¹Lyubov P. Ogorodova, ^1,2Yuliya D. Gritsenko, ¹Marina F. Vigasina, ¹Andrey Y. Bychkov, ¹Dmitry A. Ksenofontov, ¹Lyubov V. Melchakova
American Mineralogist 103, 1945-1952 Link to Article [https://doi.org/10.2138/am-2018-6475]
¹Geological Faculty, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow, 119234, Russia
²Fersman Mineralogical Museum RAS, Leninskiy prospect 18-2, Moscow, 117071, Russia
Copyright: The American Mineralogical Society

In the present study, four samples of natural melilites were characterized using electron microprobe analysis, powder X-ray diffraction, FTIR, and Raman spectroscopy, and their thermodynamic properties were measured with a high-temperature heat-flux Tian-Calvet microcalorimeter. The enthalpies of formation from the elements were determined to be: –3796.3 ± 4.1 kJ/mol for Ca_1.8Na_0.2(Mg_0.7Al_0.2Fe2+0.1Fe0.12+⁠)Si₂O₇, –3753.6 ± 5.2 kJ/mol for Ca_1.6Na_0.4(Mg_0.5Al_0.4Fe2+0.1Fe0.12+⁠)Si₂O₇, –3736.4 ± 3.7 kJ/mol for Ca_1.6Na_0.4(Mg_0.4Al_0.4Fe2+0.2Fe0.22+⁠)Si₂O₇, and –3929.2 ± 3.8 kJ/mol for Ca₂(Mg_0.4Al_0.6)[Si_1.4Al_0.6O₇]. Using the obtained formation enthalpies and estimated entropies, the standard Gibbs free energies of formation of these melilites were calculated. Finally, the enthalpies of the formation of the end-members of the isomorphic åkermanite-gehlenite and åkermanite-alumoåkermanite series were derived. The obtained thermodynamic properties of melilites of different compositions can be used for quantitative modeling of formation conditions of these minerals in related geological and industrial processes.

¹Chi Ma, ¹John R. Beckett
American Mineralogist 103, 1918-1924 Link to Article [https://doi.org/10.2138/am-2018-6599]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

Nuwaite (Ni6GeS2, IMA 2013-018) and butianite (Ni6SnS2, IMA 2016-028) are two new chalcogenide minerals, occurring as micrometer-sized crystals with grossular, Na-bearing melilite, heazlewoodite, and Ge-bearing Ni-Fe alloys in veins and as mono-mineralic crack-filling material in igneous diopside in the Type B1 Ca-Al-rich inclusion (CAI) ACM-2 from the Allende CV3 carbonaceous chondrite. The chemical composition of type nuwaite is (wt%) Ni 65.3, S 10.3, Ge 8.2, Te 7.9, Sn 5.1, and Fe 1.7, with a sum of 98.5 and an empirical formula of (Ni5.95Fe0.16)(Ge0.60Sn0.23)(S1.72Te0.33). The simplified formula is Ni6(Ge,Sn)(S,Te)2, leading to an end-member of Ni6GeS2. The chemical composition of type butianite is (wt%) Ni 62.1, Sn 8.9, Te 10.3, S 8.9, Ge 5.3, Fe 1.3, sum 99.1, giving rise to an empirical formula of (Ni5.93Fe0.13)(Sn0.52Ge0.41)(S1.56Te0.45). Butianite’s simplified formula is Ni6(Sn,Ge) (S,Te)2 and the end-member formula is Ni6SnS2. Both nuwaite and butianite have an I4/mmm inter-growth structure with a = 3.65 Å, c = 18.14 Å, V = 241.7 Å3, and Z = 2. Their calculated densities are 7.24 and 7.62 g/cm3, respectively. Nuwaite and butianite are the first known meteoritic minerals with high Ge and Sn concentrations.
Nuwaite and butianite are very late-stage, vapor-deposited, alteration products, filling in pores within preexisting grossular-rich alteration veins and cracks in igneous Al,Ti-diopside. These phases and associated heazlewoodite and Ge-bearing alloys are observed only within the Ca-,Al-rich inclusion (CAI) and not outside it or at the inclusion-matrix interface. As only sections in one half of ACM-2 contain nuwaite/butianite, they were probably derived through a relatively low fO2-fS2 sulfidation process, in which a highly localized, low-temperature Ge-, Sn-bearing fluid interacted with a portion of the host CAI. It is likely that the fluid became relatively more Sn- and Te-enriched with time and that crack fillings post-date vein fillings, possibly due to a late remobilization of vein sulfides.

¹Christian Mavris, ¹Javier Cuadros, ²José Miguel Nieto, ³Janice L. Bishop, ⁴Joseph R. Michalski
American Mineralogist 103, 1877-1890 Link to Article [https://doi.org/10.2138/am-2018-6330]
¹Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
²Department of Earth Sciences, University of Huelva, 21071 Huelva, Spain
³SETI Institute, Mountain View, California 94043, U.S.A.
⁴Department of Earth Sciences and Laboratory for Space Research, University of Hong-Kong, Pokfulam Road, Hong-Kong, China
Copyright: The Mineralogical Society of America

Earth analogs are indispensable to investigate mineral assemblages on Mars because they enable detailed analysis of spectroscopic data from Mars and aid environmental interpretation. Samples from four sites in the Iberian Pyrite Belt (El Villar, Calañas, Quebrantahuesos, and Tharsis) were investigated using mineralogical, chemical, and spectroscopic techniques, with a focus on clay minerals and alteration environments. They represent Earth analogs of areas on Mars that underwent acidic alteration. X-ray diffraction and transmittance mid-infrared data indicate that the rocks were subjected to several degrees of acid alteration corresponding to assemblages characterized by the following mixtures: (1) illite, chlorite, interstratified chlorite-vermiculite, kaolinite-smectite, and kaolinite; (2) illite, kaolinite, and alunite; and (3) jarosite and goethite. According to mineral stability data, these three assemblages correspond to pH values 7–5, 5–3, and <3, respectively. The lack of goethite in the illite-kaolinitealunite assemblage suggests an alteration in reducing conditions. Illite was progressively dissolved by acidic alteration but is sufficiently resilient not to be diagnostic of the intensity of the alteration. Illite and kaolinite were the two most abundant phyllosilicate minerals observed, and the main reaction involving phyllosilicates was the alteration of illite to kaolinite. Mixed-layer phases appeared mainly in the mildest degree of acid alteration, with few exceptions. This suggests a transition from a mechanism dominated by transformation to a mechanism dominated by dissolution-precipitation as the intensity of the acid alteration increases. Our results highlight the sparse kaolinite-alunite occur-rences on Mars as worthy of specific investigation. Acid alteration on Mars is expected to be patchy and/or consisting of fine alteration rims. Alunite occurrences on Mars in the absence of goethite may indicate an acid alteration in reducing conditions. Kaolinite produced through acid alteration on Mars is expected to exist mainly as an end-member phase of low crystallinity, which would enhance IR absorption and increase its visibility.