KO. 2840. NOTES ON MIMETITE, ETC—WHERRY. 379 



development of molecules of this structure in solutions led to the 

 crystallization of the mineral in the first place. 



Thaumasite is, accordingly, regarded as a sulfate, and it is recom- 

 mended that it be described chemically as " di-hj^droxy-tricalcium 

 carbono-silico-sulfate, crystallizing with 14 molecules of water in the 

 hexagonal system," It probably belongs in the same group as con- 

 nellite and hanksite, which are similar in crystallization. 



CRYSTALLOGRAPHIC MEASUREMENTS ON WAVELLITE FROM HBL- 

 LERTOWN. PENNSYLVANIA. 



In an abandoned iron mine 1 mile southeast of Hellertown, North- 

 ampton County, Pennsylvania, the locality of the beraunite described 

 in an earlier paper in this series,^ wavellite has long been known to 

 occur, and in 1910 the writer found two specimens containing meas- 

 urable crystals, which are rarely met with in this mineral. 



The wavellite is in acicular crystals in divergent groups in cavities 

 in ferruginous sandst^one. These are very minute, rarely exceeding 

 0.1 mm. in diameter, but their faces are brilliant and yield fairly 

 good reflections, although subparallel intergrowth renders the angles 

 somewhat variable. The indices of refraction, measured by the 

 immersion method, are a=1.52.5, ^=1.535, and 7=1.550, alldzO.005; 

 the specific gravity is 2.325. The results of the crystallographic meas- 

 urements are tabulated below. The form p, (121), is best developed, 

 and gives reflections which can be read accurately to about 5 ^ ; but the 

 results vary 30' or more from one crystal to another, so that the axial 

 ratio can not be determined beyond the third decimal place. The 

 average angles for this form proved to be: 9=41° 45': p=47° 15', 

 Avhence the ratios are : a:b:c: =:0.564 : 1 : 0.404. In all 8 certain and 

 several dovibtful forms are present; one of their modes of combina- 

 tion is shown in figure 4; other crystals are like those figured by 

 Dana. 



The forms are : 



b (010) well developed. 



a (100) traces, in the midst of striations of prism zone. 



I (430) traces, in the midst of striations of prism zone. 



m (110) well developed. 



n (340) traces, in striations. 



p (101) prominently developed, but dull. 



s (111) minute, though fairly bright. 



o (121) fairly well developed, brilliant. 

 All the material which could be spared without destroying the 

 specimens, amounting to 0.4 gram, was submitted to the firm of 



1 Proc. U. S. Nat. Mas., vol. 47, 1914, p. 507. 



