12 



PROCEEEHNGS OP THE NATIONAL MUSEUM 



VOL. 66 



calcite mixed with some fibrous silicate. After treatment with cold 

 dilute acid there remains a residue of fibrous material of very low 

 birefringence with a refractive index below 1.50, This may be 

 silica from the decomposition of thin wollastonite fibers. The inter- 

 stices between the xonotlite areas are filled with pearly granular 

 wollastonite. 



This xonotlite, like those previously described from other locali- 

 ties, is densely fibrous and very tough. When freshly broken the 

 mineral is distinctly pink in color and somewhat translucent but 

 upon exposure to air the pink color gradually fades and the mineral 

 becomes more opaque at the surface with a chalky appearance. A 

 selected fragment from the center of one of the purer masses was 

 analyzed yielding the results given in the following table. The 

 sample was not of very pure material as it was shown by micro- 

 scopic examination to contain two minerals as impurity, amounting 

 to several per cent. The most abundant of these was apparently 

 diopside, the second probably thaumasite. 



Analysis and ratios of compact xonotlite from Leesburg 



Constituent 



SiOi 



(Al, Fe)j08 

 CaO 



MgO 



Per 



cent 



45.62 

 2.05 



41.28 

 2.26 



Ratios 



0;757}o. 110X7 

 . 73R\ 

 .056/ 



,113X7 



Constituent 



HjO+llO" C 

 HjO-110° C 



Total . 



Per 

 cent 



6.00 

 1.00 



Ratios 



0.333 0.111X3 



8.21 



The ratios give the formula TCaSiOs.SHgO as compared with the 

 4CaSi03.H20 or SCaSiOs.HgO of previous analyses. This may be 

 due to water absorbed in the fine fibrous mass. The material used 

 for analysis was too impure to do more than establish the identity 

 of the species. 



Under the microscope the mineral is fimely fibrous with parallel 

 extinction and positive elongation. The refractive indices are some- 

 what variable, the average being, «= 1.580 y= 1.592. 



The xonotlite-bearing mass found loose and its original position 

 in the quarry is not known. It was near the eastern wall and may 

 have come from the vicinity of the basaltic dikes. Although spe- 

 cially sought, none of the mineral could be found on several sub- 

 sequent visits. 



When the field trip party of the Mineralogical Society of Amer- 

 ica visited this locality following the Christmas, 1923, meeting, a 

 very different type of xonotlite was found, in thin seams in relatively 

 unaltered limestone in the north end of the quarry. This formed 

 veinlets up to 5 mm. wide filled with flaky fibrous xonotlite with 

 pearly luster and pale pinkish color which greatly resembles the 



