Ardennite in a quartz schist from the Asemi-gawa area in the Sanbagawa metamorphic terrain, central Shikoku, Japan.

Abstract

Ardennite is found to occur in a piemontite-bearing quartz schist from the Asemigawa area in the Sanbagawa metamorphic terrain, central Shikoku. The ardennite-bearing quartz schist contains quartz, piemontite and garnet as major constituents with subordinate amounts of talc, chlorite, albite, calcite, hematite, braunite, rutile and apatite, together with ardennite. Ardennite is pale golden yellow and occurs as subhedral and prismatic crystals up to 1 mm in length. It is optically positive with 2V_Z=57±3°. It is orthorhombic and unit cell parameters are a=5.828(2) Å, b=18.540(8) Å, c=8.692(3) Å and V=939.1(6) ų. As₂O₅ and V₂O₅ contents of ardennite reach 10.8 and 1.3 wt.% in maximum, respectively. The empirical formula of the ardennite is (Ca_0.53Mn_3.67)(Mg_1.02Cu_0.03Fe_0.20³⁺Al_4.47Ti_0.01)(As_0.80V_0.09) Si_5.25O₂₂ (OH)₆. It is slightly poor in (As + V) and rich in Si compared with the ideal formula of ardennite. Piemontite contains SrO of 0.7wt.% in maximum.