Orange Pickeringite from the Algares 30-Level Adit, Aljustrel Mine, Iberian Pyrite Belt, Portugal
Teresa P. Silva,
João X. Matos,
Daniel de Oliveira,
Igor Morais,
Pedro Gonçalves,
Luís Albardeiro,
Fernanda Carvalho,
Ugur D. Menda,
João P. Veiga
Affiliations
Teresa P. Silva
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, 2610-999 Amadora, Portugal
João X. Matos
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Bairro de Val d´Oca, 7601-909 Aljustrel, Portugal
Daniel de Oliveira
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Estrada da Portela, 2610-999 Amadora, Portugal
Igor Morais
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Bairro de Val d´Oca, 7601-909 Aljustrel, Portugal
Pedro Gonçalves
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Bairro de Val d´Oca, 7601-909 Aljustrel, Portugal
Luís Albardeiro
LNEG (National Laboratory for Energy and Geology), Mineral Resources and Geophysics Research Unit, Bairro de Val d´Oca, 7601-909 Aljustrel, Portugal
Fernanda Carvalho
CENIMAT/i3N (Materials Research Centre), Department of Materials Science, NOVA School of Sciences and Technology (FCT-NOVA), NOVA University of Lisbon, 2829-516 Caparica, Portugal
Ugur D. Menda
CENIMAT/i3N (Materials Research Centre), Department of Materials Science, NOVA School of Sciences and Technology (FCT-NOVA), NOVA University of Lisbon, 2829-516 Caparica, Portugal
João P. Veiga
CENIMAT/i3N (Materials Research Centre), Department of Materials Science, NOVA School of Sciences and Technology (FCT-NOVA), NOVA University of Lisbon, 2829-516 Caparica, Portugal
The sheltered environment of the Algares +30 level adit (underground mine gallery) contributes to the preservation of secondary water-soluble minerals formed on the tunnel walls. The massive sulphide and related stockwork zone are hosted by the Mine Tuff volcanic unit and are exposed in the walls of the gallery, showing intense oxidation and hydrothermal alteration. Minerals from the halotrichite group were identified on the efflorescent salts, typically white fine-acicular crystals but also on aggregates with dark orange/brownish colour. Mineral characterization was performed using several methods and analytical techniques (XRD, XRF-WDS, SEM-EDS, DTA-TG), and the chemical formulas were calculated maintaining the ratio A:B ≅ 1:2 in accordance with the general formula of the halotrichite group, AB2(SO4)4·22H2O. This methodology allowed the assignment of the orange colour to the presence of trivalent iron on iron-rich pickeringite in partial substitution of aluminium.