Crystal-chemical study of ecdemite from Harstigen, a new natural member of the layered lead oxyhalides group

Abstract

Crystal chemical study of ecdemite from Harstigen, a new natural member of the layered lead oxyhalides structural group Short Title:Crystal chemical study of ecdemite from Harstigen, Sweden.

Heliophyllite: a discredited mineral species identical to ecdemite

“…A set of 18 progressively smaller cleavage laminas were investigated by the single-crystal intensity data collection, performed at the Earth Science Department of the University of Pisa, employing a Bruker SMART BREEZE diffractometer equipped with an air-cooled charge-coupled device (CCD) detector. Heliophyllite diffraction patterns showed the same features already reported for ecdemite by Perchiazzi et al (2019), namely the presence of complex intergrowths, multiple twinning and a variable degree of structural disorder, highlighted by reflections streaked along c * . Unsuccessful trials were made to solve the heliophyllite structure in the orthorhombic system: the assumption of a pseudo-tetragonal unit cell with a = b ∼ 10.8 and c ∼ 25.65 Å, namely close to that proposed by Welin (1968), resulted in quite large R int values of ∼ 27 %.…”

“…Chemical, optical and structural data for ecdemite and heliophyllite presented in literature, especially the older datasets, give contrasting indications on the true nature of these minerals. Regarding ecdemite, the recent study of Perchiazzi et al (2019) established its true monoclinic symmetry. Previous to this study, only one X-ray single-crystal study (Weissenberg photographs) of ecdemite had been published (Sillén and Melander, 1941).…”

“…The background was modeled with a 1/x function, effective for describing background intensity at low angles due to air scattering, and with a nine-term Chebyshev function. Absorption was corrected according to the approach by Sabine et al (1998). The effects of asymmetry and zero error were accounted for and found to be negligible.…”

“…Neither space group indications nor estimated standard deviations for cell parameters were given for the two phases. A crystal-chemical study of the holotype ecdemite was recently reported by Perchiazzi et al (2019). The crystal structure of ecdemite was solved and refined through synchrotron single-crystal data, showing that ecdemite is actually monoclinic pseudo-tetragonal, P 2 1 , with a = 10.8276(38), b = 10.8265(21), c = 6.9705(14) Å and β = 113.261(24) • .…”

“…F, P, Ca, V, Mn and Fe were below their respective detection limits in all spot analyses. Chemical data for ecdemite (2) in Perchiazzi et al (2019) are given for comparison.…”