BH250-100
Title
Subject
Description
Minor Minerals: iron oxides
BH250-100 is a sample collected from the saprolite that formed from the chemical weathering of the Morton Gneiss near Redwood Falls, Minnesota. The Morton Gneiss is one of the oldest exposed rocks in North America. The saprolite represents a deeply weathered, in place alteration of this gneiss, and is primarily found in southwestern Minnesota. Despite its extensive chemical breakdown, the saprolite retains the foliation and mineral grain outlines of the parent gneiss as well as in the thin sections, preserving its original texture even as the rock has become soft, crumbly, and friable.
The Morton saprolite typically appears reddish, orange, or buff in color due to the oxidation of iron-bearing minerals such as biotite. In both outcrop and hand sample, BH250-100 shows this distinctive weathered texture. Thin section analyses confirm that the primary mineral assemblage includes quartz, microcline, plagioclase feldspar, biotite, and garnet—typical of the Morton Gneiss. As weathering progresses, feldspar transforms into secondary minerals such as kaolinite and gibbsite, while biotite alters into iron oxides like hematite and goethite. Quartz, being more resistant, remains largely unaltered.
Saprolitization of the Morton Gneiss likely occurred during the Late Cretaceous to early Paleogene, approximately 80 to 50 million years ago. In my course on the Geochemistry or Natural waters study, we attempted to use geochronology to more precisely date the weathering event by separating sericite/white mica from the sample and sending it for ^40Ar/^39Ar analysis. Although the argon retention in the sericite was insufficient for robust dating, preliminary results suggest a possible age of around 60 million years, consistent with other records of deep weathering during that time.
The geochemical weathering process responsible for forming the saprolite from feldspar-bearing rock involves a well understood sequence of hydrolysis and leaching under warm, humid, and acidic conditions. The transformation of feldspar to gibbsite follows a multi-step pathway. Initially, hydrolysis of K-feldspar (orthoclase) produces potassium ions, dissolved silica, and secondary aluminum hydroxides:
2KAlSi3O8 + 2H++ 9H2O → 2K+ + 2Al(OH) + 6H4SiO4
Typically, feldspar first weathers to kaolinite under moderate leaching conditions:
2KAlSi3O8 + 2H+ + 9H2O → Al2Si2O5(OH)4 + 2K+ + 4H4SiO4
Then, with continued silica leaching and more intense chemical weathering, kaolinite is further transformed into gibbsite:
Al2Si2O5(OH)4 + 5H2O → 2Al(OH)3 + 2H4SiO4
To better understand the conditions that allow this transformation, it's important to examine the relationship between pH and the log activity of potassium ions (log a_K⁺) in natural waters. This relationship is commonly visualized in pH – log(a_K⁺) stability diagrams. These diagrams illustrate mineral stability fields as functions of acidity (pH) and K⁺ concentration in solution. They help track the evolution of weathering processes by defining the stability domains of feldspar, kaolinite, and gibbsite. This is better explained in several textbooks especially the one by Drever "The Geochemistry of Natural Waters".
In these diagrams, the x-axis represents pH, and the y-axis represents log(a_K⁺). At higher pH and K⁺ activity, K-feldspar is stable. As K⁺ is leached and pH decreases, kaolinite becomes stable. Under intense leaching and low K⁺ activity and acidic conditions (pH < 5), gibbsite becomes the dominant stable phase.
The weathering pathway observed in the Morton saprolite thus follows a trajectory from high pH and high K⁺ activity—where feldspar is stable—toward lower pH and depleted K⁺, stabilizing kaolinite and eventually gibbsite. This path reflects a progressive loss of K⁺ to groundwater, increasing silica leaching, and acidification by carbonic acid (from CO₂ in rainwater) or organic acids in the soil. The saprolite’s mineralogical transformation and geochemical context make it an excellent natural example of deep weathering in a continental craton environment.
Chemical analyses of the un weathered Morton Gneiss (BH250-118) and the saprolite (BH250-100) reveal distinct geochemical signatures. BH250-118, representing the fresh rock, is enriched in silica, potassium (K), and sodium (Na), consistent with its original mineralogy dominated by feldspar and quartz. In contrast, the saprolite exhibits significantly lower concentrations of these elements, reflecting extensive leaching during chemical weathering. Elements such as K and Na are especially mobile in acidic, water-rich environments and are progressively removed from the weathering profile. On the other hand, relatively immobile elements particularly rare earth elements (REEs) are noticeably concentrated in the saprolite compared to the fresh Morton Gneiss. This enrichment results from the preferential removal of mobile elements, leaving behind a residual concentration of immobile components. See table and figures for comparative data.
Several student projects have explored this subject; please refer to the papers, abstracts, and posters available on the Geochemistry of Natural Waters website.