1917] Eaklc: Minerals Associated with Crystalline Limestone 



351 



one is the pyramid type with a very small or no base. Only the three 

 forms (001), (100), and (111) occur on the crystals. Some of the 

 crystals have been corroded by solutions and altered into white fibrous 

 okenite. 



Okenite. — Radiating botryoidal coatings of okenite occur on the 

 apophyllite evidently as an alteration product. The coatings consist 

 of tufts of fibers and long slender needles. The acicular crystals are 

 too slender for accurate measurements but they appear to be long 

 prisms terminated on their ends by dull domes. Some approximate 

 measurements and an analysis of the mineral were made by William 

 Foshag. 



Measured 

 (110):(1T0) 33°40' 



(010) :(011) 52 00 



(011) : (Oil) 75 00 



The analysis gave : 



Calculated for okenite 

 H 2 Ca(Si0 3 ) 2 .H 2 



Si0 2 56.17% 56.60% 



CaO 26.10 26.42 



H 2 16.83 16.98 



99.10 100.00 



The crystals have parallel extinction and are probably orthorhombic. 

 H = 4-4.5. G = 2.206. Index of refraction = 1.55. Low birefring- 

 ence, 0.008-0.009. Solution in HC1 yielding flaky silica. Fuses easily 

 and gives water in a closed tube. It has formed from apophyllite 

 through the action of carbonated waters, perhaps according to the 

 equation : 



2H,KCa 4 (Si0 3 ) 8 + H 2 C0 3 = 8H 2 Ca(Si0 3 ), + K 2 C0 3 . 



Prehnite. — Two distinct varieties occur filling cavities in the white 

 feldspar of the pegmatite. The first is the common green drusy 

 prehnite. This is intimately associated with laumontite, which may be 

 secondary from it. The second variety is unusual for prehnite. It is 

 light brown to colorless and lacks the drusy surface. In some of the 

 cavities crystals line the walls, but they are so interlocked that single 

 crystals are not separable. One crystal about five millimeters broad 

 and one millimeter thick showed the forms c (001), m (110), a (100), 

 and o (061). The base is large and bright but the edge-faces are dull. 

 Most of this brown variety is compact granular with bright basal 

 cleavage planes prominent. It is associated with massive datolite and 

 gray quartz. 



