THE OTHER STUFF: NONPRINCIPAL MINERAL COMPONENTS IN ORGANIC-MATTER-RICH MUDROCKS (Invited Presentation)

Organic-rich mudrocks are commonly described, or even named, based on three principal mineral components: quartz, carbonates, and clay minerals. However, additional components are common in mudrocks and play important roles in rock fabric, pore systems, and diagenetic reactions. The most prominent of these nonprincipal components are feldspars, Ca-phosphate, and pyrite. All three of these can be present in significant amounts in organic-rich mudrocks.

Detrital feldspars in particular can be abundant enough to produce nearly arkosic mudrocks (if the clay and carbonate components are not considered). The reactivity of feldspars as grains or in rock fragments contributes to the diagenetic complexity in mudrocks. Original detrital feldspars are generally converted to albite with increasing thermal maturity. Dissolution of feldspar is a common source of intraparticle pores that range into the size of micrometers. Partial albitization of carbonate grains is another common diagenetic reaction involving feldspar.

Ca-phosphate (apatite or collophane) can be found in mudrocks in a variety of forms including detrital grains, fossils or fossil fragments, and early diagenetic cements. Phosphatic layers or lags are common in mudrocks. Intraparticle porosity is common in collophane grains and some phosphatic fossils. Replacement of carbonate by apatite adjacent to detrital apatite has been observed rarely.

Pyrite in particular can take on many forms in organic-rich-mudrocks. Although pyrite is primarily a diagenetic product in organic-rich mudrocks, pyrite framboids can form in the water column under euxinic conditions. It can also be reworked by bottom currents and behave similar to a detrital particle. Pyrite framboids are notable for both preserving intraparticle pores within the framboid and also filling larger intraparticle pores by growing in microfossil pore spaces. Pyrite has also been observed in mudrocks as a microfracture-fill and as a replacement mineral of originally opaline fossils.

Feldspars, Ca-phosphate and pyrite do play important roles in pore systems, rock fabric, and diagenetic reactions in organic-rich mudrocks, even though they are generally not principal mineral components.