Alkali amphibole from the Djebel Drissa ring complex (Algeria): occurrence and crystal chemistry

The palaeoproterozoic Djebel Drissa ring complex (Eglab shield, Algeria) is composed of metaluminous and peralkaline granites: the first group of granites is red to pink, contains biotite (annite) as Mg-Fe constituent and forms the core of the complex. The second group is greyish and outcrops at the rim of the complex; the mafic minerals in this second group of granites are Na-Fe rich amphiboles and pyroxenes (arfvedsonite, riebeckite, aegyrine). In the peralkaline granites, the textural relations between the Na-Fe minerals are ambiguous. In some cases, amphibole seems to be replaced by pyroxene; in other cases the relations seem opposites. Optical observations suggest that these minerals crystallized at late and hydrothermal stages from alkaline/peralkaline liquids, with the appearance of isolated albite crystals, diffuse albitization, droplets of quartz around perthites and ferruginization of zircon. The Agpaitic Index (Na+K)/Al of the rocks controlled the crystallization of these minerals; both the amphiboles and the pyroxenes show, from the core of the complex to the rim, a general increase in Na and Si and a decrease in Ca + IVAl. Several amphibole crystals from the ring complex were characterized using EMP and FTIR spectroscopy. Microchemical analyses show that the Djebel Drissa amphiboles must be classified in the arfvedsonite compositional space; they are Mn- and F-rich, and less magnesian and more sodic than those from the same type of granites (type Negal and alkaline pink) outcropping at Fort Trinquet in Mauritania. The evolution of the amphibole composition can be well described (Fig. 1) by a complex relationship: ANa + BM2+ + 2CM2+ = A + BNa + 2CM32+ [1]. As experimentally demonstrated, for decreasing fO2 conditions, this substitution proceeds toward more riebeckitic compositions, and can be effectively used to monitor the oxidation conditions during crystallization[1]. In the case of the studied suite of samples, the amphibole from the core of the complex notably show higher arfvedsonite component, allowing us to infer evolution toward more reducing conditions, in agreement with petrographic and petrological observations[2,3]. Micro-FTIR OH-stretching spectra were collected on several crystal fragments manually extracted from the host-rocks, under unpolarized light. They show systematically a two-band pattern, consisting of a very sharp intense absorption at 3618 cm-1 and a second, broader and composite band centred at 3648 cm-1. On the basis of our previous experience with these amphibole compositions, the former absorption can be assigned to configurations involving Fe2+ at M(1,3), while the broader band at 3648 cm-1 can be assigned to configurations involving CLi; both configurations are associated with vacant A-sites in the structure. Calculation of the crystal-chemical formulae following the suggestions of [4] for Li-bearing, sodic amphiboles, shows that the studied samples from Drissa in fact have a composition close to potassian fluoro-ferroleakeite, (Na,K)Na2(Fe2+2LiFe3+2)Si8O22F2. Interpretation of the FTIR patterns based on the stoichiometries obtained by EMP suggests extreme short-range-order in the structure of these amphiboles, the main feature of which can be summarized as: (1) all Fe3+ is strongly ordered at M(2), (2) CLi is ordered at an OH-coordinated site (M(3), e.g. [5]), (3) ANa is strongly associated with O3F, and (4) CLi is strongly associated with vacant A-sites. Features (1), (2) and (3) are in agreement with extensive single-crystal refinement on amphiboles (e.g. [4,5]), while feature (4) is in apparent contrast with previous findings (e.g. [4]). [1] Della Ventura, G., Iezzi, G., Redhammer, G.J., Hawthorne, F.C., Scaillet, B. and Novembre, D. (2005) Am. Miner., 90, 1375-1383. [2] Kahoui, M. (1988) Unpublished PhD Thesis, University of Nancy I (France), 258 p. [3] Kahoui, M., Mahdjoub, Y. (2004) Jour. Afri. Earth Scien., 39, 115-122 [4] Oberti, R., Cámara, F., Ottolini, L. and Caballero, J.M. (2003) Eur. J. Miner., 15, 309-319. [5] Hawthorne, F.C., Ungaretti, L., Oberti, R. and Bottazzi, P. (1993) Am. Miner., 78, 733-745.