an automated symbolic addition procedure. It was refined to a residualR of 6.2% using anisotropic temperature factors for the cations and isotropic temperature factors for the oxygen atoms. Chains of octahedra about Fe extend along [101]; edge-sharing pairs of these octahedra are joined by corner sharing. The Fe−Fe distances across the shared edges are 3.05 and 3.20 A, short enough to suggest magnetic interactions. All but one H2O are involved in the chains. The Te4+ ions have a pseudotetrahedral coordination, with three oxygen ions forming one face of the tetrahedron and the lone electron pair of Te occupying the fourth corner. The O−Te−O average bond angle is 95°. The Fe chains are tied together by Te−O bonds in all three dimensions.

Ferric Tourmaline from Mexico

Dark brown crystals, up to 10 mm long, occur in rhyolite at Mexquitic, San Luis Potosi, Mexico. They are short prismatic, showing {1120}, {3030}, {1011}, {0221}, with c/a 0.4521, measured with a goniometer, and distinct {1120} cleavage. With an unusual combination of cell dimensions, high density, high refractive indices, and extreme birefringence, this tourmaline falls outside the known elbaite-schorl and schorl-dravite series. A chemical analysis, recalculated on the basis of cell volume and density, gives close to the theoretical 150 atoms per cell, whether the iron is ferrous or ferric, but the physical properties indicate a ferric tourmaline.

Ewaldite, a new barium calcium carbonate

Mackelveyite type specimens turn out to be polycrystals consisting of two phases in syntactic intergrowth. One phase, for which the name mackelveyite is retained, is lime yellow, but usually shiny black owing to amorphous carbonaceous inclusions. Optically biaxial, nearly uniaxial, it is hexagonal by x-ray diffraction and has the cell dimensions reported byMilton et al. (1965), but its space group isP31m. The other phase, to be called ewaldite, is the major constituent of the polycrystals studied byDesautels (1967) under the name mackelveyite. It is bluish green and crystallizes inP63mc witha=5.284±7 (1 σ) andc=12.78±1 (1 σ) A. The intergrown crystals are turned 30° with respect to each other about a commonc direction; their cell dimensions obey the relations 3ce∼2cm and 2ae cos 30°∼am, whence 9Ve∼2Vm. Mackelveyite shows twinning by merohedry, twin plane\((2\overline {11} 0)\) and twin symmetry 6′m′ m. Ewaldite has not been observed to twin. The ewaldite formula is

Absence of neutral alkali atoms in rhodizite.

Ba (Ca_{0.48} RE_{0.20} Na_{0.15} K_{0.11} Sr_{0.03} U_{0.02} \square _{0.01} )(CO_3 )_2