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GM75 – New minerals, nomenclature and classification – oral
490
Heteropolymolybdates: structural relationships,
nomenclature scheme and new species
Kampf, A.R.1*, Mills, S.J.2 & Rumsey, M.S.3
1Mineral Sciences Dept., Natural History Museum of Los
Angeles County, Los Angeles, CA, USA (*akampf@nhm.org)
2Dept of Earth and Ocean Sciences, University of British
Columbia, Vancouver, BC, Canada
3Mineralogy Dept., The Natural History Museum, London,
England, UK
The molybdoarsenates betpakdalite, natrobetpakdalite and
obradovicite and the molybdophosphates mendozavilite,
paramendozavilite and melkovite typically occur as coatings of
minute, poor-quality and/or complexly twinned crystals.
Heretofore, the structure of only betpakdalite had been
determined. In this investigation, we examined type material for
all of these minerals, as well as similar material from other
sources, in an effort to elucidate the interrelationships within
this enigmatic family of minerals.
Comparative powder X-ray diffraction proved effective in
identifying structurally related phases and obtaining refined cell
parameters for those phases for which single-crystal studies
could not be conducted. Electron microprobe analyses were
performed for all species studied and optimal conditions for
analysis were determined. These methods indicated that
natrobetpakdalite, mendozavilite and melkovite are isostructural
with betpakdalite and suggested that obradovicite has a closely
related structure.
Single-crystal investigations yielded several new structure
refinements and the solution of the structure of obradovicite.
Although crystals of paramendozavilite were inadequate for
collection of structure data, cell parameters were for the first
time determined. Both the betpakdalite and obradovicite
structure types are based upon frameworks containing four-
member clusters of edge-sharing MoO6 octahedra which link by
sharing corners with other clusters, with Fe3+O6 octahedra and
with PO4 or AsO4 tetrahedra. The structures differ with respect
to their linkages through the Fe3+O6 octahedra, leading to
substantially different, but closely related framework
configurations. The cell parameters of paramendozavilite
indicate that its structure may be related to those of betpakdalite
and obradovicite. We suggest that these minerals be termed
“heteropolymolybdates”.
Besides the presence of As or P in the tetrahedral site (T) of
the frameworks, the non-framework cation sites are key to
discriminating different species with these structures. One
generally smaller cation site (B) at a center of symmetry is
octahedrally coordinated to H2O molecules and two or more
disordered, partially occupied, generally larger, cation sites (A)
are coordinated to O atoms in the framework and H2O
molecules. The general formulas for minerals with either the
betpakdalite or the obradovicite structure are the same:
[A2(H2O)nB(H2O)6][Mo8T2Fe3+3O30+7(OH)7-x], where: x is the
total charge of A + B and n is variable. Dominant cations noted
in the A sites include K, Na and Ca and in the B sites Na, Ca,
Mg, Cu and Fe. The various combinations that we identified
define at least eight new heteropolymolybdate species.
We believe that a suffix-based nomenclature is most
appropriate for the heteropolymolybdates with root names based
upon the structure types and the T sites cations: betpakdalite (T
= As), mendozavilite (T = P) and obradovicite (T = As);
followed by two suffixes of the form: -AB, corresponding to the
dominant cations in the two different types of non-framework
cation sites. A new root name would be required for any new
obradovicite structure species with T = P.
Our investigation of the paramendozavilite type specimen
showed it to contain no paramendozavilite, but rather an
apparently closely related new mineral. Another sample of
paramendozavilite analyzed showed K > Na.
Qingheiite-(Fe2+), Na2Fe2+MgAl(PO4)3, a new
member of the wyllieite group
Hatert, F.1*, Baijot, M.1 & Philippo, S.2
1Laboratory of Mineralogy, University of Liège, Belgium
(*fhatert@ulg.ac.be)
2Natural History Museum, Luxembourg
Qingheiite-(Fe2+), ideally Na2Fe2+MgAl(PO4)3, is a new mineral
species from the Sebastião Cristino pegmatite, Minas Gerais,
Brazil [1]. It occurs as rims around frondelite grains, included in
a matrix of quartz and albite. The empirical formula is
(0.65Na0.35)(Na0.58Mn2+0.40Ca0.02) (Fe2+0.68Mn2+0.32) (Mg0.72
Fe3+0.23Fe2+0.05)(Al0.62Fe3+0.38)[PO4]3, and the single-crystal unit-
cell parameters are a = 11.910(2), b = 12.383(3), c = 6.372(1)
Å, β = 114.43(3)°, V = 855.6(3) Å3, space group P21/n.
Qingheiite-(Fe2+) is the Fe2+ analogue of qingheiite
[Na2MnMgAl(PO4)3], and belongs to the wyllieite group of
minerals. The mineral species and its name were approved by
the Commission on New Minerals, Nomenclature and
Classification of the International Mineralogical Association
(CNMNC-IMA) under the number 2009-076.
The crystal structure of qingheiite-(Fe2+) has been refined,
based on single-crystal X-ray diffraction data, to R1 = 2.91 %.
The basic structural unit is identical to that of other members of
the wyllieite group, and consists of kinked chains of edge-
sharing octahedra stacked parallel to {101}. These chains are
formed by a succession of M(2a)-M(2b) octahedral pairs, linked
by highly distorted M(1) octahedra. Equivalent chains are
connected in the b direction by the P(1), P(2a) and P(2b)
phosphate tetrahedra to form sheets oriented perpendicular to
[010]. These interconnected sheets produce channels parallel to
c, channels that contain the large X sites. The X(1a) site is a
distorted octahedron, whereas the X(1b) site can be described as
a very distorted cube. The morphology of the X(2) site
corresponds to a very distorted gable disphenoid with a [7+1]
coordination, similar to the X(2) site of rosemaryite [2] and to
the A(2)’ site of the alluaudite structure.
The structural features of qingheiite-(Fe2+) are compared to
those of other wyllieite-type phosphates: ferrorosemaryite from
the Rubindi pegmatite, Rwanda (NaFe2+Fe3+Al(PO4)3, R1 =
2.43 %, a = 11.838(1), b = 12.347(1), c = 6.2973(6) Å, β =
114.353(6)°) [3], rosemaryite from the Buranga pegmatite,
Rwanda (NaMnFe3+Al(PO4)3, R1 = 4.01 %, a = 12.001(2), b =
12.396(1), c = 6.329(1) Å, β = 114.48(1)°) [2], wyllieite from
the Buranga pegmatite (Na2MnFe2+Al(PO4)3, R1 = 2.74 %, a =
11.954(2), b = 12.439(2), c = 6.406(1) Å, β = 114.54(1)°), and
qingheiite from the Santa Ana pegmatite, Argentina (R1 = 2.65
%, a = 11.878(3), b = 12.448(2), c = 6.438(2) Å, β =
114.49(1)°). These new structural data indicate that Al is
predominant on the M(2a) site in the investigated samples, not
on the M(2b) site as observed in ferrowyllieite [4]. The
morphologies of the X(1a), X(1b), and X(2) crystallographic
sites are also compared among the different minerals of the
wyllieite group.
[1] Hatert, F. et al. (2010) Eur. J. Mineral., 22(3), 459-467. [2]
Hatert, F. et al. (2006) Eur. J. Mineral., 18, 775-785. [3] Hatert,
F. et al. (2005) Eur. J. Mineral., 17, 749-759. [4] Moore, P.B. &
Molin-Case, J. (1974) Am. Mineral., 59, 280-290.