What is "authigenic"

a. (context geology English) Generated where it is found or observed.

An authigenic mineral or sedimentary rock deposit is one that was generated where it is found or observed. Authigenic sedimentary minerals form during sedimentation by precipitation or recrystallization instead of being transported from elsewhere (allogenic) by water or wind. Authigenic sediments are the main constituents of deep sea sedimentation. Authigenic clays tend to reduce the porosity of sediments, thus reducing permeability.

Common sedimentary authigenic minerals include calcium carbonate, apatite, and clays.

In metamorphic petrology an authigenic mineral is one formed in situ during metamorphism, again by precipitation from fluids or recrystalisation.

For any mineral to be precipitated, the water must be oversaturated with respect to that mineral. For calcite, this means that the area of deposition must be above the carbonate compensation depth, or that the pore waters are sufficiently saturated due to dissolution of other grains that precipitation can begin. The alkalinity can also be reduced by microbial sulphate reduction.