Williams — Eudialyte and Eucolite from Arkansas. 461 



only inclusions which have been observed. It appears as if 

 sometimes eudialyte and sometimes orthoclase was formed 

 first, for first one and then the other is found in idiomorphic 

 crystals. On the whole the eudialyte appears to be the earlier 

 of the two. Decomposition takes place very rapidly. 



Eucolite. — According to Brogger,* all those crystals which 

 have the form of eudialyte, and essentially its chemical compo- 

 sition, but negative double refraction in place of positive, are 

 to be considered eucolite. It appears therefore that the yellow- 

 ish brown crystals, bearing a great resemblance in form and 

 size to eudialyte, but which are characterized by their negative 

 double refraction, are to be classed under this head. The 

 crystals which appear in the Arkansas rock are of a much 

 lighter brown or brownish yellow color than those from 

 ISorway. The cleavage parallel to the base (0001) is much 

 more pronounced than in eudialyte, but that in other directions 

 is about equally poor with that already noted. 



The following faces have been observed (fig. 3) : _c = OR 

 (0001), R== + R (1011), d=-\R (01 12), g = <*P (1010), a = 

 oc_l*2 (1120). Some of the angles between these faces and the 

 base have been measured, but the poor 

 reflections prevent the obtaining of accu- 

 rate results. The angles recorded are 

 probably not nearer than from 15' to 30' 

 to the true angles. Owing to these large 

 limits of error, it is impossible to calcu- Ti3 . s 



late any axial ratio for this mineral, but 



the angles as measured do net differ very much from the cor- 

 responding angles on eudialyte. 



Angle. . 

 c: d (0001): (0112) 

 c:R (0001): (1011) 

 c: a (0001): (1120) 

 c: g (0001): (lOlo) 



The specific gravity taken in the same way as that of eudia- 

 lyte gave 2-6244 to '2-6630 at 15° C. This is extremely low, 

 but even after bringing in a correction for adhering orthoclase 

 (sp. gr. == 2 - 55 — 2 : 62) the figures would not be much higher. 

 Hardness 5 to 5*o. 



In thin sections this mineral appears semi-transparent, and is 

 of a white to a very light yellow color. In convergent polar- 

 ized light, a section perpendicular to the principal axis shows 

 a black cross which when tested with a mica plate establishes 

 the negative character of the double refraction very plainly. 

 The double refraction appears weaker than in eudialyte. Mag- 

 netite and segyrite appear to be the only inclusions. Some 



* L. c. page 489. 



Measured. 



Calculated. 



50° 40' 30" 



50° 43' 6' 



67 12 



67 45 24 



89 53 



90 



90 25 



90 



