MINERALOGIA POLONICA Vol. 32, No 1, 2001 PL ISSSN 0032-6267 Ewa KOSZOWSKA1, Dorota SAŁATA1 MINERALS OF THE HYDROTALCITE GROUP IN METASOMATICALLY ALTERED CARBONATE ROCKS FROM ZAWIERCIE, S POLAND A b s t a c t . Minerals of the hydrotalcite-manasseite group were identified in samples from two borehols in Zawiercie (ZMZ-9, RK-1). The minerals were found in calciphire bodies (RK-1) and in one small, metasomatic veinlet (ZMZ-9) formed in Middle Devonian dolomites. Alteration of dolomitic sediments was genetically connected with infiltration fluids that caused formation of a gamet-pyroxene skam. Inves­ tigations have revealed the presence of both hydrotalcite and manasseite. Besides, in few places of the veinlet there occurs a mineral, which has been recognized as iowaite. Key-words: hydrotalcite-manasseite group, calciphires, ska ms, metasomatic veins, Zawiercie, S Poland INTRODUCTION The hydrotalcite group minerals belong to a large group of natural and synthetic dihydroxides named also as "layered double hydroxides" or "anionic clays". Their general formula can be written as: M |2XM (0 H)2 (Am“)x/mn H 2 0 (where M+2, M +3 are cations in the hydroxide layers and Am_ is the interlayer anion) and is based on positively charged brucite-like layers with C 03-like anions and water molecules in interlayer positions (Drits et al. 1987) (Fig. la). Within the group, depending on the composition of the octahedral brucite-type cationic layers, three subgroups can be distinguished in which the cations are: a) M g +2 + Al+3, b) Mg +2 + Fe+3 , c) M g + 2 + C r+3. In the subgroup of the (Mg,Al) hydroxides the 2H (hexagonal) polytype is ma­ nasseite and the 3R (rhombohedral) is hydrotalcite. In the subgroup of the (Mg,Fe) hydroxides the 2H polytype is sjogrenite and the 3R polytype is pyroaurite. The 1 Jagiellonian University, Institute of Geological Sciences, Department of Mineralogy and Petrography, Oleandry 2a, 30-063 Krakow, Poland; e mail: [email protected]; [email protected] 69 Fig. 1. General structure of minerals of the hydrotalcite group (A) — the scheme of ordered arrangement of atoms in the layer structural elements (Drits et al. 1987) (B) — the structure of a mineral of the hydrotalcite group with the M g/A l ratio 2:1 proposed by Arakcheeva et al. (1996) corresponding minerals in the subgroup of the (Mg,Cr) hydroxides are stichtite and barbertonite (Arakcheeva et al. 1996). Both hydrotalcite and manasseite are relatively rare in nature. The majority of oc­ currences of the minerals of the hydrotalcite group are associated with serpen tinites, however they were also described from contact rocks (skams) and saline deposits (Ćemy 1963; Scaini et al. 1967). In Poland they have been identified in Kłodawa salt deposit (Wachowiak 1999), in the contact altered carbonate rocks in the Dubie area and in the Dębnik area (Muszyński, Wyszomirski 1998). Minerals of the hydrotalcite-manasseite group were also identified in altered carbonate rocks in Zawiercie (Koszowska, Salata 1997a, b). Although it is almost a rule that hydrotalcite and manasseite form sub- microscopic intergrowths, their separate occurrence is also known (Drits et al. 1987). Hydrotalcite is a scarcely studied mineral, though it plays a significant role in cement production, metal technology, Mg and Al corrosion studies, and is an important acid sorbent and catalyst (Moroz, Arkhipenko 1991; Cavani et al. 1991). STRUCTURE OF MINERALS OF THE HYDROTALCITE-MANASSEITE GROUP Both 3R and 2H polytypes of minerals of the hydrotalcite group can have a different M g/A l ratio. The crystal structure of minerals of the hydrotalcite group with the M g/A l 70 ratio = 2:1, proposed by Arakcheeva et al. (1996), consists of octahedral brucite-type cationic layers in which cations (Mg,Al) are present. Layer 1 is of the brucite type and has the composition [AlMg 2 (OH)6], Layer 2 is a carbonate net of the [C 03] composition, whereas layer 3 is a network of H20 molecules. The layer structural elements along the hexagonal axis form the sequence -2-1-3-1-2- (Fig. lb). All the minerals generally contain only (C03)2- in their interlayers, however varieties containing (S04)2-, CL, (OH)’, (N 03)" and (Cr04)2- anions were also described (Drits et al. 1987). E.g. Koritning and Siisse (1975) described a hydrotalcite containing (OH)- instead of (C03)2- anions in interlayers having the composition: [Mg 6 A l2 (O H ) 1 6 ] + 2 [(0 H ) 2 -4H20 ]-2. Iowaite, a magnesium-ferric iron oxychloride having the composition Mg 4 (0 H ) 8F e0 C L H 2 0, was described by Kohls and Rodda (1967) (fide Drits et al. 1987). SAMPLING AND METHODS OF INVESTIGATIONS Samples of veinlets and calciphires have been collected from two boreholes ZMZ-9 and RK-1, in Zawiercie (northwest of Kraków, Fig. 2). Minerals from veinlets and calciphires were separated by handpicking under a binocular. Microscopic observations of thin sections were performed with an AMPLIVAL petrographical microscope. X-ray diffraction powder patterns were obtained with a Philips diffractometer using Ni-filtered CuKa radiation. The morphology of minerals and their chemical composition were studied by means of a scanning electron microscope (SEM) JEOL 5410 equipped with an energy dis­ persive spectrometer (EDS) Voyager 3100 (NORAN). Fresh surfaces of rock pieces as well as polished thin sections coated with the carbon film were examined and the contents of cations were evaluated according to the "standardless" procedure of calcu­ lation in Voyager software (i.e. using standards from the software library supplied by the manufacturer). Chemical composition of selected samples of hydrotalcites was determined by means of an EDS microprobe ISIS system connected with a JEOL JSM 35 scanning Fig. 2. Location of the area investigated 71 microscope operating at accelerating voltage of 20 kV and sam ple current of 30 nA with ZAF/FLS corrections. Biotite (for Mg, Si and Fe), chlorite (for Al), almandine (for Ca), rhodonite (for Mn and Zn) were used as calibration standards. The results are given in Table 2 with all the iron recalculated as FeO. RESULTS Minerals of the hydrotalcite-manasseite group were found in calciphire bodies, drilled in the borehole RK-1 in Zawiercie and in one small, metasomatic veinlet formed in Middle Devonian dolomites (ZMZ-9, Zawiercie). Both occurrences are genetically connected with metasomatic gamet-pyroxene skarn associated with Cu mineralization. The main compounds of the calciphires are: neomorphic sparitic calcite, euhedral pink or greenish spinel sensu stricto (as disseminated crystals in sparitic calcite), mag- nesioferrite and oval aggregates built mainly of fine-flaky serpentines and small amounts of chlorites, within which occur well-preserved small fragments of forsterite and minerals of the humite group (Koszowska 2001). Reddish-brown veinlets cutting calciphires consist mainly of forsterite and minerals of the humite group. The latter show pleochroism with an absorption scheme: from colourless to yellow and from yellow to orange-yellow and repeated twinning. These minerals in comparison to forsterite are characterized by an admixture of 5.5 wt.% TiC^. Forsterite and minerals of the humite group are preserved in various degrees. They are often altered into serpentine. Minerals of the hydrotalcite group in the calciphires occur in envelopes of spinel crystals (Phot. 1,2,4; Fig. 3) or in spaces among fractures of spinels in association with chlorite, serpentine, and sometimes brucite. Microscopical (optical and electron) investigations allowed to observe the following regularity: minerals of the hydrotalcite group form rims directly around spinel crystals, while outside of the hydrotalcite rims, chlorite and occasionally brucite occur. Hydrotalcite-like minerals were also found as inclusions in magnesioferrite. The metasomatic veinlet cutting dolomites is filled with chlorite, forsterite, mi­ nerals of the humite group, serpentine, dolomite and opaque minerals represented by mag- netite, pyrite and small amounts of chalcopyrite. Chlorite (penninite) occurs in two varieties: a fine-flaky one, about 0 . 0 1 mm in size, forms nests between larger flaky crystals up to 0.2 mm in size. Forsterite and minerals of the humite group undergo alteration into serpentines. Minerals of the hydrotalcite group in the veinlet occur mostly in association with the fine-flaky chlorite. They form "nest" aggregates surrounded by the coarse-flaky chlorite (Phot. 3, 5) and can be present in the "nests" alone, or in association with the fine-flaky chlorite. In both occurrences they form plates up to 20 pm in size, rarely showing weakly visible hexagonal habit (Phot. 6 ). A mineral recognized as iowaite, which appears only in the veinlet, reveals combination of short, hexagonal prism and pinacoid about 10 pm long (Phot. 7; Fig. 4), and is often intergrown with serpentine. 72 4 5 0 0 - o M9 4 0 0 0 - 3500 M C : o 0 Energy (keV) Fig. 4. Iowaite crystals showing combination of short hexagonal prisms and pinacoid. SEM image. EDS spectrum of iowaite 73 Because it was impossible to separate the monomineral hydrotalcite fraction, further investigations were carried out on polymineral samples. X-ray diffraction patterns from oriented preparations of the investigated veinlet (ZMZ-9 125.7a, b) show two different types of basal reflections (7.84, 3.91-3.87, and 7.66-7.64,3.82-3.81 A, etc.) what indicates the presence of both 3R (hydrotalcite) and 2H (manasseite) polytypes of the hydrotalcite group (Fig. 5). In the patterns of calciphires, mainly reflections of hydrotalcite (7.84-7.76,3.91-3.87 A) were identified. Moreover, an (Mg,Fe)-mineral, whose composition is similar to iowaite, has been identified in the veinlet only on the basis of EDS investigations. This "iowaite" appears there sporadically only in few places.