WEATHEEING OF PYEOXENITES 



211 



Weathering of Soapstone. — That pyroxenic rocks often un- 

 dergo alteration into taleose aggregates to whieli the name soap- 

 stone is applied, has been noted on p. 95. Such rocks on weath- 

 ering give rise to smooth-feeling, almost soap-like residues, which 

 like those derived from the peridotites are almost completely lack- 

 ing in lime and other alkalies. Two examples are given below: 



Analyses of Peesh and Decomposed Soapstone, Albemarle County, 



yirginia 



CoNSTmraNTs 



Silica (Si02) 



Alumina (AljOa) . . . 

 Iron sesquioxide (FeaOs) ^ 



Lime (CaO) 



Magnesia (MgO) . . . 



arOuaSJl { JCvgw ) » • • • 



Soda (NaaO) 



Ignition .-•... 



t4 



o 

 o 



n 



00 



38.85 % 



12.77 



12,86 



6.12 

 22.68 



0.19 



0.11 



6.52 



100.00 % 



II 



o 



g 



M 



38.82% 



22.61 



13.33 



6.13 



9.52 



0.18 



0.20 



100.00 % 



III 





O 



M 



O 



W Q O 



f^ r~l f"^ 





16.92 % 



0.00 



6.33 



2.66 

 17.20 



9.03 



0.00 



1.32 



62.46 % 



lY 



Em 

 O 





66.42% 

 100.00 



68.52 



65.55 



23.81 



52.94 

 100.00 



79.74 



rt CO 



^ 5:; H 



i ?? ^ 



Ee) < g 



43.68% 



0.00 

 41.48 

 44.45 

 76.19 

 47.05 



0.00 

 20.26 



The fresh rock is of a blue-gray color, close textnre, and 

 consists, as shown by the microscope, of elongated crystals of 

 colorless tremolite, with folia of talc and chlorite, and occasional 

 opaque granules of chromic iron. The general petrologic feat- 

 ures are those of an altered pyroxenite. The residual soil is of 

 a dull, ochreous, brown-red color, somewhat lumpy, but with 

 no appreciable grit when rubbed between the thumb and fingers. 



Recalculated as before, the analyses give the results shown in 

 columns III, IV, and V. 



The total loss of material amounts to 52.46%, including water 

 of hydration. The most striking feature brought out is the fact 

 that the magnesia has been carried away in greater proportional 

 quantity than has the lime. A like result was noted by Ebelman 



All iron calculated as FeaOs. 



