Crystal structure and bonding in the new mineral

Crystal structure and bonding in the new mineral As. Sb. O 3. Marcus J. Origlieri 1*, Robert T. Downs 1§, Michael D. Carducci 1 Kevin M. Rosso 2, G. V. Gibbs 3 1 Department of Geosciences, University of Arizona Tucson, Arizona 85719 -0077 USA 2 Pacific Northwest National Laboratory P. O. Box 999, K 8 -96, Richland, WA 99352 USA 3 Department 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

crystal morphology Palache (1934)

microprobe chemical analysis Average of 10 standardized WDS analyses: Sb 2 O 3 As 2 O 3 total 55. 77% 45. 15% 101. 92% EMPIRICAL FORMULA = As 1. 088 Sb 0. 912 O 3 standards enargite stibiotantalite Cu 3 As. S 4 Sb. Ta. O 4

X-ray diffraction • streaky data • merged well for space group P 21/n (Rsym = 2. 71%)

crystal structure solution • Matches synthetic As. Sb. O 3 (Bodenstein et al. 1983) • Trigonal pyramids of As. O 3 and Sb. O 3 link corners to form infinite sheets of composition As. Sb. O 3 stacked along b

crystal structure solution

new mineral vs. claudetite new mineral chemistry As. Sb. O 3 space group P 21/n a 4. 5757(4) Å b 13. 1288(13) Å c 5. 4216(5) Å b 95. 039(4)° V 324. 44(5) Å3 Z 4 dcalc 5. 009 g/cm 3 claudetite As 2 O 3 P 21/n 4. 5460(4) Å 13. 0012(14) Å 5. 3420(5) Å 94. 329(2)° 314. 83(5) Å3 4 4. 174 g/cm 3

bond distances new mineral As−O 11. 773(7) Å As−O 21. 781(6) Å As−O 31. 792(6) Å <R(As−O)> 1. 782 Å claudetite As 1−O 1 As 1−O 2 As 1−O 3 <R(As 1−O)> 1. 772(5) Å 1. 788(4) Å 1. 790(5) Å 1. 783 Å Sb−O 11. 978(7) Å Sb−O 22. 006(6) Å Sb−O 31. 995(7) Å <R(Sb−O)> As 2−O 1 As 2−O 2 As 2−O 3 <R(As 2−O)> 1. 993 Å 1. 783(5) Å 1. 805(5) Å 1. 790(5) Å 1. 793 Å

bond angles new mineral O 1−As−O 2 O 1−As−O 3 O 2−As−O 3 < O−As−O> 100. 8(3)° 101. 1(3)° 97. 7° O 1−Sb−O 2 O 1−Sb−O 3 O 2−Sb−O 3 < O−Sb−O> 92. 2(3)° 93. 0(3)° 84. 8(3)° 90. 0° claudetite O 1−As 1−O 2 O 1−As 1−O 3 O 2−As 1−O 3 < O−As 1−O> 100. 8(2)° 102. 1(2)° 91. 3(2)° 98. 1° O 1−As 2−O 2 O 1−As 2−O 3 O 2−As 2−O 3 < O−As 2−O> 95. 2(2)° 97. 9(2)° 91. 3(2)° 94. 8°

substitution of Sb into claudetite Sb in As. Sb. O 3 structure preferentially occupies the As 2 site of claudetite <R(As 2−O) ~ <R(As 1−O)> < O−As 2−O> < < O−As 1−O> 94. 8° < 98. 1° Sb prefers a smaller O−M−O for MO 3 than As

ordering of As and Sb synthetic natural Bodenstein et al. (1983) this study <R(As−O)>1. 80 Å <R(Sb−O)> 1. 95 Å 1. 782 Å 1. 993 Å The more extreme <R(As−O)> and <R(Sb−O)> indicate a higher degree of ordering in natural As. Sb. O 3 than synthetic material

formula of new mineral • Natural As. Sb. O 3 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 = As. Sb. O 3

bonding in arsenites • Between sheets of the leiteite (Zn. As 2 O 4) 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−O 1 2. 947 Å (intralayer) Sb−O 2 3. 237 Å (interlayer)

quantum calculations • Follow Density Function Theory • Linear combinations of numerically solved wave functions • Basis sets optimized for Crystal 98 (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 r(rc) = 0. 984− 1. 012 As−O r(rc) = 0. 730− 0. 757 Sb−O – intra-layer bonds r(rc) = 0. 169− 0. 134 – inter-layer bonds r(rc) = 0. 084− 0. 062 – close contacts

intra-layer bonds responsible for the corrugation of the sheet Three separate bonds: Sb−O 3 2. 791 Å As−O 2 2. 903 Å Sb−O 1 2. 947 Å

inter-layer bonds Two weakest bonds in the structure are between sheets: Sb−O 2 3. 237 Å As−O 3 3. 346 Å Responsible for perfect (010) cleavage of the mineral

related structures • Cubic As 2 O 3 (arsenolite) and Sb 2 O 3 (senarmontite) have structures consisting of M 4 O 6 molecular units. • Oxygen atoms form corners of octahedra with metal atoms centered above alternating faces of the octahedron • Cubic As. Sb. O 3 is a solid solution between As 2 O 3 and Sb 2 O 3

crystal structure of cubic As 2 O 3 view down [010] view down [110]

cubic As 2 O 3 and Sb 2 O 3 • As 2 O 3 (Ballirano & Maras, 2002) – a = 11. 074 Å – R(As−O) = 1. 786(2) Å – O−As−O = 98. 4(2)° • Sb 2 O 3 (Whitten et al. 2004) – a = 11. 116 Å – R(Sb−O) = 1. 978(1) Å – O−Sb−O = 95. 9(1)°