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Basaltic melts at Racos penetrated a series of sedimentary rocks in their ascension to the surface, and in basalts of intrusive structures the most xenoliths were transformed resulting in various mineralogical compositions. The most important are the transformation products of limestone or marl xenoliths, resulting mineral rarities like larnite, brownmillerite (6th occurrence), thaumasite, or afwillite.
This paragenesis and type of occurring makes the Racos occurrence the 4th in the world, after those from Eifel-Belleberg Mts (Germany), Sky Island (Ireland) and Crestmore Creek (USA), and 5th if we count the Hatrurim formation (Israel). First indication of the rare calcium-silicate paragenesis from Racos was made in 2006 , and since than completed with new species like vaterite.
Identification of different phases is easily made by powder X-ray diffraction and scanning electron microscopy (SEM) combined with energy dispersive spectrometry, in the case of problematic compounds by wavelength dispersive spectrometry. The structural and textural features of xenoliths are described by optical and back-scattered electron imaging by SEM.
Results show an implication of rock-forming minerals (particularly anorthite) in metasomatic reactions of the basalt and xenoliths. Based on the thermal stability of matrix minerals, more groups can be set up. The highest temperature phases in the paragenesis are larnite and brownmillerite, with thermal stability between 900 and 1000 °C. Assemblages with gehlenite and diopside are indicating temperatures between 800 and 900 °C. Some xenoliths have a wollastonite matrix, yielding towards temperatures of 600°C. The pirometasomatic type paragenesis indicates a fast cooling of intrusive bodies. Dissolution-recrystallisation structures are preserved in the microstructure of high temperature phases, at the outer core of xenoliths.
A late hydrothermal phase generated the hydrous phases afwillite, thaumasite, tobermorite, ettringite, plombierite, which are stable between 100 and 400°C, and forms on the expense of high temperature phases.
As samples were collected from an operating basalt quarry, the chances of increasing the list of mineral species, and understanding the reactions which created the paragenesis, are high.
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