Zhenruite, (MoO ₃ ) ₂  ⋅ H ₂ O, and tianhuixinite, (MoO ₃ ) ₃  ⋅ H ₂ O, two new minerals in the MoO ₃ –MoO ₃  ⋅ 2H ₂ O system

Abstract. Two new minerals, zhenruite, ideally (MoO3)2⋅H2O, and tianhuixinite, ideally (MoO3)3⋅H2O, were discovered, respectively, from the Freedom #2 mine in the central part of the Marysvale volcanic field, Utah, USA, and an unnamed short adit on the Summit group of claims near Cookes Peak, Luna County, New Mexico, USA. Zhenruite occurs as acicular or prismatic crystals (up to 0.06×0.01×0.01 mm). Associated minerals include alunogen, anhydrite, coquimbite, fluorite, liangjunite, quartz, and raydemarkite. Zhenruite is colorless in transmitted light and transparent with a white streak and vitreous luster. It is brittle with a Mohs hardness of 1 1/2–2; cleavage is perfect on {001}. The calculated density is 4.081 g cm−3. Tianhuixinite occurs as nanometric crystal aggregates, 10–70 µm in size, intergrown with virgilluethite. Associated minerals include barite, fluorite, ilsemannite, jordisite, powellite, pyrite, quartz, raydemarkite, sidwillite, and virgilluethite. Tianhuixinite is dark blue-green and translucent in transmitted light. It has a white streak and vitreous luster. Tianhuixinite is brittle with a Mohs hardness of ∼2; no cleavage was observed. The calculated density is 4.131 g cm−3. At room temperature, neither zhenruite nor tianhuixinite is soluble in water or hydrochloric acid. Electron microprobe analyses yielded an empirical formula (Mo1.00O3)2⋅H2O for zhenruite and (Mo1.00O3)3⋅H2O for tianhuixinite, calculated on the basis of 7 and 10 O apfu, respectively. Zhenruite and tianhuixinite are the natural counterparts of synthetic (MoO3)2⋅H2O and hexagonal (MoO3)3⋅H2O, respectively. Zhenruite is monoclinic with space group P21/m and unit-cell parameters a=9.6790(6), b=3.70653(19), c=7.1029(4) Å, β=102.391(5)°, V=248.89(2) Å3, and Z=2. Its crystal structure is characterized by two kinds of topologically identical octahedral double chains extending along [010], one consisting of edge-sharing Mo1O6 octahedra only and the other Mo2O5(H2O) octahedra only. These two kinds of chains are linked together alternately through sharing corners to form layers parallel to (001), which are interconnected by hydrogen bands along [001]. Tianhuixinite is hexagonal with space group P63/m and unit-cell parameters a=10.5963(12), c=3.7216(4) Å, V=361.88(9) Å3, and Z=2. Its crystal structure is composed of double chains of edge-sharing MoO6 octahedra extending along [001], which are corner-connected with one another to form hexagonal channels with H2O residing at the center. The double chains of edge-sharing MoO6 octahedra in zhenruite and tianhuixinite are topologically identical to those in molybdite and raydemarkite, and zhenruite can be regarded as a combination of molybdite and raydemarkite both structurally and chemically. The discovery of tianhuixinite implies the likelihood of finding the ammonia analogue, (MoO3)3⋅NH3, in nature.