Herderite: Gemstone Information
Herderite is recorded by Anthony et al. in Handbook of Mineralogy (2000) as CaBe(PO4)F but there is some doubt about its species status as reported by Mandarino and Back, Fleischer’s Glossary of Mineral Species (2004). Here the same composition is given but queried. There have been no reports of data with F OH. Anthony et al. (2000) report an electron microprobe of a gemstone ‘from Brazil’ which showed F 7% (thus with F 5.86%, the series [with hydroxylherderite] midpoint). Herderite forms pseudo-orthorhombic crystals of the orthorhombic system by analogy to hydroxylherderite with a hardness 5–5.5. Gem material is transparent and pale in its range of colours which include pink, violet and green, some crystals are colourless. The SG 2.95–3.02 and the RI for the alpha, beta and gamma rays is 1.556–1.59, 1.578–1.61 and 1.589–1.62 respectively, biaxial negative, birefringence 0.029–0.030.A green gemstone from Brazil is noted by Arem, Color Encyclopedia of Gemstones, 2nd edition, 1987; the RI is given as 1.581, 1.601, 1.610 for the three rays with a birefringence of 0.029–0.030.
The Fe content was 7%. Perhaps this was the specimen described by Anthony et al. (op. cit.). Some herderite shows a green or violet fluorescence in both LW and SWUV; some specimens are reported to have shown orange fluorescence and persistent phosphorescence. Herderite is believed to occur in complex granite pegmatites. A transparent green specimen (perhaps the stone already described) weighing 5.90 is in the National Museum of Natural History, Smithsonian Institution, Washington DC, USA. Most gem herderite has come from locations in Minas Gerais, Brazil. Specimens from Maine,USA, described by Bank and Becker (1977) were violet or colourless with RI 1.587–1.590, 1.609–1.610 and 1.619–1.621 with SG fairly constant at 3.00.Brown material from the Virgem do Lapa mine in the same Brazilian state has the high RI 1.610, 1.630 and 1.642. F content was reported to be 0.2%.