Weissite from Gambatesa mine, Val Graveglia, Liguria, Italy: occurrence, composition and determination of the crystal structure

Abstract

Weissite, Cu2–xTe (x ≈ 0.21), a very rare copper telluride, occurs in a sample from the Gambatesa mine, Val Graveglia, Liguria, Italy, where it occurs as purplish black anhedral grains up to 0.1 mm in length and shows a black streak. No cleavage is observed and the Vickers hardness (VHN100) is 142 kg/mm2. Weissite is dark bluish black, weakly pleochroic, and moderately anisotropic in bluish tints. Reflectance percentages in air for Rmin and Rmax are 37.0, 38.4 (471.1 nm), 33.2, 34.2 (548.3 nm), 31.2, 32.1 (586.6 nm), and 28.6, 31.0 (652.3 nm), respectively.Weissite is trigonal and belongs to the space group P3m1 with the following unit-cell parameters: a = 8.3124(7) Å, c = 21.546(1) Å, V = 1289.3(2) Å3, and Z = 24. Electron microprobe analyses gave the chemical formula (Cu1.62Ag0.04Au0.04Fe0.04Sb0.04)Σ=1.78(Te0.96S0.02Se0.02). The crystal structure has been solved and refined to R = 1.95%. It consists of Cu and Te polyhedra forming complex crystal-chemical environments as is typical of many intermetallic compounds. The exceedingly short bond distances observed among the metals are discussed in relation to other copper tellurides and pure metals.

Spiridonovite, (Cu1-xAgx)2Te (x ≈ 0.4), a New Telluride from the Good Hope Mine, Vulcan, Colorado (U.S.A.)

“…The spiridonovite structure is shown in Figure 2. The presence of Ag affects the bond distances (in the range 2.82-2.84 Å; Table 6) that are larger than those reported for Cu tellurides, such as vulcanite (CuTe) [13], rickardite (Cu 3-x Te 2 ) [14], cameronite (Cu 5-x (Cu,Ag) 3+x Te 10 [4]), weissite (Cu 2-x Te) [3], and henryite ((Cu,Ag) 3+x Te 2 ) [5]. Conversely, they are smaller than those observed in Ag-tellurides, such as empressite (AgTe) [11] and hessite (Ag 2 Te) [15].…”

“…On the contrary, Te atoms exhibit a highly distorted hcp arrangement in weissite, thus producing short Cu-Cu distances and partially occupied Cu sites. Weissite has a complex structure with 24 independent Cu and 12 Te atoms showing significantly different crystal-chemical environments [3], while the crystal structure of spiridonovite is far simpler and comprises two Te sites and three M sites, which are occupied by Cu and minor Ag, as reported in Table 5. All the minerals belonging to the Cu-Ag-Te system are shown in Table 8, together with some crystallographic detail.…”

“…Spiridonovite is a new member of the Cu-Ag-Te system, together with rickardite [1], (Cu 3-x Te 2 ), vulcanite (CuTe) [2], weissite (Cu 2-x Te) [3], cameronite [Cu 5-x (Cu,Ag) 3+x Te 10 ] [4], and henryite [(Cu,Ag) 3+x Te 2 ] [5]. The new mineral species was discovered in a fragment from the cameronite holotype material, sample R-934, from the Good Hope mine (38 • 20' 35" N, 107 • 0' 26" W), Vulcan, Colorado (U.S.A.) obtained in 1974 by the late William W. Pinch on exchange from the Smithsonian Institution, Washington D.C. (U.S.A.) [4].…”

“…However, as demonstrated by [18], the β III synthetic phase is actually an intermetallic compound, with short Cu-Cu and Te-Te contacts, and thus different from spiridonovite. Cameronite (Cu 5-x (Cu,Ag) 3+x Te 10 [4]) and weissite (Cu 2-x Te) [3] have a similar intermetallic nature, showing short bond distances between the metals, characterized by the presence of typical "interpenetrated tetrahedra" due to the short Cu-Cu, Te-Te, and Cu-Te distances. Funding: The research was funded by MIUR-PRIN2017, project "TEOREM deciphering geological processes using Terrestrial and Extraterrestrial ORE Minerals", prot.…”

Chemical and structural characterization of henryite, (Cu,Ag)3+xTe2 (x ≈ 0.40): A new structure type in the (Ag)–Cu–Te system

“…The occurrence of complex superstructures in the CueTe system is indeed well known: weissite [6] and cameronite [12] can be considered superstructures where the arrangement of the Cu (and Ag) ions depends on the composition of the phase and thermal history. The formation of such superstructures, as most of the others observed in the CueTe system [16,17], is generally ascribed to the mobility of the copper ions in different valence states, Cu þ or Cu 2þ , in a rigid Te framework.…”