Crystal structure and bonding in the new mineral AsSbO3.

Marcus J. Origlieri1*, Robert T. Downs1§, Michael D. Carducci1

Kevin M. Rosso2, G. V. Gibbs3

1Department of Geosciences, University of Arizona Tucson, Arizona 85719-0077 USA2Pacific Northwest National Laboratory P.O. Box 999, K8-96, Richland, WA 99352 USA3Department of Geological Sciences, Virginia Polytechnic Institute Blacksburg, VA 24061-0420 USA*marcus@mineralzone.com; §downs@geo.arizona.edu

unknown mineral

• EDS indicated only major As, Sb

Raman spectrum

100 200 300 400 500 600 700 800 900

Raman shift (rel. cm- 1)

Inte

ns

ity

(arb

itra

ry u

nits

)

new mineral

leiteite

crystal morphology

Palache (1934)

microprobe chemical analysis

Average of 10 standardized WDS analyses:

Sb2O3 55.77%

As2O3 45.15% total 101.92%

EMPIRICAL FORMULA = As1.088Sb0.912O3

standards enargite Cu3AsS4

stibiotantalite SbTaO4

X-ray diffraction

• streaky data

• merged well for space group P21/n (Rsym = 2.71%)

crystal structure solution

• Matches synthetic AsSbO3 (Bodenstein et al. 1983)

• Trigonal pyramids of AsO3 and SbO3 link corners to form infinite sheets of composition AsSbO3 stacked along b

crystal structure solution

new mineral vs. claudetite

new mineral claudetite

chemistry AsSbO3 As2O3

space group P21/n P21/n

a 4.5757(4) Å 4.5460(4) Å

b 13.1288(13) Å 13.0012(14) Å

c 5.4216(5) Å 5.3420(5) Å

b 95.039(4)° 94.329(2)°

V 324.44(5) Å3 314.83(5) Å3

Z 4 4

dcalc 5.009 g/cm3 4.174 g/cm3

bond distancesnew mineralAs−O1 1.773(7) Å Sb−O11.978(7) ÅAs−O2 1.781(6) Å Sb−O22.006(6) ÅAs−O3 1.792(6) Å Sb−O31.995(7) Å<R(As−O)> 1.782 Å <R(Sb−O)> 1.993 Å

claudetiteAs1−O1 1.772(5) Å As2−O1 1.783(5) ÅAs1−O2 1.788(4) Å As2−O2 1.805(5) ÅAs1−O3 1.790(5) Å As2−O3 1.790(5) Å<R(As1−O)> 1.783 Å <R(As2−O)> 1.793 Å

bond angles

new mineralO1−As−O2 100.8(3)° O1−Sb−O2 92.2(3)°O1−As−O3 101.1(3)° O1−Sb−O3 93.0(3)°O2−As−O3 91.1(3)° O2−Sb−O3 84.8(3)°<O−As−O> 97.7° <O−Sb−O> 90.0°

claudetiteO1−As1−O2 100.8(2)° O1−As2−O2 95.2(2)°O1−As1−O3 102.1(2)° O1−As2−O3 97.9(2)°O2−As1−O3 91.3(2)° O2−As2−O3 91.3(2)°<O−As1−O> 98.1° <O−As2−O> 94.8°

substitution of Sb into claudetite

Sb in AsSbO3 structure preferentially occupies the As2 site of claudetite

<R(As2−O) ~ <R(As1−O)>

<O−As2−O> < <O−As1−O>

94.8° < 98.1°

Sb prefers a smaller O−M−O for MO3 than As

ordering of As and Sb

synthetic naturalBodenstein et al. (1983) this study

<R(As−O)>1.80 Å 1.782 Å <R(Sb−O)> 1.95 Å 1.993 Å

The more extreme <R(As−O)> and <R(Sb−O)> indicate a higher degree of ordering in natural AsSbO3 than synthetic material

formula of new mineral

• Natural AsSbO3 shows a higher degree of As/Sb ordering than synthetic material

• Crystal structure refinement gives lower residual value (5.66%) with idealized chemistry than with microprobe chemistry

ACTUAL CHEMISTRY = AsSbO3

bonding in arsenites

• Between sheets of the leiteite (ZnAs2O4) structure, Ghose (1987) argues “long As-O interactions must be considered as weak bonds, which hold the composite layers together.”

• Pertlik (1975) notes that As-O distances of 3.15 Å in trippkeite result from steric effects.

definition of bonding

• Bader (1990) defines a bonded interaction exists when electron density shows both:– BOND PATH – a continuous path of local

maxima of electron density in the perpendicular plane between two maxima of electron density (i.e. atoms)

– BOND CRITICAL POINT – a (3,−1) saddle point of electron density along the bond path located between the atoms

electron density distribution

Sb−O12.947 Å (intra-

layer)

Sb−O23.237 Å(inter-

layer)

quantum calculations

• Follow Density Function Theory

• Linear combinations of numerically solved wave functions

• Basis sets optimized for Crystal98 (Pisani et al. 2000)

• Uses coordinates of atoms and unit cell from crystal structure refinement

• Search radius 9 Å

bonding topology

• three groups of bonds distinguished their electron densities at the bond critical points

– close contacts (rc) = 0.984−1.012 As−O

(rc) = 0.730−0.757 Sb−O

– intra-layer bonds (rc) = 0.169−0.134

– inter-layer bonds (rc) = 0.084−0.062

intra-layer bonds

responsible for the corrugation of the sheet

Three separate bonds:

Sb−O3 2.791 Å

As−O2 2.903 Å

Sb−O1 2.947 Å

inter-layer bonds

Two weakest bonds in the structure are between sheets:

Sb−O2 3.237 Å

As−O3 3.346 Å

Responsible for perfect (010) cleavage of the mineral

related structures

• Cubic As2O3 (arsenolite) and Sb2O3 (senarmontite) have structures consisting of M4O6 molecular units.

• Oxygen atoms form corners of octahedra with metal atoms centered above alternating faces of the octahedron

• Cubic AsSbO3 is a solid solution between As2O3 and Sb2O3

crystal structure of cubic As2O3

view down [110]view down [010]

cubic As2O3 and Sb2O3

• As2O3 (Ballirano & Maras, 2002)

– a = 11.074 Å– R(As−O) = 1.786(2) Å O−As−O = 98.4(2)°

• Sb2O3 (Whitten et al. 2004)

– a = 11.116 Å– R(Sb−O) = 1.978(1) Å O−Sb−O = 95.9(1)°