A new oxy-hydroxy iron(III) sulphate from the small magnetite-pyrite ore body at Fornovolasco (Apuan Alps, Tuscany, Italy) was identified. The origin of this mineral is related to the action of strongly acid solutions derived from the alteration of pyrite in the old tunnels of Fornovolasco mines. It is strictly associated with other sulphates, like alum-(K), copiapite, halotrichite, melanterite, römerite, and voltaite. The new mineral occurs as bladed crystals, elongated on 010, up to 50 microns long and less than 5 microns across. XRPD showed that this mineral is the natural analogous of the synthetic compound Fe4(SO4)(OH)10. This new sulphate is monoclinic, with cell parameters a 16.085(2), b 3.054(1), c 10.929(2) Å, beta 93.78(1)°, space group C2/m. Electron microprobe analyses give Fe2O3 63.33, SO3 14.07, H2O 17.18, sum 94.58. The sum of the wt of oxides is low even after the addition of the calculated water content. This may be a consequence of the very small crystal size, which have a thickness of less than 5 microns. Owing to the very small size, intensity data were collected at the XRD beamline of the Elettra synchrotron facility (Basovizza, Trieste, Italy). The crystal structure was solved and refined to R 0.078 for all 259 reflections with Fo > 4sigma(Fo). Figure 1 shows the crystal structure of this oxy-hydroxy iron(III) sulphate. The crystal structure is composed by ribbons of edge-sharing Fe-octahedra running along 010, linked together by corner-sharing to form wavy layers. As it is not possible to locate two SO4 groups at distance of only 3 Å, it is probable that a regular alternation of SO4 groups and H2O molecules takes place along the b direction. Such ordered sequence should double the b parameter, with the occurrence of additional superstructure reflections. Actually, only continuous streaks were detected in the diffraction patterns. According to the structural refinement and the bond-valence balance, the chemical formula of this new oxy-hydroxy iron(III) sulphate is Fe3+4(SO4)O2(OH)6.2H2O.
Occurrence of a new oxy-hydroxy iron(III) sulphate from Fornovolasco (Apuan Alps, Italy)
BONACCORSI, ELENA;BIAGIONI, CRISTIAN;ORLANDI, PAOLO
2010-01-01
Abstract
A new oxy-hydroxy iron(III) sulphate from the small magnetite-pyrite ore body at Fornovolasco (Apuan Alps, Tuscany, Italy) was identified. The origin of this mineral is related to the action of strongly acid solutions derived from the alteration of pyrite in the old tunnels of Fornovolasco mines. It is strictly associated with other sulphates, like alum-(K), copiapite, halotrichite, melanterite, römerite, and voltaite. The new mineral occurs as bladed crystals, elongated on 010, up to 50 microns long and less than 5 microns across. XRPD showed that this mineral is the natural analogous of the synthetic compound Fe4(SO4)(OH)10. This new sulphate is monoclinic, with cell parameters a 16.085(2), b 3.054(1), c 10.929(2) Å, beta 93.78(1)°, space group C2/m. Electron microprobe analyses give Fe2O3 63.33, SO3 14.07, H2O 17.18, sum 94.58. The sum of the wt of oxides is low even after the addition of the calculated water content. This may be a consequence of the very small crystal size, which have a thickness of less than 5 microns. Owing to the very small size, intensity data were collected at the XRD beamline of the Elettra synchrotron facility (Basovizza, Trieste, Italy). The crystal structure was solved and refined to R 0.078 for all 259 reflections with Fo > 4sigma(Fo). Figure 1 shows the crystal structure of this oxy-hydroxy iron(III) sulphate. The crystal structure is composed by ribbons of edge-sharing Fe-octahedra running along 010, linked together by corner-sharing to form wavy layers. As it is not possible to locate two SO4 groups at distance of only 3 Å, it is probable that a regular alternation of SO4 groups and H2O molecules takes place along the b direction. Such ordered sequence should double the b parameter, with the occurrence of additional superstructure reflections. Actually, only continuous streaks were detected in the diffraction patterns. According to the structural refinement and the bond-valence balance, the chemical formula of this new oxy-hydroxy iron(III) sulphate is Fe3+4(SO4)O2(OH)6.2H2O.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
