196 Subsurface Geologic Methods 



Lemberg Method ^^ — Tlie solution to be used is prepared by boiling 

 for 20 to 30 minutes a mixture of 4 grams of AICI2, 6 grams of logwood, 

 and 60 grams of water; the mixture is filtered and the filtrate diluted 

 with 1,000—1,200 CO. of water. Calcite when immersed in the solution 

 is stained light purple after five to ten minutes of boiling; dolomite 

 remains unchanged. This reaction causes a film of aluminum hydroxide 

 on the calcite grains and this film absorbs the logwood dye. 



Potassium Ferricyanide Method — The potassium ferricyanide method 

 was developed by Heeger.^*^ It consists of immersing the rock sample first 

 in a dilute solution of hydrochloric acid (1:100) containing a few drops 

 of postassium ferricyanide. If the dolomite contains ferrous iron, the min- 

 eral assumes a deep-blue color; calcite is not affected. This test is con- 

 sidered satisfactory only if the dolomite contains ferrous iron; otherwise, 

 it fails to produce results. 



Identification of Feldspars 



Twenhofel and Tyler ^^ summarized the method of distinguishing 

 quartz from feldspar as follows: 



A few drops of hydrofluoric acid are placed on a thin section, or on grains 

 mounted in Canada balsam with their upper surfaces exposed, and allowed 

 to remain one or two minutes before being gently washed off. The acid produces 

 a thin, gelatinous film of aluminum fluorosilicate on the feldspar and other 

 aluminous minerals but leaves the quartz clear. After washing, the specimen 

 is immersed in a water-soluble organic dye for about five minutes and then 

 again washed. Fuchsine, methylene blue, safranine, or malachite green may 

 be used as a stain. . . . The depth of color retained on staining is greatest 

 with anorthite; becomes successively lighter with less calcic feldspars; and 

 is lightest with orthoclase or microcline. 



The degree of staining is improved if the grains are exposed to the 

 fumes of hydrofluoric acid for two or three minutes. Care should be exer- 

 cized in washing after staining, as the stain is easily removed from the 

 corroded feldspar grains. 



According to Gabriel and Cox,^^ the potash feldspars may be iden- 

 tified by exposing the rock to hydrofluoric-acid fumes and then staining 

 it with a diluted solution of sodium cobalt nitrite, which is prepared by 

 adding 15 cc. of glacial acetic acid and 25 cc. of water to 12.5 grams of 

 Co(N03)2.6H20 and 20 grams of NaNOs. The potash feldspars assume 

 a strong yellow color from the formation of potassium cobalt nitrite. 

 Quartz and plagioclase grains are not affected. 



Potash feldspars (orthoclase, microcline) may be differentiated from 

 the calcic plagioclase feldspars (laboradorite, bytownite, and anorthite) 



*° Lemberg, J., Zut microchemischen Untersuchung von Calcit, Dolomit, un Predazzpit: Zeitschr. CeoL 

 Gesell., Band 39, pp. 489-492, 1887. 



** Heeger, J. E., Ueber die Mikrochemische Untersuchung jein verteiler Carbonate im Gesteinssckliff : 

 Centralbl. Mineralogie 1913, pp. 44-51, 1913. 



S' Twenhofel, W. H., and Tyler, S. A., op. cit., p. 131. 



*^ Gabriel, A., and Cox, E. P., A Staining Method for the Quantitative Determination of Certain Rock 

 Minerals: Am. Mineralogist, vol. 14, pp. 290-292, 1929. 



