CRYSTAL STRUCTURES OF TWO SYNTHETIC ISOTYPES OF MINERAL NIACHITE
Karimova, O.V.1 ,Yakubovich, O.V.,1 Massa, W.,2 Urusov, V.S.,1 Melnikov, O.K.,3 Dimitrova, O.V.1
1 Geological Faculty, Moscow Lomonosov
State University, Russia
2 Philipp-University, Marburg, Germany
3 Institute of Crystallography, Russian
Acadamy of Scince, Moscow, Russia
Keywords : crystal structure, hydrothermal synthesis,
mixed anionic layers, niachite, isotype, structure transformation
Mineral niachite NH4(Mn,Mg,Ca)PO4H2O
was found in 1983 [1]. Single crystal X-ray studies could not be
made due to the very soft and fragile nature of species. Two
synthetic isotypes of niachite : NH4[FePO4HF]
[2] and NH4[CoPO4H2O] have been
synthesized under hydrothermal conditions in the systems FeO-NH4F-(NH4)2HPO4-H2O
(T = 200oC, P = 1000 atm) and Co3(PO4)2-KF-NH4H2PO4-H2O
(T = 250oC, P = 100 atm). An X-ray crystal structure
investigation of these compounds has been carried out (Sp.gr.
Pnm21, Z=2).
| Compound | Colour | Unit cell parameter, A | V, A3 | rcalc, g/cm3 | Refl. number | Rhkl | S |
| NH4[FePO4HF] | dark green | a=8.958(2) b=5.649(1) c=4.811(1) | 243.4(1) | 2.58 | 424 | 0.0335 | 1.28 |
| NH4[CoPO4H2O] | deep pink | a=8.766(2) b=5.621(1) c=4.797(1) | 236.4(1) | 2.67 | 580 | 0.0145 | 1.14 |
In the crystal structures of NH4[FePO4HF] and NH4[CoPO4H2O] cobalt or iron octahedra share common vertices to form a 4-gon net parallel to (bc) plane of the orthorombic unit cell. Each square mesh is capped by a PO4 tetrahedron sharing one edge and one vertex. The NH4+ ions are placed in the interlayer space. In the a axis direction the mixed Co,P or Fe,P layers are linked by hydrogen bonds of N-H...O (N-H...F) and O-H...O type.
The plate like habitus of the crystals corresponds to the layer structure : the a axis is the crystal plates normal. The layers of transition metal octahedra sharing common vertices may cause two-dimentional magnetic properties of the crystals. The magnetic structure of the K,Mn analogue K[Mn(PO4)H2O] was reported [3].
The mixed Fe,P (Co,P) layers that characterize the niachite
structure can be interpreted as fragments of the triphilyte
LiFePO4 structure type that unites the minerals and
synthetic compounds described with the formula LiMPO4,
where M=Fe,Mn,Co,Mg. Two Fe,P layers of the same topology are
parallel to (011) plane of the unit cell of the triphilyte
structure. These layers have connetion in [100] direction through
common vertices of Fe and P polyhedra as well as through chains
of Li octahedra parallel to [010]. If one of the Fe,P layers with
the associated chains of Li octahedra is extracted out of the Li2Fe2(PO4)2
structure, one gets the unbalanced model of [Fe(PO4)]-
layer that has to be stabilized by terminal aqua ligands and the
NH4+ cations between the sheets which
provide the interlayer connection along the [100] direction to
form structure of NH4[Fe(PO4)H2O].
References
1. Bridge, P.J. & Robinson, B.W.
Mineral. Magaz., 1983, V. 47, P. 79-80.
2. Yakubovich, O.V., Karimova, O.V., Melnikov, O.K. &
Urusov, V.S. Dokl. Acad. Nauk, 1995, V. 342, P. 40-44.
3. Visser, D., Carling, S.G., Day, P. &
Deportes, J. Journ. of Appl. Physics, 1991, V. 69, P.
6016-6018. Issue 8.