Publication Date:
2019
Description:
〈span〉〈div〉Abstract〈/div〉Tamboite (〈span〉x〈/span〉 = 3; 〈span〉y〈/span〉 = 2) and metatamboite (〈span〉x〈/span〉 = 3; 〈span〉y〈/span〉 = 0), Fe〈sup〉3+〈/sup〉〈sub〉3〈/sub〉(SO〈sub〉4〈/sub〉)(Te〈sup〉4+〈/sup〉O〈sub〉3〈/sub〉)〈sub〉3〈/sub〉(Te〈sup〉4+〈/sup〉O(OH)〈sub〉2〈/sub〉)(OH)(H〈sub〉2〈/sub〉O)〈span〉x〈/span〉{H〈sub〉2〈/sub〉O}〈span〉y〈/span〉, are new tellurite minerals from the Tambo mine, Coquimbo Province, Chile. The two minerals transform to each other reversibly with changes in ambient humidity. They occur as pale-yellow clusters of radiating fiber bundles on the surface of a compact aggregate of silicified tuff. Tamboite and metatamboite are optically biaxial, and their calculated mean index of refraction is greater than 1.80. The calculated densities are 3.648 g/cm〈sup〉3〈/sup〉 for tamboite and 4.053 g/cm〈sup〉3〈/sup〉 for metatamboite. Tamboite and metatamboite are monoclinic, space group 〈span〉P〈/span〉2〈sub〉1〈/sub〉/〈span〉c〈/span〉, 〈span〉Z〈/span〉 = 4. Unit-cell parameters for tamboite are 〈span〉a〈/span〉 16.879(10), 〈span〉b〈/span〉 7.310(4), 〈span〉c〈/span〉 16.666(9) Å, β 108.857(11)°, 〈span〉V〈/span〉 1958(3) Å〈sup〉3〈/sup〉; for metatamboite they are 〈span〉a〈/span〉 14.395(5), 〈span〉b〈/span〉 7.296(4), 〈span〉c〈/span〉 16.411(6) Å, β 98.909(10)°, 〈span〉V〈/span〉 1703(2) Å〈sup〉3〈/sup〉. Chemical analysis by electron microprobe gave the empirical cations [calculated on the basis of 22 anions 〈span〉pfu〈/span〉 with OH = 3 and H〈sub〉2〈/sub〉O = 5 〈span〉pfu〈/span〉 (tamboite) or H〈sub〉2〈/sub〉O = 3 〈span〉pfu〈/span〉 (metatamboite)] as (Fe〈sup〉3+〈/sup〉〈sub〉3.10〈/sub〉Al〈sub〉0.15〈/sub〉)〈sub〉Σ3.25〈/sub〉(S〈sup〉6+〈/sup〉〈sub〉0.75〈/sub〉Se〈sup〉6+〈/sup〉〈sub〉0.05〈/sub〉)〈sub〉Σ0.80〈/sub〉Te〈sup〉4+〈/sup〉〈sub〉4.11〈/sub〉. The seven strongest lines in the X-ray powder diffraction patterns [listed as 〈span〉d〈/span〉 (Å), 〈span〉I〈/span〉, (〈span〉hkl〈/span〉)] are as follows: metatamboite: 14.221, 100, (100); 2.874, 13, ; 3.140, 12, (221); 3.423, 11, ; 3.400, 11, (312); 3.012, 11, ; 4.054, 9, ; tamboite: 16.068, 100, (100); 3.425, 9, ; 2.999, 8, ; 3.171, 6, (221); 2.853, 5, ; 4.153, 4, ; 3.943, 4, (004). The crystal structures were solved by direct methods and refined to 〈span〉R〈/span〉〈sub〉1〈/sub〉 indices of 4.3 and 3.0%. The structures consist of virtually identical ferric-sulfate-tellurite-hydrate slabs that are constructed from strands of ferric-sulfate-hydrate polyhedra linked by Te〈sup〉4+〈/sup〉 cations. In metatamboite, the slabs are linked directly by hydrogen bonds whereas in tamboite, interslab linkage occurs by hydrogen bonds through interstitial {H〈sub〉2〈/sub〉O}〈sub〉4〈/sub〉 clusters known as 〈span〉Ci〈/span〉 cyclic tetramers. Exposure of a crystal to a desiccant at room temperature resulted in a third variant (〈span〉x〈/span〉 = 2; 〈span〉y〈/span〉 = 0) with the structural formula Fe〈sup〉3+〈/sup〉〈sub〉2〈/sub〉Fe〈sup〉2+〈/sup〉(SO〈sub〉3〈/sub〉(OH))(Te〈sup〉4+〈/sup〉O〈sub〉3〈/sub〉)〈sub〉3〈/sub〉(Te〈sup〉4+〈/sup〉O(OH)〈sub〉2〈/sub〉)(OH)(H〈sub〉2〈/sub〉O)〈span〉x〈/span〉, space group 〈span〉P〈/span〉2〈sub〉1〈/sub〉/〈span〉c〈/span〉, 〈span〉Z〈/span〉 = 4, 〈span〉a〈/span〉 16.879(10), 〈span〉b〈/span〉 7.310(4), 〈span〉c〈/span〉 16.666(9) Å, β 108.857(11)°, 〈span〉V〈/span〉 1958(3) Å〈sup〉3〈/sup〉, calculated density 4.176 g/cm〈sup〉3〈/sup〉. This lower-hydrate variant has less cation-bonded (H〈sub〉2〈/sub〉O) than metatamboite and tamboite, and the ferric-sulfate-tellurite-hydrate slabs are polymerized to form a framework structure. Attempts to transform the lower hydrate back to tamboite or metatamboite at room temperature and elevated humidity were unsuccessful.〈/span〉
Print ISSN:
0008-4476
Electronic ISSN:
1499-1276
Topics:
Geosciences
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