Single-crystal neutron diffraction and Mössbauer spectroscopic study of hureaulite, (Mn,Fe)5(PO4)2 (HPO4)2 (H2O)4

Abstract

The crystal chemistry of hureaulite from the Jocão (Cigana) pegmatite, Conselheiro Pena, Doce Valley, Minas Gerais (Brazil), was investigated by electron microprobe analysis in wavelength-dispersive mode, single-crystal Laue (at 293 K) and monochromatic neutron diffraction (at 2.3 K), and 57 Fe-Mössbauer spectroscopy [ M(1),M(2),M(3) (Mn 2þ 3.61 Fe 2þ 1.21 Ca 0.11 Mg 0.03) P ¼4.96 (P(2) PO 4) 2 (H 1.04 P(1) PO 4) 2 (H 2 O) 3.92 , Z ¼ 4, a ¼ 17.603(6), b ¼ 9.087(2), c ¼ 9.404(4) Å , b ¼ 96.66(4) , and V ¼ 1494.1(9) Å 3 at 293 K, space group C2/c]. The neutron refinements confirm the general structure model previously reported, showing that five independent H sites (with full site occupancy) occur in the hureaulite structure, one of them as member of the unique hydroxyl group (i.e., O(1)-H(1)) and the other four belonging to two independent H 2 O molecules (i.e., H(2)-O(9)-H(3) and H(4)-O(10)-H(5)). The hydroxyl group is the vertex of one of the two independent P-tetrahedra (i.e., HOPO 3), whereas the two H 2 O molecules are the vertices of the (Mn,Fe)-octahedra. No zeolitic H 2 O occurs in hureaulite structure. The complex hydrogen bonding scheme in hureaulite is now well defined, with five hydrogen bonds energetically favorable. The element distribution among the octahedral sites, deduced on the basis of the neutron structure refinements, shows that the highest fraction of Fe populates the M(2) site ($36%), whereas lower fractions are observed at M(1) and M(3) (i.e., $13-15 and $11-12%, respectively). The 57 Fe Mössbauer spectrum of hureaulite confirms the absence of ferric iron in the sample. The spectrum shows four well-separated absorption lines which are best fitted with three doublets. The well-resolved doublet (with the largest quadrupole splitting) is here assigned to the M(3) site, in line with the pronounced preference of Mn 2þ for the M(3) site as shown by the neutron structure refinements. The two innermost doublets have almost identical isomer shifts, though with slightly different quadrupole splitting values; the smaller quadrupole splitting value is assigned to the M(2) site, which is expected to have a slightly lower polyhedral distortion than the M(1) site, according to the experimental findings based on the neutron refinements.

A single-crystal neutron diffraction study of wardite, NaAl3(PO4)2(OH)4·2H2O

“…In the framework of a long-term project on the crystal-chemistry of hydrous phosphates (e.g., Gatta et al 2013aGatta et al , 2013bGatta et al , 2014aGatta et al , 2014bGatta et al , 2015, we have reinvestigated the crystal structure and crystal chemistry of wardite by single-crystal neutron diffraction and electron microprobe analysis in wavelength-dispersive mode (EPMA-WDS), in order to provide: i) the reliable location of the proton sites and the real topological configuration of the OH-groups and H 2 O molecules, for a full description of the atomic relationship via the H-bonds; ii) the anisotropic displacement parameters of all the atomic sites, H-sites included. To carry out this objective, single-crystal neutron diffraction data were collected at low temperature (20 K) in order to reduce the thermal displacement of the H-sites.…”

Structural Investigation into Magnetic Spin Orders of a Manganese Phosphatic Oxyhydroxide, Mn5[(PO4)2(PO3(OH))2](HOH)4

“…The current study aimed to determine magnetic structures of the pure manganese hureaulite-type solid solution Mn 2+ 5 (PO 4 ) 2 ((PO 3 (OH)) 2 (HOH) 4 (hereafter denoted as Mn-hureaulite). Characteristically, the Mn-hureaulite structure contains pentamers of three edgesharing unique Mn1O 6 , Mn2O 4 (HOH) 2 , and Mn3O 5 (HOH) octahedra at Mn2-Mn3-Mn1-Mn3-Mn2 atomic sites [3,9,10]. These pentamers are connected to each other via cornersharing Mn2O 4 (HOH) 2 and Mn3O 5 (HOH) along with PO 4 and PO 3 (OH) tetrahedra to form zigzag-like interconnected pentamer slabs parallel to the a axis (Figure 1a).…”

On the labyrinthine world of arsenites: a single-crystal neutron and X-ray diffraction study of cafarsite

“…To achieve this goal, we used electron microprobe analysis in wavelength-dispersive mode (EPMA-WDS), single-crystal Raman spectroscopy, and single-crystal X-ray (SCXRD) and neutron diffraction (SCND). This study was performed in the framework of a series of studies devoted to understand the labyrinthine world of hydrous minerals, mainly based on single-crystal neutron diffraction (e.g., Gatta et al 2015Gatta et al , 2016aGatta et al ,b, 2017.…”