Bringing knowledge to life MINERALOGICAL CONTROL OF THE DISPERSION OF ANTIMONY IN THE NATURAL ENVIRONMENT Peter Leverett, Adam J. Roper and Peter A. Williams School of Natural Sciences, University of Western Sydney, Locked Bag 1797, Penrith NSW 2451, Australia
Several hundred Sb-bearing primary sulfides and sulfosalts all essentially insoluble in H 2 O stibnite (Sb 2 S 3 ) is most common by far Some 40 secondary minerals formed by oxidation all more soluble in H 2 O Conflicting reports in the literature concerning mobility How soluble? Which are important? What environments? ph, redox potential and other dissolved components all play a role
A simple, typical oxidation sequence would be stibnite (Sb 2 S 3 ) kermesite (Sb 2 S 2 O) senarmontite/valentinite (Sb 2 O 3 ) cervantite (Sb 2 O 4 ) antimonic acid (HSbO 3 nh 2 O n = 0.5-6). Natural salts of the latter may be members of the roméite group, or stibiconite, bindheimite, bismutostibiconite, stetefeldtite, cubic pyrochlores, A 2-p B 2 X 6-q Y 1-r, space group Fd3m, Z = 8.
1. hydroxycalcioroméite (Ca,Sb 3+ ) 2 (Sb,Ti) 2 O 6 (OH) CaSb 2 O 5 (OH) 2 or Ca(H 3 O)Sb 2 O 6 (OH) 2. oxycalcioroméite Ca 2 Sb 2 O 7 3. oxyplumboroméite Pb 2 Sb 2 O 7 Atencio et al. (2010) Canadian Mineralogist, 48, 673-698.
E/V ph Vink, B.W. (1996) Chemical Geology, 130, 21-30. Sb 2 O 5 as a proxy; Sb(OH) 6 at all ph values; problems with thermochemical data better now
E(V) 1.4 1.2 0 2 4 ph 6 8 10 O 2 H 2 O 1.0 0.8 0.6 Sb 2 O 4 CaSb 2 O 6 0.4 0.2 Sb 2 O 3 0.0 H 2 O H 2 Sb 2 S Sb 2 S 2 O -0.2-0.4-0.6 Pourbaix diagram for simple Sb mineral phases for a(ca 2+ ) = 10 4 ; data for CaSb 2 O 6 from measurements of amorphous material by Johnson et al. (2005) Journal of Environmental Quality, 34, 248-254.
Solution chemistry is comparatively simple Good and reliable thermochemical data for hydrolysed species Oxidized systems dominated by [Sb(III)]: Sb(OH) 2+, Sb(OH) 3o, Sb(OH) 4 ; [Sb(V)]: Sb(OH) 4+, Sb(OH) 5o, Sb(OH) 6 Other inorganic ligand complexes are negligible Organic complexes are enigmatic
Simple oxide solubilities at 298.15 K at circumneutral ph 0.5Sb 2 O 3 (senarmontite, s) + 1.5H 2 O(l) Sb(OH) 3o (aq); lg K = 4.98; ca 1.3 ppm Sb Sb 2 O 4 (cervantite s) + 1.5H 2 O(l) Sb(OH) 3o (aq) + Sb(OH) 5o (aq); lg K = 10.84; ca 2 ppt Sb Sb(V) species, are rather more varied
BSE images of synthetic brandholzite (left, FOV 1.7 mm) and synthetic bottinoite (centre, FOV 5 mm). The bottinoite crystals (blue) altering from ullmannite (NiSbS, dark grey) are ca 2 mm across. Smaller cations with Sb(OH) 6 give species like Bottinoite Ni[Sb(OH) 6 ] 2 6H 2 O Brandholzite Mg[Sb(OH) 6 ] 2 6H 2 O Mopungite NaSb(OH) 6
Aqueous solubilities (mol dm 3 ) at 298.15 K are mopungite: 3.18 ± 0.2 x 10 3 ; 387 ppm Sb brandholzite: 1.95(4) x 10-3 ; 475 ppm Sb bottinoite: 3.42(11) x 10-4 ; 83 ppm Sb Blandamer et al. (1974) Journal of the Chemical Society, Dalton Transactions, 1084-1086; Diemar et al. (2009) Pure and Applied Chemistry, 81, 1547-1553. These are ephemeral phases and react to give much less soluble species Antimonic acid (HSbO 3 nh 2 O) solubility is about 10 ppb Sb at natural ph values, including hyperacidic environments
Larger cations stabilize the pyrochlore structure of the roméite group Solubility studies complicated by ion exchange phenomena Oxyplumboroméite, FOV 5 mm, Broken Hill, Australia (left); stibiconite, probably oxycalcioroméite, 5 cm, San Luis Potosí, Mexico (right); images courtesy of Mindat.org
At 298.15 K, oxyplumboroméite, Pb 2 Sb 2 O 7, in 0.0100 M HNO 3 gives a final ph of 2.05 0.05 (n = 6) and total dissolved Sb equal to 7.7 2.1 x 10 8 mol dm 3 (ca 9.3 ppb) At 298.15 K, oxycalcioroméite, Ca 2 Sb 2 O 7, in 0.0100 M HNO 3 gives a final ph of 2.23 0.01 (n = 6) and total dissolved Sb equal to 3.3 1.0 x 10 7 mol dm 3 (ca 40 ppb) Diemar et al. (2009) Pure and Applied Chemistry, 81, 1547-1553.
Other phases are worthy of attention Tripuhyite, FeSbO 4, and schafarzikite, FeSb 2 O 4 Mindat lists 38 localities for tripuhyite and 9 for schafarzikite; others are known Tripuhyite (left), Clara mine, Oberwolfach, Germany, FOV 10 mm; schafarzikite (right), Krížnica, Slovakia; FOV 4 mm; images courtesy of Mindat.org.
At 771-981 K, ΔG fө (FeSbO 4,s) = 976.9 + 0.3289T (K) ± 5.5 kj mol 1 and ΔG fө (FeSb 2 O 4,s) = 1068.7 + 0.3561T (K) ± 3.5 kj mol 1 Extrapolation to 298.15 K gives ΔG fө (FeSbO 4,s) = 878.8 ±5.5 and kj mol 1 and ΔG fө (FeSb 2 O 4,s) = 962.5 ±3.5 kj mol 1 Swaminathan and Sreedharan (2003) Journal of Alloys and Compounds, 358, 48-55. These values are quite startling if the extrapolation is reliable!
FeSbO 4 (s) + 3H 2 O(l) FeOOH(s) + Sb(OH) 5o (aq); lg K = a(sb(oh) 5o ) = 18.3 at 298.15 K FeOOH(s) + 2Sb(OH) 3o (aq) + H + (aq) + e FeSb 2 O 4 (s) + 4H 2 O(l); E o = 1.38 V at 298.15 K Goethite vanishes from considerations! Is the extrapolation reliable? We had better find out
Stabilities were thus determined using solution methods Derived ΔG fө (s, 298.15 K) values for FeSb 2 O 4 and FeSbO 4 are 959.4 4.3 and 836.8 2.2 kj mol 1, respectively These values compare favourably, and unexpectedly, with those in the literature a(sb(oh) 5o ) = 10 11 at 298.15 K reacts with goethite to form tripuhyite Leverett et al. (2011) Chemical Geology, submitted for publication.
E (Volt) 1.2 1 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6 0 2 4 6 8 10 ph 298.15 K; total dissolved Sb and Fe = 10 6 mol dm 3
Why has tripuhyite been overlooked? Physical characteristics Tripuhyite rosettes up to 0.5 mm across, Clara mine, Oberwolfach, Germany; images courtesy of Mindat.org. Majzlan et al. (2011) American Mineralogist, 96, 1-13; Diemar (2008) BSc(Hons) thesis. University of Western Sydney; Diemar et al. (2009) Pure and Applied Chemistry, 81, 1547-1553; Leverett et al. (2011) Chemical Geology, submitted for publication.
So where do we stand? Sb is not going anywhere in solution and we have shown that Sb(OH) 6 (aq) reacts with goethite, ferrihydrite, akaganéite and NH 4 Fe(SO 4 ) 2 12H 2 O to give tripuhyite Colloids, suspensions, organics, adsorption Montserrat Filella, Juraj Majzlan, and others In my geochemical world, soil is a wonderful filter and geochemical anomalies are very tight
The Bayley Park prospect, Hillgrove, NSW, Australia.
Bayley Park soil Sb (C); 20 ppm contours; X = shallow prospecting pit.
Pearse Pearse South Au-Ag-Sb deposit, Mineral Hill, NSW, Australia; images courtesy of Kimberley Metals PL
0.5 1 1 0.5 0.5 1 2 Pearse South soil Sb (ppm) 5 1 0.5 0.5 5 4 3 3 4 5 3 3 4 2 0.5 0.5 0.5 Sample at point of refusal of auger; hot 6 M HCl extraction Drill holes in blue; high grade Au intersections in red Au and Sb mineralization is not coincidental The economic resource (Sb+Ag+Au) is coincidental with the N S anomaly Soil Au and Ag anomalies are confined to the same trend but do not overlay perfectly 1 0.5 2 2 1 0.5
The Wild Cattle Creek deposit, Dorrigo, NSW, Australia the mineralization is confined to the shear zone (left) and sections in red (right) carry >1% Sb as stibnite; images courtesy of Anchor Resources Limited.
N 300 m A 700 m soil Sb anomaly to the E of the Wild Cattle Creek mine (contours to 700 ppm; cut off 100 ppm; A horizon, 0 25 cm) Subsequent drilling (1972) confirmed the continuation of mineralization in the sub vertical structure
Tripuhyite abundant in all three deposits Bringing knowledge to life Sb isn t very widely dispersed But, of course, while we now know more, there is much more to know! Stibnite specimen 30 cm across. Xikuangshan deposit, Hunan Province, China; image courtesy of Mindat.org.