**4. рН of feldspar suspension**

pH is a parameter showing the acidity of feldspathic rocks and the engineering properties of clay slurry (viscosity, the density of clay slurry, tixotropy for molding ceramic products. As рН rises, the tixotropy of the clay slurry increases. Interacting with water, feldspathic minerals change the acidity of ceramic shlicker, depending on their constituent cations and anions. The optimum viscosity of clay slurry for refractory products is achieved at рН 5.5 and that for porcelain at рН 7.5–9. According to GOST 21119.3-91 for feldspathic fillers in varnish and paint production, рН is 6-9.

The рН of suspensions was measured using the potentiometer method on an I-120.1-type ionometer. Suspensions were prepared from из finely dispersed samples (grain size 0.063 mm) at solid:fluid ratio of 1:2.5 in accordance with the procedure described in [6,7]. The results of measurements of the рН of feldspar rock suspensions are presented in Table 1, and Figure 4 shows the dependence of рН on total microcline and plagioclase concentration in the rock.

Our results have shown that a common pattern of variations in the рН of feldspathic rock suspensions is a rise in рН with increasing alkaline oxide concentration.

Fig. 4. Dependence of variations in pH on microcline and plagioclase.

Table 1 shows that suspensions of Khetolambino and Lupikko microcline pegmatites have the highest рН values: 9.67 and 9.82, respectively. Potassic halleflinta, a nonconventional type of feldspathic raw material, has a high рН (10.0). Comparison of the рН of quartzfeldspathic rock suspensions has shown that large quantities of quartz (рН of quartz is 6.4- 6.9) decrease their рН considerably, as in Roza-Lambi volcanics (рН 7.1).

Figure 4 shows that rocks that contain a total microcline and plagioclase concentration of 75.8–78.0 mass. % and quartz concentration of 22.0–24.2 mass. % have the highest pH.

Thus, the рН of feldspar suspensions depends on total microcline and plagioclase concentrations and quartz concentration in the rock. The pH of Elisenvaara syenite is 8.25 is consistent with feldspathic raw material according to GOST 21119.3-91 (рН 6-9) and Kemio feldspar from Finland (рН 8.3).
