Boracite Mg3[B7O13Cl] from the Zechstein salt deposits

Abstract

AbstractAmong the borates in the Middle European Zechstein Salt Succession boracite MgBoracite is to be found in two varieties: individual crystals in cubic, tetrahedral or dodecahedral habit on the one hand and fibrous crystals so-called “stassfurtite” on the other hand.The formation conditions such widely spread borates in the salt succession are ambiguous in two respects. First of all the synthetic formation of boracites is to be made by hydrothermal or melt conditions. Both processes can be suspended for the salt succession. Furthermore the cubic modification is stable above 265 ºC for the Mg-boracite. The cubic, tetrahedral or dodecahedral habit could be used as a geothermometer, but such conditions can be exclude by the paragenetic minerals, esp. carnallite (MgKClThe chemical composition of orthorhombic, pseudo-cubic boracite depends on the location.Pure Mg-boracite in hexahedral habit and in fibrous habit, so-called “stassfurtite”, occurs in the North Harz region, whereas the Fe-, Mn-, Mg-boracite appears in the South Harz region.Until now the source of boron, the time of formation of crystals, but also the reasons for the differences in habit of the single hexahedral crystals are still unclear. The formation during a diagenetic/metamorphic process is evident. However, the preferred formation in Stassfurt seam could be an indication for the boron enrichment in an early diagenetic process.Furthermore permit the determination of the thermal stability and the volatile content of crystals conclusions to the chemical composition of the fluid.The observed variation suggests that the condition of crystal growth as well as the chemical composition of fluid repeatedly changed over the time. Randomly occuring xenomorpheous anhydrite and magnesite inclusions within single boracite crystals have been interpreted as an indication to factors of chemical milieu during the formation of crystals.The reversible phase transition temperature of the boracite is a linearly function of the iron and manganese content and varies from 265 ºC for Mg-boracite to 330 ºC for Fe(Mn)-boracite.The thermal decomposition of boracite is determined by two processes. The decomposition started with a boronchlorine release (BOCl?), having a maximum rate at 1050 ºC. Additionally to this release one observes a simultaneous emission of HThe results give evidence for the aged approach of a secondary formation of boracite within the complete Stassfurt seam, possibly in connection with the formation of salt diapirs in the Jura and Cretaceous period.The wider environmental distribution of borates is an indication of chemical transport processes within the salt succession. This should be a more important issue in the discussion about the utilisation of salt diapirs for the storage of nuclear waste.