• antimony
• crystal-chemistry
• crystallography
• mineralogy

The chemical element antimony (Z = 51) has been known since 4000 BC and its minerals and compounds have always been widely used. Four different oxidation states are known: -III, 0, +III, and +V. The more common oxidation states are +III and +V and these two different chemical species display different crystal chemical behaviours. The former is characterized by asymmetric coordinations, due to the occurrence of an active lone-electron-pair, whereas Sb5+ typically occurs in octahedral coordination.
Currently, more than 300 mineral species containing Sb as an essential chemical constituent are known. Among them, 67% belong to the crystal chemical class of sulfides and sulfosalts (in agreement with the chalcophile behaviour of Sb), whereas the remaining 33% belong to the other classes, that, with the exclusion of native elements and alloys and halides, can be indicated with the general term of oxysalts. In this PhD thesis, this latter group of minerals will be one of the main topics, as sulfides and sulfosalts have been widely studied by other authors in the last decades.
Antimony oxysalts occur as primary minerals in hydrothermal ore deposits, in metamorphic deposits, in pegmatites, as well as in metamorphosed Mn ore bodies (in some cases belonging to the so-called Långban-type ore deposits, being similar to the large Swedish ore deposit). Moreover, Sb can occur in several secondary phases related to the alteration of primary Sb minerals (mainly sulfides and sulfosalts).
This PhD thesis focuses on the mineralogical investigation of selected Sb minerals belonging to different crystal-chemical classes. In particular, the full characterization of four Sb oxysalts (apuanite, versiliaite, melanostibite, and scorticoite) has been performed, and six new mineral species belonging to the tetrahedrite group, i.e., tetrahedrite-(Cu), tetrahedrite-(Cd), hakite-(Cd), hakite-(Fe), hakite-(Zn), and stibiogoldfieldite, have been described. The study of these species was performed through a multi-technique approach, integrating data collected in the laboratories of the University of Pisa (in particular those of the Centro per l’Integrazione della Strumentanzione Scientifica dell’Università di Pisa, C.I.S.U.P.) as well as in foreign laboratories, thanks to a series of collaborations with colleagues of the Natural History Museums of Prague (Czech Republic) and Stockholm (Sweden). The following techniques were used: X-ray diffraction (both single-crystal and powder techniques), EDS and WDS chemical analyses, micro-Raman spectroscopy, and Mössbauer spectroscopy.
Apuanite, (Fe2+4Fe3+8)(Fe3+8Sb3+16)O48S4 (Z = 1), and versiliaite, (Fe2+4Fe3+4)(Fe3+4Sb3+12)O32S2 (Z = 1), are two very rare thio-oxides so far known only from the Buca della Vena mine, Apuan Alps, Tuscany, Italy. These species, first identified at the end of the 1970s, were studied by several authors who highlighted their non-stoichiometry, mainly due to a S deficit. This deficit was interpreted as related to the nanometric intergrowth of apuanite and versiliaite with S-poorer phases. In particular, versiliaite was intergrown with schafarzikite, Fe2+Sb3+2O4 (Z = 4), whereas an hypothetical phase with composition Fe2+8Fe3+4Fe3+4Sb20O48S2 was suggested to be intimately associated with apuanite. The reexamination of holotype material of both apuanite and versiliaite and the study of seven additional specimens allowed to collect new crystal-chemical data, pointing out the occurrence of Cl. This halogen, so far undetected, partially justified the non-stoichiometry reported by previous authors. The occurrence of Cl is in keeping with the identification of chloro-sulfosalts and oxy-chloro-sulfosalts at the Buca della Vena mine.
Melanostibite, ideally Mn2Fe3+Sb5+O6 (Z = 3), is a very rare Sb oxide first found at Sjögruvan, Sweden, at the end of the 19th century. During a systematic investigation of a suite of specimens collected in the Mn ore deposit of Scortico-Ravazzone (Apuan Alps, Tuscany, Italy), pseudo-hexagonal crystals, red in color, up to 0.1 mm in size, were identified. Single-crystal X-ray diffraction study allowed their identification as melanostibite, supporting the acentric nature of this species, even if previous structural investigations performed on the Swedish material suggested the centric nature of melanostibite. Owing to this discrepancy, and considering the technological importance of the Mn2Fe3+Sb5+O6 compound, the Swedish type material was reexamined, in order to ascertain the correct space group symmetry. The study confirmed the acentric nature of melanostibite also for the sample from Sjögruvan.
Scorticoite, ideally Mn6(Sb□)2(SiO4)2O3(OH)3 (Z = 1), is a new silicate belonging to the welinite group and first found in the Scortico-Ravazzone deposit, Apuan Alps, Tuscany. It occurs as blackish prismatic crystals, up to 0.1 mm, associated with sarkinite and baryte. The crystal structure of this minerals was refined and its isotypic relations with welinite-group minerals was confirmed. The mineral and its name, honouring its type locality, were approved by the IMA-CNMNC under the IMA number 2018-159.
During this PhD work, structural data on a series of specimens of tetrahedrite-group minerals previously chemically characterized by the colleagues of the National Museum of Prague were collected, allowing the identification of six new mineral species. Tetrahedrite-(Cu), ideally Cu6(Cu4Cu2)Sb4S13 (IMA 2022-078) and tetrahedrite-(Cd), Cu6(Cu4Cd2)Sb4S13 (IMA 2022-115), were described on samples from the ore deposits of Bankov (Slovakia) and Radětice (Czech Republic). They are two new members of the tetrahedrite series within the tetrahedrite group and their name is in accord with the nomenclature of this group. Stibiogoldfieldite, ideally Cu6Cu6(Sb2Te2)S13, was found on samples from the Mohawk mine, Nevada, U.S.A., classified as goldfieldite. It is a new member of the goldfieldite series, approved by the IMA-CNMNC (IMA 2022-104). Finally, three new members of the hakite series, the Se-analogues of the members of the tetrahedrite series, were found, namely hakite-(Cd), Cu6(Cu4Cd2)Sb4Se13 (IMA 2022-090), hakite-(Fe), Cu6(Cu4Fe2)Sb4Se13 (IMA 2022-082), and hakite-(Zn), Cu6(Cu4Zn2)Sb4Se13 (IMA 2022-083). These species were discovered on samples collected in the Příbram ore district, Czech Republic. The characterization of these selenides allowed the full description of their crystal-chemistry, for the first time obtained using single-crystal X-ray diffraction, and increased our understanding on these compounds.
Along with the crystal-chemical investigations on the mineral species described above, a revision of the species belonging to the pyrochlore supergroup was performed, with a focus on the members of the roméite group (i.e., the species dominated by Sb), common accessory species in metamorphosed Mn deposits and weathering products of Sb ore deposits. During the recalculation of the end-member formulae of the different members of the pyrochlore supergroup, some issues have been highlighted and a proposal for a new nomenclature scheme of this supergroup has been finalized.
The aim of this work is to add new data to the crystal chemistry of oxysalts and sulfosalts of antimony, owing to their increasing importance from a technological and environmental perspective in the 21st century society. A better understanding of their crystal chemistry would be useful for understanding the release and dispersion of Sb in the environment, as well as to give new ideas on potential technological application of Sb minerals. Moreover, the study contributes to the mineral systematics of Sb, owing to the description of seven new mineral species.