Mineraloška, kristalografska in geokemična analiza wulfenitnih zrn iz rudnika Mežica

Gračanin, Nik (author), Rogan Šmuc, Nastja (mentor), Šturm, Sašo (comentor)

Nekdanji rudnik svinca in cinka Mežica je svetovno znano nahajališče minerala Wulfenit (PbMoO4). V magistrski nalogi smo analizirali tabularna in bipiramidalna wulfenitna zrna iz Mežice z namenom postavitve mehanizma njihove rasti. Pri tem smo uporabili komplementarni pristop, ki je temeljil na pregledu dosedanjih dognanj pri geološkem razvoju in opisu mineralne združb v Mežici ter na izsledkih raziskav laboratorijskih sintez wulfenitnih zrn skupaj z naprednimi analitskimi tehnikami, s katerimi smo analizirali izbrana zrna in obruse mineralnih združb, ki nastopajo v Mežici. Pri analitskih tehnikah smo se odločili za pristop od makroskopskega do mikroskopskega oz. atomskega nivoja, pri čemer smo uporabili tehnike odsevne optične mikroskopije, vrstične elektronske mikroskopije (SEM in HAADF–STEM), analize kemijske sestave (SEM–EDS in HR–ICP–MS) in kristalografske analize (SCXRD). V obrusih mineralnih združb smo potrdili prisotnost galenita, sfalerita, anglezita, karbonatnih mineralov, wulfenita in deskloizita. Rezultati analiz wulfenitnih zrn so pokazali, da so kristalne rešetke wulfenitnih zrn iz Mežice kemijsko čiste, vsebujejo pa številne vključke predvsem karbonatnih mineralov, ostali vključki pa spadajo med svinčeve in železove okside, redki so vključki deskloizita. Vključki se navadno pojavljajo v pasovih. Wulfenitna zrna spadajo v prostorsko skupino I4/a1 in nimajo dvojčkov, dislokacij ali ionskih zamenjav v kristalnih rešetkah. Glede na dobljene rezultate trdimo, da je zunanja morfologija odraz spremembe v okolju tekom izločanja in rasti wulfenitnih zrn iz raztopin, bogatih s Pb2+in MoO42- Mehanizem rasti wulfenitnih zrn iz Mežice sestoji iz treh stopenj. V prvi stopnji so se iz raztopin izločala tabularna zrna, koncentracija Pb2+ je bila nižja od koncentracije MoO42-. V drugi stopnji so raztopine postale bogate s Ca in Mg ioni, kar privede do sunkovitega izločanja karbonatnih vključkov. V tretji stopnji je koncentracija Pb2+ višja od MoO42-, kar favorizira rast bipiramidalnih zrn.

• Language: Slovenian
• Keywords: wulfenit, Mežica, kristalografija, mineralogija, geokemija, mehanizmi rasti zrn
• Work type: Master's thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2024
• PID: 20.500.12556/RUL-160669
• Publication date in RUL: 03.09.2024

Secondary language

• Language: English
• Title: Mineralogical, crystallographic and geochemical analysis of wulfenite crystals from Mežica mine

Abstract

The former lead and zinc mine in Mežica is a world-renowned deposit of the mineral wulfenite (PbMoO4). In this master's thesis, we analyzed tabular and bipyramidal wulfenite crystals from Mežica to propose the mechanism of their growth. We employed a multi-layered approach based on a review of previous findings regarding the geological development and description of the mineral assemblage in Mežica, as well as the results of studies of wulfenite crystals grown in labs together with advanced analytical techniques used to analyse samples of mineral assemblage and wulfenite crystals. For the analytical techniques, we chose an approach from the macroscopic to the microscopic and atomic level, using techniques such as reflective optical microscopy, scanning electron microscopy (SEM and HAADF–STEM), chemical composition analysis (SEM–EDS and HR–ICP–MS), and crystallographic analysis (SCXRD). Samples of mineral assemble were confirmed to consist of galena, sphalerite, anglesite, carbonates, wulfenite and descloisite minerals. The analyses showed that the wulfenite crystal lattices from Mežica are chemically pure, however they contain numerous inclusions of mostly carbonate minerals, together with less common lead and iron oxides and descloisites. Inclusions usually occur in bands. The wulfenite crystals belong to the spatial group I4/a1 and do not exhibit twins, dislocations, or ionic replacements in the crystal lattices. Based on the obtained results, we argue that the external morphology reflects changes in the environment during the precipitation and growth of wulfenite grains from solutions rich in Pb2+ and MoO42-. The growth mechanism of wulfenite grains from Mežica consists of three stages. In the first stage, tabular grains precipitated from solutions, where the concentration of Pb2+ was lower than that of MoO42-. In the brief second stage, the solutions became enriched with Ca and Mg ions, leading to the sudden precipitation of carbonate inclusions. In the third stage the concentration of Pb2+ is higher than that of MoO42-, favouring the growth of bipyramidal grains.

• Keywords: wulfenite, Mežica, crystallography, minerology, geochemistry, mechanisms of crystal growth