Low‐Temperature Synthesis of Hexagonal Anorthite via Hydrothermal Processing

Abstract

Hexagonal anorthite (CaAl2Si2O8) has been prepared by hydrothermal processing of monocalcium aluminate and quartz at temperatures as low as 200°C. The successful development of this phase is dependent upon several processing parameters, including the hydration of the calcium aluminate precursor material to the hydrogarnet phase (Ca3Al2O6·6H2O) prior to hydrothermal treatment and the use of quartz as opposed to amorphous sources of SiO2. Quartz has partial solubility in the hydrogarnet lattice for additions up to 40 wt%. Increased SiO2 substitution has been shown to reduce the conversion of hydrogarnet to Ca4Al6O13·3H2O, thereby increasing its thermal stability and improving its strength characteristics at temperatures greater than 200°C. Quartz additions greater than 43 wt% lead to the formation of CaAl2Si2O8 as the sole reaction product. The moderate temperatures involved in forming this anhydrous material are an order of magnitude lower than those necessary to form this phase by melt crystallization, making it a true chemically bonded ceramic. The reaction can form a bonded matrix with strengths up to 40000 psi (280 MPa). Strengths are limited due to density changes during anorthite formation, but the matrix is thermally stable up to 1000°C.

Hydrothermal origin of hexagonal CaAl2Si2O8 (dmisteinbergite) in a compact type A CAI from the Northwest Africa 2086 CV3 chondrite

“…Together with synthesis experiments of dmisteinbergite under hydrothermal conditions (Borglum et al. ; Dimitrijević et al. ; Hong et al.…”

“…The experimental studies of synthesis of dmisteinbergite and its natural occurrences in terrestrial rocks suggest at least two possible formation mechanisms of this mineral: crystallization from a supercooled melt (Davis and Tuttle 1952;Abe et al 1991;Abe and Sunagawa 1995;Daniel et al 1995;Sokol et al 1998) and formation in a hydrothermal environment (Borglum et al 1993;Dimitrijevi c et al 1996;Hong et al 1999;Downs 2006). Coarse-grained dmisteinbergite discovered in the FUN CAI STP-1 most likely formed by crystallization from a supercooled melt .…”

“…) and of a mixture of calcium monoaluminate (CaAl 2 O 4 ) and quartz (Borglum et al. ) at approximately 200–350 °C. Finally, transformation of Ca‐zeolite to dmisteinbergite and anorthite by annealing at approximately 800 °C was reported by Dimitrijević et al.…”

“…It was shown that in a supercooled melt, hexagonal CaAl 2 Si 2 O 8 nucleates and grows prior to anorthite. Dmisteinbergite was also synthesized by hydrothermal processing of CaAl 2 Si 2 O 8 powder (Hong et al 1999) and of a mixture of calcium monoaluminate (CaAl 2 O 4 ) and quartz (Borglum et al 1993) at approximately 200-350°C. Finally, transformation of Ca-zeolite to dmisteinbergite and anorthite by annealing at approximately 800°C was reported by Dimitrijevi c et al (1996).…”

Discovery of dmisteinbergite (hexagonal CaAl2Si2O8) in the Allende meteorite: A new member of refractory silicates formed in the solar nebula

“…Unpublished data online at www.webmineral.com and www.mindat.org also describe low-temperature hydrothermal dmisteinbergite in Kurumazawa gabbro quarry, Katashina, Gumma, Japan. Synthetic hexagonal CaAl 2 Si 2 O 8 is well studied (e.g., Takeuchi and Donnay 1959;Abe et al 1991;Borghum et al 1993;Abe and Sunagawa 1995). We report here the first occurrence of dmisteinbergite in a meteorite as a new refractory mineral in a CAI and consider its origin and implication for the formation of STP-1.…”