The Bi sulfates from the Alfenza Mine, Crodo, Italy: An automatic electron diffraction tomography (ADT) study

Abstract

We report about three bismuth sulfates from mineralized quartz dikes from Alfenza (Crodo, Italy), two new phases and a rare mineral, cannonite, all growing on bismuthinite. The first new phase occurs as white, “hortensia-like” aggregates of pseudo-hexagonal platelets, with perfect basal cleavage, ~20 μm wide and few micrometers thick. The approximate composition is Bi2O2(SO4), and cell parameters and symmetry, as determined by automatic diffraction tomography, are a = 22.0(4), b = 16.7(3), c = 15.9(3) Å, β = 102.9(5)°, space group Pc or P2/c. A major stacking disorder is detected by HR-SEM images and electron diffraction data. The second new phase was detected only by TEM. It can be distinguished by its random orientation on the TEM grid (i.e., absence of preferential parting), the higher resistance under the electron beam, and different cell parameters and structure, whereas the composition is similar (Bi/S ~ 2.2/1), apart for the presence of tellurium up to ~6 cations percents. The unit cell is hexagonal, space group P62c, a = 9.5(2) and c = 15.4(3) Å. In this case, a structure model was obtained ab initio from electron diffraction data. Interestingly, the mineral has a porous structure with one dimensional porosity (diameter of the channel ~7 Å). Finally, within the same centimeter sized hand-specimens, we detected also cannonite. Its identification was done by automatic diffraction tomography. The measured cell parameters are a = 7.7(2), b = 13.9(3), c = 5.7(1) Å, β = 109.8(5)°, the space group P21/c. Cannonite at Alfenza forms radiating, acicular aggregates of colorless, transparent crystals with “scalpel-like” habit, elongated along c, up to 200 μm in length

Fast electron diffraction tomography

“…For all these reasons EDT has received a lot of interest from the community involved in structure resolution and may almost be considered as a routine technique. The structures solved with PEDT cover different areas of crystallography: inorganic chemistry (Mugnaioli et al, 2012), mesoporous materials (Jiang et al, 2011;Mugnaioli & Kolb, 2013), metalorganic frameworks (Feyand et al, 2012), modulated structures (Palatinus et al, 2011), new minerals (Rozhdestvenskaya et al, 2010;Gemmi et al, 2012;Capitani et al, 2014;Plá šil et al, 2014) and biomineralization (Mugnaioli et al, 2014), to name only a small selection of all publications. Although REDT is more recent, it seems to be as promising as PEDT.…”

Solving Challenging Crystallographic Problems with Automated Electron Diffraction Tomography (ADT)

“…For mineralogical quests, ADT turned out to be extremely helpful for detecting new minerals existing only in very small amounts [8]. Recent structure solutions cover the investigation of samples synthesized under high pressure in multi anvil cells.…”

Daliranite, PbHgAs2S5: determination of the incommensurately modulated structure and revision of the chemical formula

“…3D ED methods have been successfully applied in solving new mineral crystal structures. Charoite-90 (Rozhdestvenskaya et al, 2010), charoite-96 (Rozhdestvenskaya et al, 2011), sarrabusite (Gemmi et al, 2012), widenmannite (Plá šil et al, 2014), karibibite (Colombo et al, 2017), denisovite (Rozhdestvenskaya et al, 2017) and a still unnamed mineral with presumed composition (Capitani et al, 2014) are some examples. In experimental petrology 3D ED has been crucial in the identification and structure determination of three new high-pressure phases in the MgO-Al 2 O 3 -SiO 2 -H 2 O system: HAPY (Gemmi et al, 2011), HYSO and 11.5 Å -phase (Gemmi et al, 2016).…”