Original paper

Investigation of fluorite from the Slavyanka deposit, Bulgaria as a material for application in the optics

Zidarova, Bogdana

Neues Jahrbuch für Mineralogie - Abhandlungen Band 189 Heft 3 (2012), p. 223 - 262

published: May 1, 2012

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ArtNo. ESP154018903001, Price: 29.00 €

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Abstract

From a geodynamic point of view the Slavyanka deposit belongs to a region of autonomous Alpine tectono-magmatic activation, namely the Ograzdenian block of Serbo-Macedonian massif. It is a result of the activity of a paleohydrothermal gradient system. Its spatial development has been controlled by the local structure-deformational pattern and the lithological situation determining the morphogenetic type of the deposit. This is a case of vein-type deposit formation, which is due to the presence of strike-slip faults (Central and Mikhaletz) as ore-bearing structures and tensile stresses. The temporal evolution of the system has been controlled by the generation of discrete impulse post-volcanic hydrothermal activity, fixed by a four-stage release of the mineral-forming impulses. The paleohydrothermal activity has been organized in convective cells of several orders hierarchically. Temperature gradient, the orientation of the heat flow and the inhomogeneities in the local thermal fields determine the convective mass-heat transport, which appear as zonality in the distribution of the fluorite formation temperature and as regular variation of its constitutive peculiarities, thus leading to a direction-dependent change of the fluorite properties. Inverse temperature zoning is caused by an upward heat flow in the deposit which had been shielded by Tertiary sediments, while the strong fracturing of the underlying gneissic massif makes it a heat collector, producing an inverse temperature gradient. Directed and non-reversible variations in temperature, concentration, pH and Peq. are the indicators of zone variation in the constitutional peculiarities of the fluorite. The mechanism of formation of fluorite bodies depends on local peculiarities of the hydrothermal system, namely, the m ineral composition of the host rocks and the degree of their tectonic pretreatment, the degree of opening of the system, chemistry of solution, thermal gradient between the front of crystallization and the solution. The hydrothermal solutions depositing fluorite in this deposit have low salinity (below 1 % NaCl-equivalent), pH ∼6, relatively constant Eh, Th = 100-200°C (± 5°C) and Peq. from 1 to 20 MPa. Most probably, the fluorite in this case came from a complex form. Complexes of the type AlFn3− play an important role as intermediate form for the fluorine transport in an acid medium. Their destruction in the presence of SiF62− and Ca2+ and in the solutions with pH ∼6 leads to deposition of fluorite, alkalization of the solutions and subsequent deposition of montmorillonite and quartz. The mechanism of growth of fluorite crystals and aggregates on a microscale depends on the degree of opening of the hydrothermal system and on the equilibrium state in the crystal-solution system. If the temperature gradient between the front of crystallization and the solution in an open system far for equilibrium exceeds certain extreme values (e.g. 22°C as is the case in the Slavyanka deposit), synergetic effects are effective, i.e. self-organization of the medium through dissipation originating from convective mass-heat transport in dissipative cells with polygonal cross-section of the front of crystallization (instability of Benar). These are revealed in the anatomy of growing fluorite aggregates through specific dissipative structures. The model proposed for the formation of fluorite aggregates from the deposit is an alternative possibility to the mineralogical concepts for the growth of mineral aggregates, which relates their structure with the dynamics of the forming medium. In open hydrothermal systems local variations in the concentration of Ca2+ and F− ions take also place along the front of crystallization. The ratio of their activities and the product of solubility [(aCa2+) × (aF−)2/KspCaF2 ≷ 1] determine the processes of dissolution ( 1) of fluorite crystals and aggregates and reflect in their zonal structure. One can assume the zoned deposition of montmorillonite on the pyramids of crystal growth or along the front of crystallization of dissipative cells has resulted from self-vibration reactions of the solution.

Keywords

fluoritefluid-inclusiondatarare-earthelementsoptical characteristicsapplications