X-RAY-DIFFRACTION STUDY OF MINERAL COMPONENTS IN CALCAREOUS ALGAE (CORALLINACEAE, RHODOPHYTA)

The mineral composition of nine species of red calcareous algae (Coral linaceae, Rhodophyta) collected in the Adriatic Sea in 1987 and 1988 w as examined by X-ray powder diffraction (counter diffractometer, monoc hromatized CuK alpha radiation). In addition, a comparison between the calcareous algae from the north Adriatic (Rovinj area) and the centra l Adriatic (Kornati Islands) with regard to genus, species and environ mental factors was undertaken. All analyzed samples contained magnesiu m calcite, which was dominant in all but in two cases, where aragonite was the main phase. Diffraction lines of magnesium calcite were broad ened and shifted toward higher Bragg angles in relation to pure calcit e. Supposing that in the calcite crystal lattice only magnesium replac es calcium, it follows that the fraction of magnesium in magnesium cal cite, found from diffraction line shifts, would be 18 molar % (M%). Ar agonite was dominant (75 to 80 M%) in two samples of Pseudolithophyllu m expansum collected at Kornati Islands in 1987 and 1988. These two sa mples also contained magnesium calcite and a small fraction of calcite (5 to 10 M%). In other studied samples aragonite was detected in smal l fractions, up to 10 M%. The elemental analysis of corralinacean alga e obtained by X-ray spectroscopy showed that the fraction of the metal s Sr, Fe, Mn, Zn, Pb, Br, Cu and Rb was very small (15 to 2000 ppm). I n most samples other expected minerals were detected in small fraction s, such as sylvite (KCl, up to 2 M%), quartz (alpha-SiO2, up to 2 M%) and magnesite (MgCO3, only in one sample, 1 M%). The results show that calcareous algae are able to deposit a mixture of magnesium calcite, calcite and aragonite. Such a large molar fraction of aragonite in the alga P. expansum, or in any other corallinacean algae, has not been n oted in recent literature. It seems that a complexity of microclimatic and oceanographic factors may influence the diversity of two localiti es and cause some exchange in living organisms. In addition, the fact that under certain conditions the same organism is capable of forming different minerals from the same tissue (McConnaughey 1989) confirms o ur opinion that environmental effects are imprinted in the skeletal co mposition of calcareous algae P. expansum.