266 Eakle and Rogers — Wilkeite, a New Mineral. 



NaF, northupite, MgCO s .Ka a CO i .NaCl, tychite, 2MgC0 3 . 

 2Na 3 C0 3 .Na 2 SO< and tliaumasite, CaSi0 3 .CaC0 3 .CaS0 4 .15II 3 0, 

 but as far as we know wilkeite is the only mineral with four 

 acid radicals. 



Summary of the properties of wilkeite. — Hexagonal, c = 

 0-730 approximately. Forms (1010), (1120), (1011), cleavage 

 imperfect basal. H = 5. Gr = 3 - 234. Color pale rose-red. 

 Uniaxial, optically negative, n = l - 640 ± "005. (w, — n) = 

 0*001. Fusibility = 5£. Soluble in dilute hydrochloric and 

 nitric acids with the separation of flocculent silica. Gives wet 

 tests for sulphate and phosphate radicals and for calcium. 



Chemical formula : 3Ca 3 (P0 4 ) 2 .CaC0 3 + 3Ca 3 [(SiO' 4 )(S0 4 )]. 

 CaO. 



Occurrence. — In a crystalline limestone contact zone, associ- 

 ated with diopside and vesuvianite. 



Okenite. 



Properties. — The wilkeite is often altered to a white opaque 

 mineral with a pearly luster and fibrous structure. This 

 mineral was identified as okenite by its optical properties. 

 The index of refraction determined by the Becke test with 

 reference to liquids of known index of refraction is 1-565 ± 

 •005. The double refraction determined by the maximum 

 interference color in a thin section of known thickness is "009 

 or *010. Only a very few minerals have this combination of 

 optical characters* and of these okenite is the only one that 

 agrees at all in chemical properties. The okenite is easily 

 fusible to a glass and gives abundant water in the closed tube. 

 In thin sections the mineral is translucent and shows a fibrous 

 aggregate structure with parallel extinction and elongation 

 parallel to the slower ray. The relief is low compared with 

 wilkeite. All stages in the alteration of the wilkeite to 

 okenite can be observed, from wilkeite with a narrow border 

 of okenite to specimens of okenite with minute remnants of 

 wilkeite. Several distinct pseudomorphs of okenite after 

 wilkeite were noted. 



The accompanying figure is a camera lucida sketch of the 

 wilkeite in process of alteration to okenite. The okenite is 

 the fibrous mineral with low relief. The two parts of the 

 wilkeite are in optical continuity. The penetration of the 

 wilkeite by okenite stringers and the ragged edges of the wilke- 

 ite makes it certain that the okenite is an alteration 

 product of the wilkeite. The other minerals associated with 

 the wilkeite show no evidence of alteration. Wollastonite 



* See Table of Refringence and Birefringence in the back of WinchelFs 

 Elements of Mineralogy. 



