Slavikit, magnesiocopiapit a alunogen od Nezabudic v CHKO Krivoklatsko

Samples of recently formed secondary sulphates from the site known since the mid-19^th century as Nezabudické skály (e.g., Feistmantel 1856) were studied (Fig. 1–3). The site is situated in the Berounka valley, near the village of Nezabudice in the Rakovník district, and lies in the Křivoklátsko Protected Landscape Area (GPS 50° 01’ 23.22” N; 013° 50’ 21.66” E). The samples were taken from places where the schists and graywackes of Neoproterozoic age contain inserts of black pyritised graphitic schists, usually strongly weathered, and where water occasionally flows or seeps. Here, sulphates form efflorescences directly on the walls, or thicker crusts under overhangs, where mineralised water drips. The most abundant is gypsum, forming hard grape-like crusts composed of small colourless crystals. Very brittle, fibrous, dry, powdery, white crusts and deposits were identified as hexahydrite with a small admixture of epsomite and gypsum. These very brittle, white aggregates of Mg-salts are mostly formed separately from the gypsum crusts. At the foot of the wall, brittle, grey-white aggregates of alunogen with slavíkite and earthy, yellow crusts of magnesiocopiapite, up to several cm thick, are formed (see Fig 4–5). X-ray diffraction data were obtained on a Bruker D8 Advance powder diffractometer; CuKα radiation and a position-sensitive Lynx Eye XE detector were used. To verify the chemical composition, the samples were studied on a Jeol 6490LV electron microscope under the following conditions: accelerating voltage 15 kV, electron beam current 1 nA, and working distance 15 mm. The chemical composition was determined using an INCA Act-X energy-dispersive detector (EDX) (Oxford Instruments). Abundant alunogen, sometimes intergrown with yellow copiapite on the surface of the aggregates, forms soft when wet, brittle when dry, grey or yellowish crusts, up to several cm thick. After drying, the crusts form brittle, strongly porous aggregates composed of intergrown, colourless, thin alunogen plates, several tenths of a millimetre in size. Only Al and S were detected chemically, the calculated lattice parameters for the space group P1¯ are: a = 7.4258(8) Å, b = 26.951(1) Å, c = 6.0571(8) Å, α = 89.99(1)°, ß = 97.55(1)°, γ = 91.87(1)°, V = 1201.1(1) Å³, and Z = 2. Groups of clear alunogen tables are covered in places by inconspicuous, pale yellow-green microcrystalline aggregates of slavíkite in the deeper parts of the crust. Aggregates of slavíkite are also composed of hexagonal tabular crystals, but only 5–10 µm in size (Fig. 5a–c). The partial empirical formula of slavíkite is Mg_4.47Al_1.37Fe_13.91(SO₄)₂₁. The calculated lattice parameters of slavíkite for the space group R3¯ are: a = 12.1883(2) Å , c = 34.955(4) Å, V = 4497.0(6) Å³, and Z = 1. Magnesiocopiapite mostly occurs separately as earthy sulphur-yellow nodules and forms distinctly elongated, parallel-growing hexagonal tables, 20–40 µm in length (Fig. 5d). An indicative analysis on an EDX detector would reveal a minor content of Mg in addition to dominant Fe and S. The calculated lattice parameters of magnesiocopiapite for space group P1¯ are: a = 7.348(1) Å, b = 18.770(2) Å, c = 7.394(1) Å, α = 91.36(1)°, ß = 102.16(1)°, γ = 98.86(1)°, and V = 983.4(2) Å³. Other sulphates identified via XRD that were studied in more detail are halotrichite-pickeringite, hexahydrite, epsomite, and gypsum.

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