Crystal data for ammoniojarosite. NH₄Fe₃(OH)₆(SO₄)₂

Jarosite minerals are of widespread occurrence in nature and have assumed increasing prominence in metal extraction studies (see, e.g. Haigh, 1967). Minerals in this group ideally conform to the formula MFe₃(SO₄)₂(OH)₆, and possess a hexagonal, pseudo-cubic structure similar to that of alunite (Wyckoff, 1965). The monovalent ion M⁺ can be Na⁺, K⁺, H₃O⁺, NH₄⁺ or Ag⁺, but solid solutions are easily formed and pure jarosites rarely occur in nature. They are difficult to synthesize. Attempts to synthesize them normally result in specimens deficient in (for example) Na⁺, because of the readiness with which H₃O⁺ ions occupy some of the available M-type sites. Brophy & Sheridan (1965) have studied in detail the X-ray diffraction patterns given by the continuous series of solid solutions formed by Na⁺/K⁺/H₃O⁺ substitution in jarosites. Much less attention has been paid to the X-ray data for ammoniojarosite; Sabina & Traill (1960) report the powder diffraction pattern very briefly and Hendricks (1937), working with a natural sample, gives values for the hexagonal unit cell constants a₀ and c₀ which fall within the range of variation of the corresponding parameters established for the Na⁺/K⁺/H₃O⁺ system by Brophy & Sheridan (1965). In this work the powder diffraction pattern of a synthetic sample of ammoniojarosite has been studied in detail to assess the value of the X-ray data for analytical work; the results differ significantly from those given by Sabina & Traill (1960), and lead to a value of the Co constant which exceeds that of any member of the Na⁺/K⁺/H₃O⁺ system.