Acta Musei Moraviae

Nordstrandite and zeolite mineralization from Jedlka near Benešov nad Ploučnicí (Czech Republic)

The rare Al-hydroxide, nordstrandite, has recently been found in a Cenozoic tephrite on the southeastern slope of Hlídka Hill (480 m a.s.l.), 500 m N of Jedlka near Benešov nad Ploučnicí, northern Bohemia (Czech Republic). It occurs in cavities up to 10 cm in size as white, light grey, pinkish, yellow to brown hemispherical to spherical aggregates up to 4 mm across in association with phillipsite-Ca, thomsonite-Ca, mesolite, natrolite and calcite. Nordstrandite is triclinic, space group P-1 with the following unit-cell parameters: a 5.108(2), b 5.083(2), c 5.125(2) Å, α 70.43(2)°, β 74.06(2)°, γ 58.51(2)° and V 106.09(13) ų. Nordstrandite aggregates are sometimes slightly zoned and rimmed or partly replaced by smectite and chlorite. Chemical analyses of nordstrandite yielded, in addition to prevailing Al, also variably elevated (but mutually correlated) contents of Si, Mg, Fe, Ca and, in part of analyses, K and/or Mn, which are likely attributed to admixtures of smectite and chamosite. The most abundant zeolite is phillipsite-Ca, which forms white crystalline coatings in rock cavities composed of well-developed crystals up to 0.3 mm in size. Its unitcell parameters are: a 9.8648(15), b 14.3106(16), c 8.7458(13) Å, β 124.881(7)° and V 1012.8(3) ų. Its chemical composition corresponds to the empirical formula Ca_1.96K_1.51Na_0.04Ba_0.01(Al_6.14Si_10.03O₃₂)·12H₂O (mean n=10). Thomsonite-Ca occurs as colourless, white, yellowish or pinkish radially arranged aggregates up to 5 mm in size composed of flattened acicular crystals up to 3 mm in length. Its unit-cell parameters are: a 13.1145(17), b 13.0783(18), c 13.215(2) Å and V 2266.6(4) ų and its chemical analyses correspond to the empirical formula Na_1.11Ca_1.62Sr_0.21(Al_4.88Si_5.15O₂₀)·6H₂O (mean n=23). Mesolite forms irregular to elongated fibrous aggregates up to 400 μm in size between crystals of thomsonite-Ca. Its empirical formula is Na_2.04Ca_1.83(Al_6.29Si_8.86O₃₀)·8H₂O (mean n=8). Natrolite occurs as thin acicular colourless crystals up to 1 mm in length growing on the centre of the basal face of a flattened acicular crystals of thomsonite-Ca. Its chemical analyses correspond to the empirical formula Na_1.57Ca_0.04(Al_2.07Si_3.03O₁₀) ·2H₂O (mean n=8). The low formation temperatures of the mineralization are indicated by both chlorite thermometry (<35 °C) and natrolite-thomsonite assemblage (<100 °C).