2015, vol. 41 (1): 114–115

Pickeringite from the Pieprzowe Mts. (the Holy Cross Mts., Central Poland)

Beata Naglik, Lucyna Natkaniec-Nowak

AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Department of Mineralogy, Petrography and Geochemistry; al. Mickiewicza 30, 30-059 Krakow, Poland; e-mail: [email protected], [email protected]

© 2015 Authors. This is an open access publication, which can be used, distributed and reproduced in any medium according to the Creative Commons CC-BY 4.0 License requiring that the original work has been properly cited.

The Pieprzowe Mts., which constitute the east- and . Crystallization and preservation of ern part of the Holy Cross Mts. (Central Poland), pickeringite depend on air humidity, which results are considered a large complex of mid-Cambrian from its high solubility in water (Balcerzak et al. strata (Alexandrowicz 1972). In a 2-km long ex- 1992, Parafiniuk 1991). The aim of this study is to posure of the Pieprzowe Mts succession, along the describe conditions of pickeringite crystallization Vistula River bank near the town of Sandomierz, in the weathering zone of the Pieprzowe Mts. shale shales, quartz-mica shales, quartzites, sandstones, formation. mudstones and conglomerates are outcropping. SEM-EDS analysis made it possible to detect All shaly facies are enriched in . The oc- three types of sulphide minerals occurring with- currence of secondary minerals in the weather- in aggregates. The most common is pickeringite ing zone of the mid-Cambrian shale formation forming elongated crystals with smooth surfaces, in the Pieprzowe Mts. is known for an extended which demonstrate absence of dissolution pro- period of time. Differences in color efflorescence cesses. Fibers of pickeringite are associated with observed on the weathered surface were consid- a mineral of tabular shape named – ered as alum incrustations; hence, the term “alum Al(SO4)3. Epsomite was detected in subordinate shale” has been used with reference to these rocks. quantities. XRD analysis confirmed the polymin- The phase composition of fine-crystal aggregates eral character of efflorescenes. The main reflex for was determined by Kuhl (1931). Based on observa- pickeringite (ca. 4.80 Å) is fragmented into two tions under polarizing microscope, he interpreted peaks: (1) 4.8061 Å and (2) 4.7958 Å. These val- them as a composition of mirabilite, epsomite and ues are reduced as compared to ICDD data, which alunogen, thus contradicting his earlier hypothesis could result from admixtures of other sulphates, about pickeringite occurrence. alunogene and epsomite. Another options is that Pickeringite is a sulphate mineral owing its or- this reduction means that pickeringite is forming igin to oxidation of pyrite in low pH conditions transitional phase into , which has its (Kruszewski 2013) and is a member of the halot- maximum reflex at 4.81 Å. This could be con- richite group with theoretical formula MgAl2(SO4)4 ∙ firmed by chemical composition (Fe impurities).

∙ 22 H2O. However, chemically pure pickering- The Pieprzowe Mts. shale formation presents ite is rare as most of its occurrences form tran- specific conditions favouring evaporation of pick- sitional phases between pickeringite and halot- eringite. The elements required for pickeringite richite (Parafiniuk 1991). This mineral is usually crystallization originate from the host rocks. The observed in aggregates with other sulphates; the source of Al3+ could be kaolinite, which is com- most common association consists of alunogen mon as a product of alteration processes as well

http://dx.doi.org/10.7494/geol.2015.41.1.114ISSN 2299-8004 | e-ISSN 2353-0790 | Wydawnictwa AGH Pickeringite from the Pieprzowe Mts. (the Holy Cross Mts., Central Poland) 115

as an authigenic compound. Processing of micas is also interesting. Alunogen associated with pick- transformation causes environmental enrichment eringite crystalize when the delivery of Mg2+ is in Mg2+. The published sources indicate that the reduced. Paragenesis consisting of alunogene, ep- presence of pyrite is a key-factor for pickering- somite and pickeringite could be a result of differ- ite crystallization. However, this mineral is also ent solubility of these phases or changing of Mg2+ typical for shales present in the Carpathian rock and Al3+ ions during evaporation processes. The units, where as a product of pyrite oxidation ja- following sequence of crystallization is proposed: rosite and are common. This suggests alunogene-pickeringite-epsomite. that some other initiator is required for the for- mation of pickeringite. It has been suggested that surface evaporation of groundwater causes the REFERENCES precipitation of sulphates (Cody & Briggs 1973 Alexandrowicz S.W., 1972. Góry Pieprzowe – klasyczne od- vide Parnell & Roderic 1982). The Pieprzowe Mts. słonięcie utworów kambru. Chrońmy Przyrodę Ojczystą, shale formation contains dense network of veins 28, 5–10. so it is possible that waters from deeper layers are Balcerzak E., Dobrzyński D. & Parafiniuk J., 1992. Wpływ przeobrażeń mineralnych na skład chemiczny wód able to migrate into the subsurface. Some doc- w strefie wietrzenia łupków pirytonośnych w Wieściszo- umented occurrences of pickerigite are related wicach, Rudawy Janowickie, Sudety Zachodnie, Polska. to arid regions. Certainly, an important role is Annales Societatis Geologorum Poloniae, 62, 75–93. played by humidity – southern exposure and poor Kruszewski Ł., 2013. Supergene sulphate minerals from the burning coal mining dumps in the Upper Silesian Coal plant cover of the Pieprzowe Mts. could be the Basin, South Poland. International Journal of Coal Geol- reason why pickeringite may not only crystalize ogy, 105, 91–109. but also exist there. Most of the pickeringite oc- Kuhl J., 1931. Sprawozdanie z badań petrograficznych nad currences in Poland are associated with rocks of utworami środkowokambryjskimi Gór Pieprzowych koło Sandomierza. Posiedzenie Naukowe PIG, 30, 24–26. old units (mid-Cambrian in age in the Pieprzowe Parafiniuk J., 1992. Fibroferrite, slavikite and pickering- Mts., Proterozoic in Wieściszowice and in Krobi- ite from the oxidation zone of pyrite-bearing schists in ca quarry). Older rocks are poor in carbonates so Wieściszowice (Lower Silesia). Mineralogia Polonica, 22, it could mean that pH is a more important fac- 1, 3–15. Parafiniuk J., 1994. Siarczanowe minerały wietrzeniowe tor conditioning pickeringite crystallization than z kamieniołomu łupków łyszczykowych w Krobicy pyrite occurrence. Pickeringite was found also on (Sudety Zachodnie). Przegląd Geologiczny, 42, 7, 536–538 burning coal-mining dumps in the Upper Silesian [with English summary]. Coal Basin (Kruszewski 2013), what confirm that Parnell JR. & Roderic A., 1982. Weathering processes and pickeringite formation in a sulfidic schist: a consider- pH determines crystalization of this mineral. Cor- ation in acid precipitation neutralization studies. Envi- relation between pickeringite and other sulphates ronmental Geology, 4, 3–4, 209–215.

Geology, Geophysics and Environment, 2015, 41 (1): 114–115