Geology presents a well-defined family of minerals in the case of crystalline magnesium silicates. However, in industrial water processes deposits that contain both "Mg" and "Si" are usually referred to as "magnesium silicate" scales. In this paper we attempt to delineate the true nature of such Mg- and Si-containing precipitates formed under the severe stresses simulating geothermal-like conditions. The purpose of this work is to deliberately form precipitates in the presence of Mg2+ ions and soluble silica (silicic acid) and study these with a variety of analytical techniques, to reveal morphology, texture, structure and composition. Such precipitates are better described as "magnesium-containing amorphous silica".