Crystals of Rare-earth-ferrous BoratesCrystals of rare-earth-ferrous borates (structure of carbonate mineral huntite, space group I3 32) were obtained from the solution-melt with the Bi203-B203 base. It was established by the methods of X-ray structural and chemical analyses that large amounts of Bi from the solution became part of the crystals affecting their solubility, morphology and hardness. Solubility curves for YFe-, NdFe-, and ErFe-borates in the 3 B&03 . 5 B,O, melt were plotted. Refinement of crystalline structure of NdFe-borate showed that bismuth takes up place of rare-earth ion in the crystal lattice. Parameters of unit cells, density and refractive indexes of t,he obtained crystals were measured. Rare-earth-ferrous borates belong to the large group of isostructural compounds with the general formula of RM,(BO,),, where R is Ln, M denotes Al, Pe, Ga, Cr. All of them are isostructural to huntite (space group R 32). These crystals are piezoelectric substances, they possess non-linear and laser properties, very hard, refractory and resistent to acids and alkali. Rare-earth-ferrous borates, with the exceptionof Ce, Pr, Yb and Lu were synthetized by the method of solid-phase reactions (with 10% excess of H,BO,) by JOUBERT in the sealed gold ampoule (JOUBERT et al.). Fine crystalls (0.5 x 0.5 x 1.0 mm) of GdPe-and YPe-borates were also obtained as a result of spontaneous crystallization by TAKAHASHI from solution in the Bi,O,-B,O, and PbF,-B,O, melts (TAKAHASHI et al.). Detailed study of RFe,(BO,), was not carried out for lack of high quality single crystals.Due to incongruent melting of RFe,(BO,), the authors studied conditions of their crystallisation from solution-melts of molybdates and vanadates of alkali elements