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Author's personal copy 1142 SCIENCE OF THE TOTAL ENVIRONMENT 407 (2009) 1135– 1152 Fig. 6 – XRD pattern from a complex assemblage including iron (melanterite and siderotil), manganese (mallardite) and aluminium sulphates (tamarugite). Pattern peaks from ICDD. found with typical acicular habit and can be seen as an exception because it forms pure concretions. This mineral is more abundant at the skarn pits resulted from open pit mining, where it grows as white masses, in coexistence with amorphous spherical iron-sulphate. Although rare, siderotil was detected in assemblages carrying melanterite and aluminium sulphates (Fig. 6). When compared with the more common Rozenite+Melanterite assemblage, the Melanterite+Siderotil showed a much intense blue colour, which may be reflecting the incorporation of cooper by melanterite, like it was demonstrated by Jambor and Traill (1963). At Valdarcas it wasn't detected the coexistence of the three minerals melanterite, siderotil and rozenite, which is consistent with the results of the cited authors that demonstrated the preferential evolution of cooper-bearing melanterite by dehydration to siderotil, in spite of rozenite under the same conditions. The assemblage Rozenite+Gypsum+Sulphur was found to form well-crystallized aggregates. They occur as very fine grained yellow-coloured powders, often fairly thick (1–2 cm), characterized by softness and by intense sulphur odour. SEM study has revealed that native sulphur occurs with euhedral crystals dispersed over the sulphate efflorescences. 4.1.2. Distribution, seasonal behaviour and paragenesis Soluble sulphates were abundantly observed at the waste-dumps and rarely along the Poço Negro creek. Here, gypsum and rozenite were the only identified soluble salts. They occur at the margins of the creek, episodically, during evaporative periods. Fig. 9 shows the distribution of the sulphate-mineral assemblages over the waste-dumps surface, for winter and summer conditions. During summer, efflorescent blooms of gypsum and rozenite were observed as thick accumulations that cover great extensions of the exposed surfaces. Occasionally, during the most prolonged dry periods, native sulphur joints to this assemblage. The way the well-developed sulphur crystals appear over sulphates suggests that sulphur belongs to a latest stage of slow deposition relative to sulphate minerals. Melanterite and Al-sulphates were only observed in narrow sheltered areas, in the presence of remnant effluent. In winter, gypsum rosettes grow even on the most exposed vertical slopes of the waste-dumps in accordance with the relative low solubility of this sulphate. Highest mineralogical diversity was observed during fairly long-lasting evaporation periods (one or two weeks) that succeed intensive rainfall periods (often in February). In these conditions, melanterite was the first Fe-sulphate to be identified, followed by the Alsulphates. If evaporation conditions were persistent, it was possible to observe the assemblage Rozenite+Melanterite (major rozenite), at the most sunny exposed conditions. These field observations, combined with the morphologic features, indicate that Al-sulphates precipitate after melanterite. Fig. 8 shows Al-sulphates over melanterite crystals, suggesting that the latest deposition of aluminium may be the result of iron depletion in the solution. Interpretation of the occurrences and distribution justify summarizing the following remarks: - There are microenvironments for mineral precipitation, related with topographic irregularities that have allowed a variety of exposure conditions to the climate elements; for instance, erosion has created cavernous shelters, even at the most vertical and climate exposed slopes, that ensure the persistence of the most soluble salts (melanterite and Alsulphates) during winter (protecting them from dissolution) and during summer (protecting them from desiccation).
Author's personal copy SCIENCE OF THE TOTAL ENVIRONMENT 407 (2009) 1135– 1152 1143 Fig. 7 – Field images of representative sulphate efflorescences. a) Fragile white rosettes of gypsum; b) Gypsum rosettes cemented by jarosite; c) Rozenite in plaques on desiccation fractures network; d) Powdered efflorescences of Rozenite+ Gypsum+Sulphur; e) General aspect of the efflorescences including rozenite and melanterite with minor oxyhydroxides; f) Detail of melanterite (green globules) and of its weathering products rozenite (white) and oxyhydroxides (brown); g) Microenvironment of salt precipitation controlled by humidity conditions comprising rozenite+melanterite+Al-sulphates. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) - Melanterite occurs preferably near the most acidic seepages, indicating the proximity to its paragenetic precursors (mainly pyrrhotite); Table 3 shows the composition of the effluent at ValdR, which represents the most acidic AMD, from which this salt precipitates; - At the most acidic seepages, melanterite precedes the Alsulphates; this sequence suggests that the latest deposition of the aluminium minerals is controlled by high Al:Fe ratios, resulting from iron depletion in the solution; this is in accordance with the paragenetic sequence observed by
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também um pouco por todo o mundo,
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Figura 1 - As iniciativas de geocon
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a maioria dos quais com relevância
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Silva J. & Gomes C. (2003) - Patrim
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constitucionais (Tabela 1) - e, de
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Para a implementação, a nível na
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Figura 3 - Representação esquemá
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REFERÊNCIAS Alfama V., Gomes A.M.,
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30º Curso de Actualização de Pro
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Tabela 1. Tamanho do grão em rocha
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aos alunos uma reflexão sobre mine
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Figura 4. Classificação de Streke
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das rochas, afim de deduzir qual a
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3. ROCHAS METAMÓRFICAS As rochas m
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Figura 11. Relação entre o tipo d
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sempre a determinação do tipo de
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Para relacionar o grau crescente de
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Rochas detríticas As rochas detrí
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Tabela 14: Componentes dos calcári
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Geomorfologia e Património Geomorf
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Norte Foz do Lima Microgeoformas al
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Fig. 4 - Geossítios do Forte do C
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algumas pias de forma lobada e late
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perfeita. Acima da Estrada Nacional
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Geossítio do Monte do Galeão - Vi
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na encosta atlântica, a 85 metros;
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Fig. 19a- MDT de declives do concel
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Fig. 22 - Soerguimento relativo dos
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tenha sido pouco intenso ou que a e
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Fig. 26 - Afloramento de geoformas
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ALVES, A.; 1996. Causas e processos
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