Hydromagnesite from the efflorescence of the Stone Town Nature Reserve in Ciężkowice, the Western Carpathians, Poland

Hydromagnesite, Mg₅(CO₃)₄(OH)₂⋅4H₂O, is a naturally occurring mineral belonging to the group of hydrated magnesium carbonates containing hydroxyl groups, part of the MgO–CO₂–H₂O system. Its formation requires high Mg/Ca ratios, typically linked to ultramafic rock weathering, and is influenced by pH, with alkaline conditions being favorable. Hydromagnesite commonly forms in alkaline lakes, as efflorescences on carbonate rocks, and even in meteorites, and plays a key role in CO₂ capture and storage. This study characterizes natural hydromagnesite forming as spring efflorescences on a sandstone tor in the Stone Town Nature Reserve, Ciężkowice, Poland. The site hosts pickeringite, MgAl₂(SO₄)₄⋅22⋅H₂O, and alunogen, Al₂(SO₄)₃∙17⋅H₂O, rich efflorescences during summer, reflecting significant seasonal pH variations. This represents perhaps the first discovery of hydromagnesite on such rocks in Poland. The paper describes mineralogical and geochemical characteristics of the efflorescence and hydromagnesite itself, using methods SEM-EDS, XRPD, EPMA, Raman spectroscopy, and STA coupled with QMS for the analysis of evolved gases. Hydromagnesite crystals exhibit an acicular to flame-bladed habit, often with irregular surfaces covered by flaky and flocculent grains, clustering in aggregates. The calculated formula is Mg₅(CO₃)₄(OH)₂⋅5.14 H₂O (based on five cations). Crystals of hydromagnesite are monoclinic (space group P2₁/c) with: a = 10.050(8) Å, b = 8.921(7) Å, c = 8.384(6) Å, β = 114.291(25)°. Raman spectra reveal intense bands at 1119 cm⁻¹ (ν₁ CO₃²⁻), weaker bands at 710, 732, 762 cm⁻¹ (ν₄ CO₃²⁻), 646 cm⁻¹ (ν₄ HCO₃⁻), and OH-stretching vibrations at 3515, 3445, and 3373 cm⁻¹. Thermal effects associated with hydromagnesite occur at 270°C and 390°C, corresponding to dehydration and overlapping dehydroxylation and decarbonation, respectively. The relatively low decarbonation temperature likely results from crystal morphology enhancing heat transfer and earlier CO₂ release. Although the efflorescence contains minor hexahydrite, gypsum, and quartz, the above parameters for the predominant hydromagnesite are consistent with literature values.