Continuous reactions are derived for the quaternary multisystem MgO - FeO - SiO2 - H2O using the phases serpentine, talc, brucite, olivine, orthopyroxene and H2O. Stoichiometric coefficients are expressed in terms of mole fractions and, if set equal to zero, define the position where a mineral changes from being produced to being consumed. ''Zero contours'' may be displayed graphically in reaction space, defined by the coordinate system with axes given by mole fractions. Zero contours in Mg-Fe systems have characteristic features. They have a linear geometry, i.e., they are lines in two-dimensional space, planes in three-dimensional space. In the absence of pure end-member phases, all zero contours intersect the two corners (0,0,....) [= Fe-corner] and (1,1,....) [= Mg-corner] of reaction (hyper-)space. The line connecting the two corners is the location where all zero contours intersect, and where no reaction is possible. In the presence of a pure phase, the zero contour of the pure phase intersects that corner of reaction space with which it is not compatible. No other zero contour may intersect that corner. The disappearance of a phase at the corner parallels the disappearance of a component, leaving the system univariant. A reaction is still possible. In the presence of two serpentine minerals, two types of zero contours are distinguished. Where the composition of the two serpentine minerals coincides, all phases except serpentine disappear. The zero contours of the two serpentine minerals are complementary to each other. Each of the two depends on the composition of coexisting non-serpentine minerals as well as on the composition of the other serpentine. The technique is applied to serpentinization in Bushveld layered rocks in South Africa.