Hydrothermally grown buddingtonite, an anhydrous ammonium feldspar (NH4AlSi3O8)

Abstract: Ammonium feldspar was grown hydrothermally from a gel, having a stchiometric Al₂O₃6SiO₂ composition. As a source for NH ₄⁺ , a 25 percent NH₃ solution was used. Internal Cr/CrN and graphite/methane buffers fixed the fugacity of NH₃ during the experiments. Unit cell parameters of the synthetic ammonium feldspar are a: 0.8824 (5) nm, b: 1.3077 (8) nm, c: 0.7186 (4) nm, β: 116.068 (12), V: 0.7448 (34) nm₃. the X-ray power diffraction pattern is measured and indexed in accordance to the space group C2/m. Infrared and thermal gravimetric analyses provide no evidence for the presence of structurally bound water molecules in the crystal structure of synthetic ammonium feldspar. Hydrothermally grown anhydrous ammonium feldspar is shown to be identical to the mineral buddingtonite by the similarity of the data between the synthetic and natural materials. There may be justification for considering natural buddingtonite as an anhydrous feldspar with the ideal formula NH₄Si₃O₈. Reexamination of natural specimens is desirable.