Antimony: Major Minerals, Chemistry Properties and Reactions

Major Minerals

The abundance of antimony in the Earth's crust is estimated to be between 0.2 and 0.5 ppm, comparable to thallium at 0.5 ppm and silver at 0.08 ppm. Although this element is not abundant, it is found in more than 200 mineral species. Antimony is occasionally found as the native metal, but more commonly it is found in the sulfide mineral stibnite (Sb2S3), which is the predominant ore mineral. China is the top producer of antimony followed at a distance by Russia, South Africa, Bolivia, and Tajikistan. Xikuangshan Mine in Hunan province has the largest deposits in China with an estimated deposit of 2.1 million metric tons.

Antimony is found in over 200 different minerals. Four alloys, paradocrasite (Sb3As), pararsenolamprite (As,Sb), sorosite (Cu11x(Sn,Sb), where 0.1 # x # 0.2, and stibarsen (AsSb), are known in addition to the native metal antimony (Sb) (Fig. 1. Crystalline and granular, silvery native antimony, Sb, 4 × 2.5 × 2 cm.).

Over 150 minerals are found in the sulfide class containing Sb in their crystal structures, for example, allargentum (Ag_1-xSb_x), aurostibite (AuSb2), berthierite (FeSb2S4), boulangerite (Pb5Sb4S11) (Fig. 2), bournonite (PbCuSbS3) (Fig. 3), cylindrite (Pb3Sn4FeSb2S14), getchellite (AsSbS3) (Fig. 4), jamesonite (Pb4FeSb6S14) (Fig. 5), kermesite (Sb2S2O), miargyrite (AgSbS2), pyrargyrite (Ag3SbS3), stephanite (Ag5SbS4), stibnite (Sb2S3) (Fig. 10), and tetrahedrite (Cu6[Cu4(Fe,Zn)2]Sb4S13) (Fig. 6). Four halides are known with Sn, such as kelyanite ((Hg²⁺)₆Sb³⁺BrCl₂O₆) and nadorite (PbSbClO2) (Fig. 7). Within the oxide class just under 50 minerals are found with Sb in their structure, for example, cervantite (Sb³⁺Sb⁵⁺O₄), senarmonite (Sb2O3) (Fig. 8), and stibiconite (Sb³⁺Sb⁵⁺₂ O₆(OH)).

FIGURE 2. Nest of boulangerite, Pb5Sb4S11, crystals covering the face in a 3.5×3 cm area associated with chalcopyrite, CuFeS2, crystals to 6 mm and dolomite, CaMg(CO3)2.

FIGURE 3. Bournonite, PbCuSbS3, silvery twinned crystals to 5 mm with quartz, SiO2.

FIGURE 4. Lamellar red getchellite, AsSbS3, to 4 mm.

FIGURE 5. Steely gray acicular or filiform crystals of jamesonite, Pb4FeSb6S14, to over 1 cm.

FIGURE 6. Tetrahedrite, Cu6Cu4(Fe21,Zn)2Sb4S12S, crystal faces from 1.5 to 3.0 cm.

FIGURE 7. Deep amber-brown plates of nadorite, PbSbClO2, about 1 cm.

FIGURE 8. Senarmontite, Sb2O3, turbid white octahedral crystals from about 0.75 to 1 mm.

FIGURE 10. Stibnite, Sb2S3, several terminated crystals, the longest to 7 cm, all diverging from a common point in the matrix. Stibnite is orthorhombic, 2/m2/m2/m (space group P21/b21/n21/m). It has a hardness of 2. The cleavage on {010} is perfect, while the fracture is uneven. It is highly flexible but not elastic, slightly sectile. The luster is metallic, splendent on cleavage surfaces, while the transparency is opaque. The color is silver or lead-gray to black, while the streak is lead-gray. Crystals are slender to stout, complexly terminated, elongated along [001], to 0.65 m; bent crystals not uncommon, rarely twisted. In radiating and confused groups of acicular crystals, or bladed forms with prominent cleavage; also occurring as columnar, granular, or very fine masses. Twinning is rare on twin planes {130}, {120}, and perhaps {310}.

Chemistry Properties

Antimony is a silvery, lustrous gray metalloid with five known forms. The α-form crystallizes in a trigonal cell, isomorphic with the gray allotrope of arsenic (Table 6.14). The other forms are metastable. Yellow Sb is unstable above 290℃; black Sb is formed by cooling gaseous Sb; a rare explosive form can be made by electrolysis of antimony(III) chloride. The remaining two forms are formed by high-pressure techniques. Antimony is a member of group 15 or pnictogens. It has 51 electrons arranged in an electronic configuration of [Kr]4d105s2 5p3 . As with other posttransition elements in period 5, the "inert pair effect" is observed and the common oxidation states are 13 and 15. Antimonide compounds where Sb takes an oxidation state of 23 are common. These compounds usually occur when antimony is combined with more electropositive metals. Some antimonides are notable for semiconducting properties (e.g., GaSb and InSb). In some of its compounds, oxidation states of 22, 21, 11, 12 14 can also be found.

Reactions

Antimony is less reactive than arsenic. It is stable to air and moisture at room temperature but oxidizes when heated under controlled conditions to give antimony(III) oxide (Sb2O3), antimony(II) oxide (Sb2O4), or antimony(V) oxide (Sb2O5). It combines with sulfur when heated at temperatures of 500℃~ 900℃ to form antimony trisulfide, Sb2S3. This is the form of antimony mostly found in minerals.

Similar to most polymeric oxides, Sb2O3 dissolves in aqueous solutions. It is prepared via two methods, re-volatilizing of crude antimony(III) oxide and by oxidation of antimony metal. In the first method step 1 entails the oxidation of crude stibnite to crude antimony (III) oxide using furnaces operating at approximately 500 to 1,000℃.

In step 2 the crude antimony(III) oxide is purified by sublimation. In the second method antimony metal is oxidized to antimony(III) oxide in furnaces. This reaction is exothermic. Antimony(III) oxide is formed through sublimation and recovered in bag filters. The size of the formed particles is controlled by process conditions in furnace and gas flow. The reaction can be schematically given as:

Sb2O3 is an amphoteric oxide, it dissolves in aqueous NaOH solution to give the meta-antimonite NaSbO2, which can be isolated as the trihydrate. Sb2O3 likewise dissolves in concentrated mineral acids to form the corresponding salts, which hydrolyze upon dilution with water. With nitric acid, the trioxide is oxidized to antimony(V) oxide. When heated with carbon, the oxide is reduced to antimony metal. With other reducing agents, e.g. sodium borohydride or lithium aluminium hydride, the unstable and very toxic gas stibine is formed.

Dilute acids do not attack Sb but concentrated oxidizing acids react readily, especially at elevated temperatures. Concentrated HNO3 gives hydrated Sb2O5; aqua regia gives a solution of [SbCl6] 2 hot concentrated H2SO4 gives the salt Sb2(SO4)3.