ORIGINAL PAPER Chemistry of Neutral and Alkaline Waters with Low Al 3+ Activity Against Hydroxyaluminosilicate HAS B Solubility. The Evidence from Ground and Surface Waters of the Sudetes Mts. (SW Poland) Dariusz Dobrzyn ´ski Received: 27 July 2005 / Accepted: 20 August 2007 / Published online: 26 September 2007 Ó Springer Science+Business Media B.V. 2007 Abstract A wide set of aqueous chemistry data (574 water analyses) from natural environments has been used to testify and validate of the solubility of synthetic hydroxya- luminosilicate (HAS B ) , Al 2 Si 2 O 5 (OH) 4 . The ground and surface waters represent regolith and/or fissure aquifers in various (magmatic, sedimentary and metamorphic) bedrocks in the Sudetes Mts. (SW Poland). The solubility of HAS B in natural waters was calculated using the method proposed by Schneider et al. (Polyhedron 23:3185–3191, 2004). Results confirm usefulness and validity of this method. The HAS B solubility obtained from the field data (logK sp = –44.7 ± 0.58) is lower than it was estimated (logK sp = –40.6 ± 0.15) experi- mentally (Schneider et al. Polyhedron 23:3185–3191, 2004). In the waters studied the equilibrium with HAS B is maintained at pH above 6.7 and at [Al 3+ ] £ 10 –10 . Silicon activity (log[H 4 SiO 4 ]) ranges between –4.2 and –3.4. Due to the calculation method used, the K sp mentioned above cannot be considered as a classical solubility constant. However, it can be used in the interpretation of aluminium solubility in natural waters. The HAS B has solubility lower than amorphous Al(OH) 3 , and higher than proto-imogolite. From water samples that are in equilibrium with respect to HAS B , the solubility product described by the reaction, Al 2 Si 2 O 5 (OH) 4 + 6H þ $ 2Al 3þ + 2H 4 SiO 4 +H 2 O is calculated to be logK sp = 14.0 (±0.7) at 7°C. Keywords Hydroxyaluminosilicate  Water chemistry  Natural water  The Sudetes Mts.  Poland 1 Introduction Silicon and aluminium are––besides oxygen––the most abundant elements of the conti- nental lithosphere (Si––27.7%, Al––8.1%). Aqueous geochemistry of both elements differs, and in most natural waters, like in low-temperature fresh ground water or surface D. Dobrzyn ´ski (&) Department of Groundwater Geochemistry, Faculty of Geology, Warsaw University, al. _ Zwirki i Wigury 93, Warsaw 02-089, Poland e-mail: dardob@uw.edu.pl 123 Aquat Geochem (2007) 13:197–210 DOI 10.1007/s10498-007-9019-7