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:
The alteration pathway commonly begins with the weathering of K-feldspar to muscovite (sericite) under conditions where potassium activity remains relatively high:
KAlSi₃O₈ + H⁺ + H₂O → KAl₃Si₃O₁₀(OH)₂ + dissolved silica + K⁺
With continued hydrolysis and leaching, feldspar alters to kaolinite under moderate silica activity and acidic conditions:
2KAlSi₃O₈ + 2H⁺ + 9H₂O → Al₂Si₂O₅(OH)₄ + 2K⁺ + 4H₄SiO₄
Under even more intense leaching, where potassium and silica are progressively removed from the system, kaolinite or feldspar may further alter to pyrophyllite in silica-rich but potassium-poor environments:
Al₂Si₂O₅(OH)₄ + 2SiO₂ → Al₂Si₄O₁₀(OH)₂ + H₂O
Finally, prolonged weathering and extreme silica depletion favor the formation of gibbsite, a highly aluminum-rich hydroxide mineral characteristic of advanced tropical or subtropical weathering profiles:
Al₂Si₂O₅(OH)₄ + 5H₂O → 2Al(OH)₃ + 2H₄SiO₄
The direct breakdown of feldspar to gibbsite may also be simplified as:
2KAlSi₃O₈ + 2H⁺ + 9H₂O → 2Al(OH)₃ + 2K⁺ + 6H₄SiO₄
These reactions illustrate the progressive loss of K⁺ and dissolved silica during weathering. As feldspar becomes unstable, secondary minerals evolve from K-bearing phases such as muscovite toward clay minerals such as kaolinite and eventually to aluminum hydroxides such as gibbsite.
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 research projects conducted in my Geochemistry of Natural Waters course focused on the sericite (white mica) present within the Redwood Falls saprolite. These studies examined the mineralogy, chemistry, and isotopic composition of the sericite in order to better understand the weathering history of the Morton Gneiss and the environmental conditions responsible for saprolite formation. Like to the papers can be found
One student investigation used X-ray diffraction (XRD), scanning electron microscopy (SEM), electron microprobe analysis, Fourier-transform infrared spectroscopy (FTIR), and wavelength-dispersive spectroscopy (WDS) to characterize the white mica. The study demonstrated that the sericite is unusually depleted in potassium compared to typical muscovite or sericite compositions. Potassium concentrations measured by electron microprobe commonly ranged from only 0.04–0.11 wt.% K₂O, far below normal muscovite values of approximately 8–11 wt.% K₂O. This extreme potassium depletion suggests that the sericite experienced prolonged or repeated weathering under conditions that strongly leached alkali elements from the weathering profile.
SEM imaging of the sericite grains revealed delicate mica-sheet textures and layered crystal structures preserved despite extensive chemical alteration. XRD analyses confirmed the abundance of kaolinite within the saprolite, while FTIR spectra suggested that portions of the sericite structure may have been partially altered toward kaolinite-like compositions. These results support the interpretation that the Morton saprolite underwent progressive chemical weathering in which feldspars first altered to sericite and later evolved toward kaolinite and gibbsite under increasingly intense leaching conditions.
Another student project investigated the oxygen isotope composition of the sericite grains in order to reconstruct the paleoenvironmental conditions associated with saprolitization. The sericite was carefully separated from the saprolite using grain-size separation, magnetic separation, heavy liquids, and hand-picking techniques prior to isotopic analysis by laser fluorination mass spectrometry. Measured δ¹⁸O values ranged from approximately 40‰ to 50‰ relative to SMOW. Although these unusually high values were interpreted cautiously because of possible analytical complications during fluorination, the work demonstrated the potential for secondary weathering minerals in the Morton saprolite to preserve isotopic evidence of ancient meteoric water interaction and paleoclimatic conditions.
Together, these student projects transformed BH250-100 into an important teaching and research specimen within the BH250 Collection. The saprolite not only illustrates large-scale chemical weathering of Archean crystalline rocks, but also serves as an exceptional example of how mineralogical, geochemical, and isotopic methods can be integrated to investigate ancient weathering processes, elemental mobility, and paleoenvironments. The studies also provided students with hands-on experience in modern analytical techniques including SEM, XRD, FTIR, electron microprobe analysis, and stable isotope geochemistry. Copies of the student papers, abstracts, posters, and related research materials can be found linked near the bottom of this page.