On the Experience of Determining Albite Content in Potassium Feldspars by the Staining Method

G. M. Ivanova

(Presented by the scientific seminar of the Department of Petrography)

In order to study the regularities in the development of albitization processes in syenite massifs of the Kondoma district (Gornaya Shoriya), we carried out mass determinations of the presence of albite in potassium feldspars. The determination was performed on rock samples according to the method proposed by P. I. Tyutin (1963). In all, about 100 powdered samples of albitized syenites from 7 boreholes of the Tashtagol massif were subjected to staining. This method consists of the following.

First, 6 standards were prepared, each weighing 0.5 g. For this purpose powders were obtained from monomineralic fractions of albite and K-feldspar. Albite and K-feldspar were mixed in the following ratios: standard No. 1—albite 100%; standard No. 2—albite 80%, K-feldspar 20%; standard No. 3—albite 60%, K-feldspar 40%; standard No. 4—albite 40%, K-feldspar 60%; standard No. 5—albite 20%, K-feldspar 80%; standard No. 6—no albite, K-feldspar 100%. The standard powders were then stained. The staining process was carried out as follows. The weighed powders were transferred into porcelain cups with a capacity of 50–100 cm³. The time was then noted, and 6 cm³ of 5% hydrofluoric acid was added to each cup. Etching continued for 10–15 min, during which the mixture was stirred several times with a glass rod. After 10–15 min had elapsed, the acid was poured off from the cups and the powders were washed with 5 cm³ of water. Thereafter the powders were again covered with 5 cm³ of water, and with a pipette 5 cm³ of a 0.025% methylene blue solution was added to each cup. To achieve the best staining, the mixture was stirred several times. After 4–5 min the powder became stained and settled to the bottom of the cup. After this the dye was poured off, and the powders were washed 2–3 times with water. The powder was then dried on a sand bath, after which the dried lumps of stained powder were lightly ground with a rubber pestle and poured into envelopes.

As a result of staining, albite acquired a blue color, and potassium feldspar a pale bluish-green color. In a mixture they gave mixed bluish-blue tones with a greenish tint, of varying degree and intensity of coloration.

For testing, syenite samples were selected from 7 boreholes, uniformly over the entire depth of the massif exposed by drilling. At the same time, samples were selected that, as far as possible, had not been affected by sericitization, chloritization, and other secondary processes. The selection of samples was carried out by examining them under a microscope. This was done so that such secondary minerals as sericite and chlorite would not affect the staining.

powders. Quartz and carbonates remain colorless during staining, so they have no effect on the intensity of the color. In all, 96 samples of syenites were taken. All the specimens tested were ground to a fine powder; then a 0.5 g weighed portion was taken from each sample and stained by the method described above.

After staining, the standard powders were examined on an FM-56 photometer, where first the working filters were selected by comparing the light reflectance of a baryte plate with the light reflectance of the test samples. Filters No. 9 and No. 10 were selected as the working filters. Then the coefficient of light reflectance of each standard was determined. It was calculated by the formula \(R = rR_{\text{pl b}}\), where \(R\) is the reflection coefficient of the specimen tested; \(R_{\text{pl b}}\) is the reflection coefficient of the baryte plate, equal to 84%; and \(r\) is the ratio of the brightness coefficient of the specimen to the brightness coefficient of the baryte plate. The value \(r\) is taken as the reading on the photometer scale. As a result of the calculations, the following reflection coefficients were obtained for the standards: standard No. 1 — 48.72; standard No. 2 — 38.64; standard No. 3 — 34.4; standard No. 4 — 32.76; standard No. 5 — 30.24; standard No. 6 — 27.72. Then, by selecting standards, the light reflectance of the test powder sample was compared with the light reflectance of the standard powder. In cases where their light reflectances were close, the photometer scale showed readings of the same magnitude for both samples, and the light-reflectance coefficients calculated by the formula \(R = rR_{\text{pl b}}\) were very close. The albite and kaliophilite contents in the test samples were taken to be the same as those of the standard whose light reflectance was close to that of the given sample.

Chemical analyses of 10 monomineralic fractions of feldspars confirmed the staining results: they gave approximately the same ratios of albite and kaliophilite content in the sample. Thus, for example, in sample No. 1211 (borehole No. 176), staining established an albite content of 80% and a kaliophilite content of 20%. Chemical analysis showed the following results: \(K_2O = 1.70\%\); \(Na_2O = 7.58\%\); \(Al_2O_3 = 15.61\%\); \(SiO_2 = 66.48\%\). Recalculation to molecular quantities and to mineral formulas yielded the following feldspar composition: the orthoclase molecule constitutes 14%, and the albite molecule 86%. Thus, the contents are fairly close.

The data we obtained as a result of staining made it possible to establish a number of regularities. The method itself is quite simple, which permits its very broad use in similar investigations.