Crystal structure determination of rosasite and mcguinnessite

N. Perchiazzi

Department of Earth Sciences, Università di Pisa
Via S.Maria 53, 56126 Pisa Italy

The carbonate group of malachite, with general formula Me²⁺₂ (CO₃)(OH)₂, includes malachite (Me²⁺= Cu), georgeite (Cu), glaukosphaerite (Cu,Zn), kolwezite (Cu,Co), mcguinnessite (Cu,Mg), nullaginite (Ni), pokrovskite (Mg), rosasite (Cu,Zn). Besides the chemical composition, the structural relationships between these minerals are demonstrated by the similarity of their powder patterns. A constant feature of these minerals is their microcrystalline fibrous habit. This feature precluded in most cases single crystal studies, which could be performed for malachite and rosasite only. For the remaining phases, space group symmetry and cell parameters were mainly derived from powder pattern indexing. Apart from that of malachite (Zigan et al., 1977), no other structural determinations are available for the minerals of the malachite group.

Spheroidal aggregates of rosasite in extremely thin fibrous crystals from Ojuela mine, Durango, Mexico, were carefully hand picked under the binocular microscope and gently hand milled in an agate mortar under acetone, to fill with the resulting powder a 0.5 mm Lindemann capillary.

Powder diffraction data were collected on a Bruker D8 Vario diffractometer, equipped with a primary Ge (111) monochromator and a Braun PSD detector, working in capillary geometry. This configuration allowed us to use a carefully selected amount of material, as well as to reduce the strong preferred orientation due to the fibrous habit of rosasite. Two scans were acquired in the interval 11-60 and 60-100 2θ°, with step 0.016°and counting times 24 and 48 s respectively.

The crystal structure of rosasite was solved through the EXPO (Altomare et al., 1999) program. A trial with a malachite-like starting model gave no results, whereas assuming the space group and the cell constants gave in the single crystal study by Roberts et al. (1986), we obtained a promising starting structural model. This model was completed through the subsequent Rietveld refinement, performed through the GSAS/EXPGUI suite of programs (Larson and von Dreele, 2000; Toby, 2001). The refined cell parameters were a=12.8976(3), b=9.3705(1), c=3.1622(1) Å, β=110.260(3)°, space group P2₁/a.

In the early stages of the refinement, constraints on the Me-O bonds were introduced and finally removed. The carbonate group was refined as a rigid body, imposing a C-O distance of 1.3 Å and a common isotropic displacement parameter for the atoms of the group. Isotropic displacement parameters were assumed also for the remaining atoms. The Rietveld refinement finally converged to Rwp=10.38% Rp=0.0751 (powder totals) and RF²= 6.12, 6.33 % (for the two used datasets).

The experimental data for mcguinnessite were acquired in the same conditions as for rosasite, and its crystal structure was successfully solved using the rosasite structure as a starting model. Trials to solve mcguinnessite through a malachite-structure model failed. The refined cell parameters for mcguinnessite were a=12.8978(6), b=9.3737(4), c=3.1558(2) Å, β=111.225(4)°, space group P2₁/a.

The Rietveld refinement (GSAS/EXPGUI package) finally converged to Rwp=8.06% Rp=0.0578 (powder totals) and RF²= 5.46, 5.16 % (for the two used datasets). The two minerals are isostructural, and the two independent metal sites are occupied in rosasite by Cu (the more distorted polyhedron) and by Zn_0.8Cu_0.2, in mcguinnessite by Cu_0.82Mg_0.18 and Mg_0.9Cu_0.1. The crystal structure of rosasite is reported in Fig. 1, as seen parallel to (001) plane.

Both the Zn and the Cu octahedra share edges forming Cu-based and Zn-based octahedral “columns”, running along [001] , and responsible for the acicular habit of the mineral.

These octahedral “columns” form by edge-sharing octahedral “ribbons”, two columns large, are directly connected by apex-sharing and through the carbonate groups, each carbonate being linked to three octahedral ribbons.

Figure 1: The crystal strucure of rosasite, as parallel to (001). In Mcguinnessite we find Mg instead of Zn, and the Cu site is partially filled by Mg.

References

Altomare, A., Burla, M.C., Cavalli, M., Carrozzini, B., Cascarano, G.L., Giacovazzo, C., Gagliardi, A., Molitemi, G. G., Polidori, G., & Rizzi, R. (1999): EXPO: a program for full powder pattern decomposition and crystal structure solution. – J. Appl. Cryst. (1999). 32, 339-340

Larson, A.C. and Von Dreele, R.B. (2000):General structure analysis system (GSAS) Los Alamos National Laboratory Report LAUR 86-748.

Roberts, A. C., Jambor, J. L., Grice, J. D. (1986): The X-ray crystallography of rosasite from Tsumeb, Namibia. Powder Diffraction, . 1, 56-57.

Toby, B.H. (2001) : EXPGUI, a graphical user interface for GSAS. J. Appl. Cryst 34, 210-221.

Zigan, F., Joswig, W., Schuster, H.D., Mason, S.A. (1977)Verfeinerung der struktur von Malachit, Cu₂(OH)₂CO₃, durch neutronenbeugung : Zeitschrift für Kristallographie 145 (1977), 412-426.