PETEOLOGY AT GOOSE CREEK SHANNON. 



75 



Constituent. 



SiO^ 



AI0O3 



CaO 



MgO 



K2O 



NaaO 



H2O+110°C 

 H2O-110°C 

 Insoluble 



Total 



Goose 

 Creek. 



54.40 



17.88 

 8. 56 



Theory. 



57.40 

 16. 30 



7. 70 



16. 00 

 2. 32 



99. 16 



1.40 

 17.20 



100, 00 



Epides- 

 mine. 



56. 66 

 16.00 



7.58 

 . 06 

 .67 

 . 88 



18. 69 



.44 



100. 98 



Enough of the Goose Creek material was not avail- 

 able for determination of the alkalies which, judg- 

 ing from the summation of the analysis, must be 

 small in amount. The analyzed material had the 

 following optical properties: Biaxial, negative, 2V 

 medium, a =1.490, /3 =1.500, 7=1-502, 7-a=.012. 

 There are two perfect cleavages parallel to the two 

 elongated pinacoids. The best of these adopted as 

 h (010), gives maximum birefringence and is par- 

 allel to the optic plane, giving extinction varying 

 from 0° to 5°. The other cleavage is perpendicu- 

 lar to the obtuse bisectrix which makes the orienta- 

 tion, like that of the smaller crystals from another 

 s ecimen described above, XAc=0°-5°,Y = 6,Z = a. 



Stilbite has been considered monoclinic and is so 

 gi en by Dana, although crystals with the symmetry 

 of that system have apparently never been found. 

 T e assignment of the mineral to the monoclinic sys- 

 tem of crystalhzation depends upon the optical struc- 

 ture of crystals which, when lying on the h (010) face, 

 show optical anomalies with two twinning planes 

 intersecting each other at right angles and divid- 

 ing the crystal into 4 identical quarters. 



Goldschmidt has disregarded the optical structure and classes 

 stilbite ( = desmin, Germ.) as orthorhombic. 



Epidesmine has been described as an orthorhombic mineral having 

 crystals bounded by three pinacoids at right angles to each other. 

 While the formula given for epidesmine is slightly different than that 

 here adopted for stilbite, the analysis, quoted above, is well within 

 the limits of variation of stilbite analyses. The only distinction 

 between stilbite and epidesmine would then appear to be that the 

 crystals of the latter are presumably homogeneous with parallel ex- 

 tinction on (010) while the crystals of stilbite are twinned vnih a 



Fig. 25.— Stilbite of 

 the usual habit 

 showing pyrami- 

 dal faces. 



