10 PROCEEDINGS OF THE NATIONAl, MUSEUM vol.73 



features would seem to indicate a biaxial positive mineral with 2Y 

 small and, assuming the direction of elongation to be parallel to 

 the c axis. c = Y. Except in that the index of refraction is approxi- 

 mately 0.01 higher, the optical properties of the mineral agree with 

 those given for mesolite in Larsen's tables.^ 



At a sharp line about 3 millimeters from the base of the zeolite 

 layer it becomes more glassy, coarser, and quickly diverges into free 

 fibers. Under the microscope these fibers differ from the more com- 

 pact material in having lower birefringence and slightly lower in- 

 dex of refrection, /3 being about 1.512 ±0.002, with the birefringence 

 approaching 0°. The elongation of these free fibers is Z, and they 

 are positive in all positions — an easy method of distinguishing 

 them from mesolite. The optical properties of the mineral agree 

 with those of okenite but, as shown by analysis of a sample from 

 another specimen having identical optical properties, the composi- 

 tion is identical with that of mesolite. This mineral is described 

 below as pseudomesolite. Each of the free needles of this specimen 

 has a narrow border of a distinct, undetermined mineral of high 

 birefringence, higher indices of refraction, and positive elongation. 



The calcite breaks cleanly away from the other minerals, leaving 

 rhon:!bic molds in the mesolite crust. As it was impossible to separate 

 the two fibrous zeolites by hand picking, they w^ere crushed together, 

 screened between 40 and 100 mesh sieves, and run several times 

 through bromoform-methylene iodide heavy solution. The sample 

 thus prepared consisted almost entirely of the pure material of Y 

 elongation, here called mesolite, with only a very little of the material 

 of Z elongation, here referred to as pseudomesolite. This sample was 

 analyzed with the following results: 



Analysis and ratios of fibrous nu^solite 



Constituent : Per cent Ratios 



SiOo 42.02 0.697 0.697 0.92X3 



AloO, 28.94 .283 .283 1.12X1 



CaO' 10.46 .1871 



MgO .06 .0011 <^7Q J Q7^j 



Na^O 3.24 .0521 ' -'" i- U/Xi 



KnO 1.92 .020j 



H2O above 125° C 13.20 .7321 -«« in9v? 



IT2O below 125° C .64 .036/ ' '^^ 1. uzxd 



100. 48 

 These results give the mesolite formula commonly adopted with 



(Ca,Na3)O.A1.203.3Si02.3H20. 

 The ratio between Ca and Na2 is nearly 2:1, so that the formula 

 may be expanded to 



2CaO.Na2O.3Al2O3.9SiO2.9H2O. 



* Esper S. I.arscn. Optical determination of the nonopaque minersls : U. S. Geol. Survey 

 Bull. 679, p. 208, 1921. 



