Читать книгу Monument Future - Siegfried Siegesmund - Страница 157

Gypsum lumps from the furnace exhibited a temperature dependent zonation with respect to the occurrence of phases and phase morphologies (Lenz, Sobott 2008) . Low fired gypsum lumps typically consist of three zones. A thin surface layer of rehydrated anhydrite III (dehydrated hemihydrate), is followed by a layer of fibrous hemihydrate, and the central part is made up of a mixture of hemihydrate and dihydrate (unconverted gypsum). Thermoanalytic measurements prove anhydrite III to be stable up to 370 °C. It forms fibrous crystals, partly pseudomorph after gypsum. With increasing firing temperature hemihydrate and anhydrite III are converted to anhydrite II (natural anhydrite). Anhydrite II formed in the temperature range between 370 and 800 °C shows elongated fibrous crystals and at temperatures above 800 °C the crystals tend to be short prismatic (Figure 5).

Figure 5: left: short prismatic anhydrite II crystals in samples fired at 1,100 °C for 5 hours; right: prismatic anhydrite crystals in samples fired at 800 °C for 5 hours; crossed polarized light.

In a laboratory sample which was fired at 600 °C for 5 hours radial fibrous anhydrite crystals were observed. Obviously the shape of anhydrite II crystals and aggregates in fired samples depends largely on how the heat treatment was executed.

In thin sections of gypsum samples which were exposed to temperatures above 600 °C isotropic crystals with a hexagon outline were observed.

They were identified by EDXRF measurements as periclase MgO (Figure 6).

Müller et al. (2009) described the formation of periclase by a contact metamorphic reaction in dolomite marble at P = 1 kbar and T > 605 °C

CaMg(CO₃)₂ → MgO + CaCO₃ + CO₂ dolomite periclase calcite

Since idiomorphic to hypidiomorphic dolomite crystals up to 400µm in size were observed in thin sections of the raw material from the gypsum quarry near the Takht-e-Soleyman, it is reasonable to assume that the dolomite disintegrated and was converted to periclase according to the above mentioned chemical reaction. Therefore the appearance of periclase in samples can be regarded as an intrinsic temperature indicator if the pressure dependence of the reaction is known.

Figure 6: Periclase crystals in anhydrite II matrix; sample from furnace, adjacent to thermocouple that recorded a maximum temperature of 800 °C; plane polarized light.