Acta Musei Moraviae

Acicular apatite and associated rock-forming minerals from the basalt quarry Bílčice in the Nízký Jeseník Upland (Czech Republic)

Rich drusy coatings of acicular apatite, with needles up to 1.5 cm long, were recently found in vesicular cavities of the Pleistocene alkaline lavas in the quarry Bílčice. The occurrence was recorded at the upper mining level, i.e., within the upper respective effusion of the Chřibský les lava flow. The mineral assemblage of apatite-bearing cavities and adjacent host rock was studied by means of an electron microprobe. The apatite from both cavities and rock matrix shows very homogeneous and stoichiometric composition with prevailing fluorapatite component (F-Ap_75-89Cl-Ap_13-17OH-Ap_0-11). The wall rock is composed of forsterite (Fo_57-79Fa_21-42Te_0-1), clinopyroxene (Wo_48-54En_33-41Fs_9-15), ulvöspinel (Ulv_48-58Mgt_23-30Mgf_8-17Spn_1-7Gal_2-3), plagioklase (Ab_33-65An_23-65Or_1-18Slw_0-2Cn_0-1), alkali feldspar (Ab_48-60An_2-7Or_28-47Slw_0-3Cn_0-4), nepheline (Ne_68-77Ks_3-12Qz_17-23), and glass enriched in P, F, and Cl. An increase of grain size of rock-forming minerals towards the vesicular cavity underlines the increasing role of volatiles during crystallization of the magma. The uniform composition of various morphological forms of apatite implies for a geochemically closed system and yields a genetic link to the magmatic processes, including the cavity filling. The external post-magmatic fluids operated later during formation of a clay mineral consuming nepheline and glass. In addition, a small quartzrich xenolith was found during our microprobe study. The contact zone between xenolith and igneous rock is mainly composed of alkali feldspar and clinopyroxene. The shifts in composition of both phases suggest that the protolith of this xenolith was characterized by elevated contents of Fe³⁺ and K.