Textural patterns, mineralogy, and chemistry of sandstone-related Calçadinha chalcedony (Piauí, Brazil)

Abstract

Paleozoic sandstones of the Parnaíba Basin, in addition to hosting opal deposits, also have occurrences of chalcedonies with potential for mineral and ornamental handicrafts, in addition to assisting the understanding of the geological evolution of the basin. However, the chalcedonies were not investigated yet, and this study intended to fulfill this gap by the investigation of the chalcedonies of Calçadinha in Piauí. Fieldwork, microtexturals analysis, X-ray diffraction and scanning electron microscopy with energy dispersive spectrometry, chemical analysis, and gemological assessments were developed. Four distinct types of chalcedonies have been distinguished. They stand out for their well distribution of Fe and Mn dendrites, which involves opal nodules, and contains microcavities with well-formed microcrystalline quartz, nontronite, and palygorskite. The mesoscopic features of these chalcedonies and cabochon and free forms cutting show potential for use in mineral crafts and semi-jewels. As expected, the chalcedonies are dominated by high contents of SiO 2 , besides the low and variable contents of Al 2 O 3 , Fe 2 O 3 , MgO, and TiO 2 . Among trace elements that show high Ba contents, bound in barite, seem also to be a geochemical signature of the country sandstones in Parnaíba basin. These chalcedonies were formed during the partial solubilization of SiO 2 of sandstones, which was promoted during their tectonic formation in faults and fractures zones. KEYWORDS:Manganese dendrites; Palygorskite; Opal; Barium; Rare earth elements. (Piauí, Brasil) RESUMO:Arenitos paleozóicos da Bacia do Parnaíba hospedam, além de excelentes depósitos de opala, ocorrências de calcedônias, ainda não investigadas, com potencial para o artesanato mineral e ornamental, e que poderão auxiliar o entendimento da evolução geológica da bacia. Trabalhos de campo, análises microtexturais, difração de raios-X e microscopia eletrônica de varredura com espectrometria de energia dispersiva, análises químicas e apreciações gemológicas foram desenvolvidas. Destacaram-se quatro modos distintos de calcedônias

Mineralogy of agate gemstone in Quaternary deposits, Southeastern Basrah, Iraq

“…6) [1,5], which gives agate stone is a distinctive brown or red color, and is sometimes in the form of successive lines with different concentrations of this element. Several researchers have suggested that agate bands form as a result of crystallization from hot aqueous fluids [15]. The combination of elements such as Al, Fe and Mn observed in the jaspar samples (Fig.…”

“…The precise mineral analysis of selected samples of agate species showed that the main mineral component in them is quartz (Fig. 4,5) [14], and it is believed that it is the main phase in these minerals, as no other phases may be associated with it, such as terdymite or cristobalite [15]. Microscopic study and electron microscopy SEM images of the studied agate by examining the geomorphology of the surface grains.…”