Ferriandrosite-(Ce), a new member of the epidote supergroup from Betliar, Slovakia

A new member of the epidote supergroup, ferriandrosite-(Ce), ideally MnCeFe3+AlMn2+(Si2O7)(SiO4)O(OH), was found at the Július manganese ore occurrence near Betliar, Rožnava Co., Košice Region, Slovakia. It occurs as subhedral grains and polycrystalline aggregates, up to 0.3 mm in size, enclosed in pyroxmangite. Other associated minerals are spessartine, rhodochrosite, quartz, baryte and pyrosmalite-(Mn). Ferriandrosite-(Ce) is dark brown, with a light brown streak and vitreous lustre. The Mohs hardness is about 6 1/2 to 7, tenacity is brittle without any observable cleavage or fracture. The calculated density is 4.321 g·cm-3. Ferriandrosite-(Ce) is optically biaxial (+), with weak pleochroism, high surface relief and the mean calculated refractive index is 1.832. The empirical structural formula of ferriandrosite-(Ce), based on 13 anions per formula unit, is A1(Mn2+0.63Ca0.35Ce0.02)S1.00A2(Ce0.53La0.27Nd0.14Pr0.05REE*0.01)S1.00M1(Fe3+0.41Al0.12V3+0.01Mg0.40Ti0.05)S0.99M2Al1.00M3(Mn2+0.75Fe2+0.22Mg0.03)S1.00T1-3Si3.00O11O4(O0.67F0.33)(OH), where REE* are minor REE. Ferriandrosite-(Ce) is monoclinic, space group P21/m, a = 8.8483(4) Å, b = 5.7307(3) Å c = 10.0314(5) Å, ß = 113.3659(15)°, V = 466.95(4) Å3and Z = 2. The crystal structure of ferriandrosite-(Ce) was refined to a final R1= 0.0210 for 1910 reflections with Fo> 4s(Fo) and 127 refined parameters. Structural features of ferriandrosite-(Ce) are discussed and compared with other members of the androsite-series.