A rounded fragment of a multicoloured tourmaline crystal (2.5 cm diameter), collected from the secondary gem deposit of Mavuco, Alto Ligoña pegmatite district, Mozambique, has been investigated using a multi-analytical approach, with the objective of reconstructing its growth history. The sample represents a core-to-rim section, perpendicular to the c axis, of a crystal characterised by a variety of colours. These change from a black core to an intermediate zone with a series of colours, yellow, blue–green and purple, to a final dark-green prismatic overgrowth. These changes are the result of a wide variation in Fe, Mn, Ti and Cu concentrations and their redox state. The black core is characterised by enrichment in Fe and Mn, with iron present in its divalent state. The yellow zone shows a progressive depletion in Fe and its colouration is caused by Mn2+ and Mn2+-Ti4+ IVCT interactions. The progressive decrease in Mn coupled with the absence of Ti, and the lack of Fe, implies that Cu2+ acts as the only chromophore in the pale blue–green zone. The dominant colour-causing agent of the purplish zone is Mn3+, denoting a change in redox environment; however, even though the amount of Cu remains significant, its chromophore effect is obscured by Mn3+. The dark-green prismatic overgrowth, characterised by a sharp increase in Fe, Mn and also Ca, is interpreted as a late-stage partial re-opening of the geochemical system. This occurrence could potentially be related to mechanical instability of the cavity in which the crystal grew.

Two tourmaline crystals with a blue growth zone at the analogous pole, respectively from the San Silvestro and the Fucili pegmatites, located in the San Piero in Campo village, Elba Island (Tyrrhenian Sea, Italy), have been described for the first time using compositional and spectroscopic data to define their crystal-chemical aspects and the causes of the colour. Compositional data obtained by electron microprobe analysis indicate that both tourmalines belong to the elbaite–fluor-elbaite series. The upper part of each crystal is characterised by an increased amount of Fe (FeO up to ~1 wt.%) and a Ti content below the detection limit. Optical absorption spectra recorded on the blue zone of both samples show absorption bands caused by spin-allowed d-d transitions in [6]-coordinated Fe2+, and no intervalence charge transfer Fe2+-Ti interactions, indicating that Fe2+ is the only chromophore. Mössbauer analysis of the blue zone of the Fucili sample confirmed the Fe2+ oxidation state, implying that the redox conditions in the crystallisation environment were relatively reducing. The presence of colour changes at the analogous termination during tourmaline crystal growth suggests a change in the composition of the crystallisation environment, probably associated with a partial opening of the system.

Fahlores [~(Cu,Ag)10(Zn,Fe)2Sb4S13] from the Keno Hill mining district, central Yukon, Canada record virtually the entire petrogenetic history of a Cretaceous hydrothermal system extending over 40 km outward from the Mayo Lake granitic pluton. These fahlores are an essential constituent of polymetallic sulphide veins developed in a graphitic Mississippian quartzite, where they occur in association with sphalerite, pyrargyrite, galena and siderite. Fahlores exhibit pronounced east-west zoning in average Ag/(Ag+Cu) and Zn/(Zn+Fe) values, with these simultaneously increasing and decreasing from east to west over 20 km of hydrothermal activity. These zonations are coupled with average Ag/(Ag+Cu) and Zn/(Zn+Fe) values in fahlore roughly paralleling the 300°C isotherm for fahlores in equilibrium with pyrargyrite, miargyrite and sphalerite in the simple system Ag2S-Cu2S-ZnS-FeS-Sb2S3. Early high-Ag, high-Zn fahlores from the eastern and western mines have Ag/(Ag+Cu) and Zn/(Zn+Fe) values requiring temperatures ≥400°C, in agreement with temperatures established from the As-content of arsenopyrite coexisting with pyrite, pyrrhotite and sphalerite. Ag/(Ag+Cu) and Zn/(Zn + Fe) values in later, main-stage fahlores are consistent with the 250–310°C range of temperatures established for boiling of Keno Hill fluids. Finally, Ag- and Fe-rich fahlores were produced by retrograde Fe-Zn exchange with sphalerite or crystallized from late-stage epithermal fluids which produced polybasite, stephanite, acanthite and wire silver. One such fahlore exhibits unmixing into high-Ag and low-Ag varieties. This is the first reported miscibility gap for freibergite fahlores and confirms the earlier prediction of such gaps.