A re-investigation on the historical cervantite-type antimony ochres

Antimony ocher are yellowish antimony oxo-hydroxides formed by weathering of stibnite (Sb2S3). They occur naturally as single crystallized phases in the isometric system with pyrochlore type structure, containing some Ca and water molecules in the structure, its range in composition can be expressed by the formula: Sb5+2-x (Sb3+, Ca)y (O, OH, H2O)6-7, in which y is generally near 1, and x ranges from 0 to 1. Furthermore, Sb-ochres use to include substitutional As, Fe, Ta, Ti, Cu and others. This chemical variability keeping the structure has generated historical confusion of names of equivalent minerals with similar X-ray diffraction patterns being necessary the use of additional techniques. The mineral -type Cervantite from Cervantes (Spain) (Ca, Sb3+)2(Sb5+)2O6(OH) was disapproved at 1954 and re-approved at 1962 as & alpha;-Cervantite Sb3+Sb5+O4 analyzing synthetic and natural antimony ochres from other localities, e.g., Brasina (Serbia) by X-ray diffraction. We herein characterize both historical specimens-type from Cervantes (Lugo, Spain) and Zajaca-Stolice (Brasina, Serbia) from the chemical-elemental, structural, thermal and speciation points of view, together with a vibrational study by Raman and FTIR, since the X-ray diffraction patterns of isometric samples with pyrochlore-type structure are excessively similar among them. The Cervantes specimen-type could be named hydroxycalcioromeite (Ca, Sb3+)2(Sb5+)2O6(OH) whereas the Brasina specimen-type Ca2(Sb5+)4O12(OH)2 is very similar but lacking Sb3+; both specimens contain Ca and hydrous components, faraway of the official anhydrous orthorhombic & alpha;-Cervantite (Sb3+Sb5+O4) setting. Micro-Raman was essential determining molecular phases and Sb-O bond vibrations, FTIR and DTA-TG finding hydroxyl groups and XPS defining Sb speciation.