This work presents mineralogical and chemical characteristics of weathering crusts developed on sandstones exposed to various air pollution conditions. The samples have been collected from sandstone tors in the Carpathian Foothill and from buildings in Kraków. It has been stated that these crusts differ in both fabric and composition. The sandstone black crust from tors is rich in organic matter and composed of amorphous silica. Sulphate incrustations accompanied by dust particles have been only sometimes observed. Beneath the black crust, a zone coloured by iron (oxyhydr)oxides occurs. The enrichment of the surface crust in silica and iron compounds protects the rock interior from atmospheric impact. The sandstones from architectonic details are also covered by a thin carbon-rich black crust, but they are visibly loosened. Numerous salts, mainly gypsum and halite, crystallise here, thus enhancing deterioration of the rock. Moreover, spherical particles originated from industrial emissions are much more common. Gypsum in natural outcrops, forms isolated and well-developed crystals, whilst these found on the architectonic details are finer and densely cover the surface. Such diversity reflects various concentrations of acid air pollutants in solutions.

Optical microscopy in transmitted and reflected light and scanning electron microscopy with a field emission gun combined with microprobe analysis energy-dispersive spectroscopy (EDS) were used for characterizing the energy-dispersive spectroscopy changes in the weathered stone layers and, particularly, in studying microscopic particles. Additionally, the electron backscattered diffraction (EBSD) technique provided crystallographic information for the particles examined. Five main categories of anthropogenic particles were distinguished: gypsum crystals, solid glass particles, dendritic iron oxides, crystalline iron oxides, and partly coked carbon particles. From the crystallographic information obtained with the EBSD technique, most of iron oxides particle was identified as hematite.

Pickeringite, ideally MgAl₂(SO₄)₄·22H₂O, is a member of the halotrichite group minerals XAl₂(SO₄)₄·22H₂O that form extensive solid solutions along the joints of the X = Fe-Mg-Mn-Zn. The few comprehensive reports on natural halotrichites indicate their genesis to be mainly the low-pH oxidation of pyrite or other sulfides in the Al-rich environments of weathering rock-forming aluminosilicates. Pickeringite discussed here occurs within the efflorescences on sandstones from the Stone Town Nature Reserve in Ciężkowice (the Polish Outer Carpathians), being most probably the first find on such rocks in Poland. This paper presents mineralogical and geochemical characteristics of the pickeringite (based on SEM-EDS, XRPD, EPMA and RS methods) and suggests its possible origin. It belongs to the pickeringite–apjohnite (Mg-Mn joints) series and has the calculated formula Mg_0.75Mn_0.21Zn_0.02Cu_0.01Al_2.02(S_{0.99 to 1.00}O₄)₄·22H₂O (based on 16O and 22H₂O). The unit cell parameters refined for the monoclinic system space group P2₁/c are: a = 6.1981(28) Å, b= 24.2963(117) Å, c = 21.2517(184) Å and β = 100.304(65)°. The Raman spectra (SO₄) bands are the intensive 994 cm⁻¹ and a low-intensive 975 cm⁻¹ (ν₁), low-intensive 1081, 1123 and 1145 cm⁻¹ (ν₃), 524, 467 and 425cm⁻¹ (ν₂), 615 cm⁻¹ (ν₄), while those at 344 and 310 cm⁻¹ are attributed to ν_g H₂O and at 223 cm⁻¹ to the lattice modes. Crystallization of pickeringite within the particular tor resulted from a certain set of conditions: climatic (e.g., season, temperature, humidity), physicochemical (e.g., pH, concentration), mineral (the presence of pyrite), and site-related (location and efflorescence protection). The sulfate ions could have been derived from oxidation of pyrite in the Ciężkowice sandstones and possibly are related to local mineral waters.