*2.1. Occurrence*

The samples of minerals studied in this work were taken from the Fersman Mineralogical Museum, Museum of Natural History in Luxembourg and private collections of authors of the current paper (V.V.G., A.V.K.). Guilleminite: from the Museum (69465 and 82312), from V.V.G. (6111). Demesmaekerite: from J.P. Haynesite: from the Fersman Museum (88922 and 94267), from V.V.G. (5767), from A.V.K. (247X). The samples of guilleminite and demesmaekerite originate from the Musonoi, DR Congo. The samples of haynesite originate from the Repete mine, Utah, UT, USA.
