Crystal Structure and Chemical Composition of Ludwigite from Vranovac Ore Deposit (Boranja Mountain,Serbia)

The crystal structure of ludwigite from Vranovac ore deposit (Boranja Mt., Serbia) was refined using the X‐ray powder diffraction (XRPD) Rietveld method in the space group Pbam to a final R_B=7.45% and R_F=5.26%. It has the unit cell dimensions of: a= 9.2515(2) Å; b= 12.3109(2) Å; c= 3.03712 (7) Å; and V= 345.91(1) ų. The calculated distances and angles are mostly in good agreement with the Mg²⁺‐Fe²⁺ substitutions across the M(1) and M(3) sites, as well as with the Fe³⁺‐Al³⁺ replacement in the M(4) site. However, the mean observed M(2)‐O distance is considerably shorter than prescribed, due to a slight increase of the Fe³⁺ content in the M(2) site. Such replacement was compensated by slight increase of the Fe²⁺ content in the M(4) site, resulting in the (Mg_1.48Fe²⁺_0.46Fe³⁺_0.05Mn_0.02)_2.01 (Fe³⁺_0.94Fe²⁺_0.04Al_0.02)_1.00B_1.00O₅ composition. The formation temperature was estimated to be about 500–600°C. The influences of the various chemical compositions to the crystallographic parameters, M‐O distances, M(3) and M(4) sites shift, distortion parameters and estimated valences, were also studied and compared with other reference samples.