Dec, 1908. Mineralogical Notes « 161 



usually give successive signals. Fig. 1, PI. LI, exhibits the usual 

 development. In addition it may be noted that one individual of 

 the twin usually shows a tendency to grow by the other, suggesting 

 a penetration twin; but the growth is never extended far. Deter- 

 mination of the specific gravity gave 4.284 The forms and measure- 

 ments observed are as follows: 



h (210) e (101) 



Observed Calculated 



3 6° 13' 36 52' 



52 10' 53 8' 



65° 3»' 65 35' 



54 12' 54° 42' 



SPHALERITE 



TUCKAHOE, MISSOURI 



As is well known, sphalerite occurs in the Joplin district in the 

 form of small crystals in clay, and occasionally in sufficient abundance 

 to be used as an ore. Mr. James Roach of Tuckahoe, Missouri, who 

 mines ore of this character, kindly selected about 25 of the best crystals 

 and presented them to the Museum, Mus. No. M 6382. The crystals 

 are of interest as showing an unusual habit for sphalerite and one 

 which is in some respects difficult of interpretation. The crystals 

 range from 5 to 20 mm. in diameter and are of a generally 

 tetrahedral form. In color some, generally the smaller ones, are 

 reddish-brown and nearly transparent, but the majority are dark- 

 colored and opaque. The development of the crystal planes varies 

 from almost indiscriminate rounding to well-defined. All the crystals 

 however, as stated, show a general tetrahedral form. Now and then 

 apparent re-entrant angles are to be seen, which suggest that the 

 crystals are probably twins ; but on breaking the crystals no differ- 

 ences of cleavage can be observed to confirm this supposition. As a 

 rule the crystals are made up of only fifteen planes, but occasionally 

 eighteen can be observed. None of the planes are sufficiently brilliant 

 to give measurements with the reflecting goniometer, but the crystals 

 are of such size that satisfactory results can be obtained with the 

 contact goniometer. By study of the crystals in this manner the 

 presence of the tetrahedron and cube can be definitely and satisfac- 

 torily determined. These forms are always present in their full num- 



