W. E. Ford — Herderite Crystals from Aicfacrn, Maine. 283 



Akt. XXXIII. — On Some Herderite Crystals from Aufoirn, 

 Maine / by W. E. Ford. 



Herderite is a rare fluo-phosphate of beryllium and calcium, 

 Ca[Be(F,OH)]P0 4 , in which the fluorine is replaced in varying 

 amounts by the hydroxyl group. A variety, in which there 

 was only a very little or no fluorine present, was described by 

 Penfield, who suggested the name hydro-herderite for this type 

 and the name hydro-fluor-herderite for the more usual occur- 

 rence in which both hydroxyl and fluorine occur.* The mineral 

 was first described from the tin mines of Ehrenfriedersdorf, 

 Saxony, on specimens that were obtained prior to 1825. It 

 was later discovered at Stoneham, Maine, and has since been 

 found at the neighboring localities of Auburn, Hebron, Paris 

 and Greenwood. It has also recently been noted on specimens 

 from Epprechtstein, Fichtelgebirge, Bavaria. \ 



The crystals from the original American locality, Stoneham, 

 were described by E. S. Dana as belonging to the orthorhoinbic 

 system 4 Later Penfield found, on studying crystals from 

 Paris, Maine, that they showed the symmetry of the mono- 

 clinic system with an axial inclination, however, of sensibly 

 90°. § He proved that the Stoneham crystals were in reality 

 penetration twins with the base, c(001), as twinning plane yield- 

 ing pseuclo-orthorhombic individuals. He further showed that 

 the crystals from Paris, Maine, belonged to the hydro-herderite 

 variety and gave slightly different crystal constants than the 

 hydro-fluor-herderite from Stoneham. 



The twin crystals so far observed, with only a few excep- 

 tions, have shown no external evidences of twinning, being in 

 appearance strictly orthorhoinbic. Recently, however, the 

 Brush Mineral Collection acquired from Mr. Lazard Cahn a 

 suite of specimens from Mt. Apatite, Auburn, Maine, that 

 included not only simple crystals of unmistakably monoclinic 

 symmetry, but also twin crystals, which through prominent 

 reentrant angles and by unequal development of the two indi- 

 viduals showed clearly their twinned character. 



These crystals varied somewhat in size but most of them 

 averaged about one-half inch in their thickest part. They 

 were colorless or white and were in part clear and transparent, 

 and in part cloudy and translucent. The following forms were 

 on them : fl_(010), c (001), tw (110), e (302), t (032), v (031), q (332), 

 n (331), n (331), r (121), g (343), A (634). These forms have all 

 been observed before with the exceptions of y (343) and 

 h (634). The form r (121) had only been described on crystals 



*This Journal, xlvii, 329, 1894. fCentralbl. Min., 294, 1908. 



% This Journal, xxvii, 229, 1884. § Loc. cit. 



