(A) Silica content control of a geothermal stream, from which energy is extracted while contorlling scale pptn., is carired out by (a) measuring the concn. of all solutes (including silica) present in individual concns. of more than 10 wt. ppm.; (b) calculating the max. silica solubility (s) in the soln. of a particular temp., from the Setchnow equation (I)    log s = log so - 8uMi (DiMi)...(I)    So = silica solubility in distilled water at the particular temp. Di = proportionality constant at a given temp. for each other solute in soln. Mi = molar concn. of respective solute, to obtain the max. non-precipitating silica content in the stream; and (c) adjusting a physical parameter, selected from temp., pressure, recycle seed rate and flow rate, to increase the silica concn. without exceeding the max. silica concn., (B) also claimed are (i) appts. for automatically adjusting physical parameters of an aq. soln. contg. silica and iron, (ii) a method of adjusting physical parameters of an aq. soln. contg. less than a satd. concn. of amorphous silica, (iii) a method of adjusting physical parameters of a flowing stream of a geothermal brine soln. processing system contg. less than a satd. concn. of amorphous silica; (iv) a method of controlling pptn. of a dissolved silicon-contg. constituent from an aq. soln. stram also contg. iron and manganese constituents; and (v) a method of controlling pptn. of a dissolved silicon-contg. constituent from an aq. soln. stream at an initial process condition in a process in which the constituent pptes. in later process steps under different process conditions, the soln. also contg. a dissolved iron-contg. constituent.    USE/ADVANTAGE -   The process is used esp. for controlling silica(-contg.) scale deposition in geothermal powr genrating systems. It allows prediction of silica pptn. from solns. contg. iron and other solutes so that system parameters can be adjusted to minimise scaling, thus allowing use of the Setchnow equation for continuous automatic system control.