Whewellite

Abstract: Whewellite and weddellite, calcium salts of oxalic acid, have been found in the litter layer of several different soils, indicating that oxalate is a major metabolic product of fungi in natural environments. The presence of oxalate in soil solution speeds weathering of soil minerals and increases the availability of nutrients to vegetation.

Abstract: Whewellite, CaCrOo'HrO, and weddellite, CaCzO+' (2 + x) HrO where x = 0'5, occur in sediments, in plants, and in urinary stones' Their crystal structures have been refined to R : 0.033 and R : 0.032 respectively, using new sets of X-ray diffraction data, collected on a single-crystal di-ffractometer. Refined cell parameterc are'. Y2,/c, a:6.29O(l), D: l4'583(l), c: 10'116(l)A' B: 109.46(2)\", Z : 8 for whewellite; I4/m, a : 12.371(3), c :7'35't(2)A, Z : 8 for weddellite' During refinement of whewellite, three out of four H atoms could be located, and split positions, p\".tiutty occupied, for the two independent water molecules were found. In weddellite refinement, it was possible to locate all the H atoms, and a split position for the \"zeolitic\" water was found: a maximum water content of 2'5 HrO was confirmed' The comparison of the structures explains the relationships existing between some repeats of the two minerals and shows the differences between the Ca coordination polyhedra. A possible correlation between the structural features and the mechanism of formation of the two mineral species is suggested. The symmetry and planarity of the oxalate groups are discussed' Introduction Whewellite and weddellite are hydrated oxalates of calcium, respect ively CaCrOo' H'O and CaCrOo'(2+x)HrO (with x = 0.5), found naturally in plant tissues, in sediments as a mineral of organic origin, and in urinary stones. About 70 percent of human urinary stones contain whewellite and/or weddellite, either singly or mixed with other components, mostly phosphates, uric acids, or urates. The frequency with which the two minerals are found in association in natural sources suggested a re-examination and a comparison of their crystal structure as the first stage of an investigation on their genetic relationshiPs. Previous structure analyses were carried out by Cocco (1961) and Cocco and Sabelli (1962) for whewellite (two-dimensional photographic data, R : 0.14) and by Sterling (1965) for weddellite (three-dimensional photographic data, R : 0.13). Experimental Single crystals of whewellite and weddellite were obtained from urinary calculi. A Philips PW ll00 single-crystal di.ffractometer was used to collect the X-ray diffraction data. Table I presents the experimenial details. On the crystals of both species three standard reflections monitored at three-hour intervals showed less than 3.5 percent intensity variation of figures by Germain et al. (1971), S'u5ing ef a/' (1g62),and Johnson (1965) were employed' The scatiering curves for neutral atoms given by the International Tablesfor X-ray Crystallography (1974) were used. High-precision methods using an co scan of four intense reflections from each of several lattice rows provided accurate data for unit-cell parameter computations. Results of the structure refinements VVhewellite The orientation and the dimensions of the cell axes (Table l) are consistent with one of the two alternaiive orientations proposed by Arnott et al' (1965) 0np3-ffi4x / 80 / 0304-0327$o2.oo 32'.1 328 TAZZOLI AND DOMENEGHETTI: Table l Crystal and diffraction data WHEWELLITE AND WEDDELLITE from them). The occupancy of these positions W(10) and W(20) and, in turn, that of the main positions W(l) and W(2) were refined with other cycles of least squares, together with the coordinates and the anisotropic thermal factors of the non-hydrogen atoms. Fixed isotropic temperature factors were used for the atoms H(ll), H(21), H(22), W(10), and W(20). The final atomic parameters are shown in Tables 2 and 3; bond lengths and angles in Table 4. The observed and computed structure factors are compared in Table 5.' The final value of the discrepancy index was R : 0.033 for the 2864 observed reflections. lleddellite The values of the cell parameters (Table l) agree well with those of Sterling (1965). The atomic coordinates of this author, with the exclusion of those relative to oxygen of the \"zeolitic\" water, were used to start the refinement. During the first cycles, carried out using isotropic thermal factors, the examination of the difference Fourier map confirmed the presence of zeolitic water, with incomplete occupancy, in the channel running along the four-fold axis, with coordinates similar to those given by Sterling. In the last stages of refinement, difference Fourier maps allowed us to locate the hydrogen atoms of the two molecules of non-zeolitic water and to find a split po-, sition W(30) for the oxygen of the zeolitic water, the occupilncy of which was refined alternately with that of the main position W(3) (he distance between the two positions was 0.574). Fixed isotropic temperature factors were used for the atoms H(5), H(6), and W(30). The final atomic parameters are given in Tables 6 and 7, lengths and bond angles in Table 8. The observed and computed structure factors are compared in Table 9.'The final value of the discrepancy index was R : 0.032 for the 889 observed reflections. Discussion and comparison of the structures lMhewellite In whewellite (Fig. l) the coordination polyhedra of the pseudo-equivalent atoms Ca(l) and Ca(2) are distorted square antiprisms: in each of them seven of the oxygens belong to five oxalic groups and one to a water molecule. Each Ca polyhedron shares three edges with three adjacent Ca polyhedra. In this way I To receive a copy of Tables 5 and 9, order Document AM-?9116 from the Business Office, Mineralogical Society of America, 2000 Florida Avenue, NW, Washington, DC 20009. please remit $1.00 in advance for the microfiche. P n o p e r t y W h e w e l l i t e W e d d e l l i t e