A Study of Rock Weathering With Reference to the Soils of the Malvern Hills

Stephen, I.

doi:10.1111/j.1365-2389.1952.tb00628.x

Cited by 49 publications

(11 citation statements)

References 11 publications

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“…Based upon literature and petrographic observations, we identified three candidate families of reaction that are observed to be altering volumetrically significant minerals at the bedrock-saprolite interface in the Rio Blanco quartz diorite: plagioclase alteration to clay, hornblende alteration to Fe oxides, vermiculite and/or smectite [26,27], and biotite alteration to hematite, vermiculite, or kaolinite [18,[28][29][30][31]. Of these three candidates, only reactions incorporating alteration of Fe minerals [32] have a positive ∆V of reaction, where V is the molar volume of solid phase products or reactants:…”

Section: Spheroidal Weathering Modelmentioning

confidence: 99%

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A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher

Buss

Brantley

2006

Earth and Planetary Science Letters

206

229

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Spheroidal weathering, a common mechanism that initiates the transformation of bedrock to saprolite, creates concentric fractures demarcating relatively unaltered corestones and progressively more altered rindlets. In the spheroidally weathering Rio Blanco quartz diorite (Puerto Rico), diffusion of oxygen into corestones initiates oxidation of ferrous minerals and precipitation of ferric oxides. A positive ∆V of reaction results in the buildup of elastic strain energy in the rock. Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock. Substitution of reasonable values for the parameters involved in the model produces results consistent with the observed thickness of rindlets in the Rio Icacos bedrock (≈ 2-3 cm) and a time interval between fractures (≈ 200-300 a) based on an assumption of steady-state denudation at the measured rate of 0.01 cm/a. Averaged over times longer than this interval, the rate of advance of the bedrock-saprolite interface during spheroidal weathering (the weathering advance rate) is constant with time. Assuming that the oxygen concentration at the bedrock-saprolite interface varies with the thickness of soil/saprolite yields predictive equations for how weathering advance rate and steadystate saprolite/soil thickness depend upon atmospheric oxygen levels and upon denudation rate. The denudation and weathering advance rates at steady state are therefore related through a condition on the concentration of porewater oxygen at the base of the saprolite. In our model for spheroidal weathering of the Rio Blanco quartz diorite, fractures occur every ~ 250 years, ferric oxide is fully depleted over a four rindlet set in ~ 1000 years, and saprolitization is completed in ~ 5000 years in the zone 2 containing ~ 20 rindlets. Spheroidal weathering thus allows weathering to keep up with the high rate of denudation by enhancing access of bedrock to reactants by fracturing.Coupling of denudation and weathering advance rates can also occur for the case that weathering occurs without spheroidal fractures, but for the same kinetics and transport parameters, the maximum rate of saprolitization achieved would be far smaller than the rate of denudation for the Rio Blanco system. The spheroidal weathering model provides a quantitative picture of how physical and chemical processes can be coupled explicitly during bedrock alteration to soil to explain weathering advance rates that are constant in time.

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Section: Spheroidal Weathering Modelmentioning

confidence: 99%

“…We can solve (27) and (28) to determine how the steady state thickness of the soil, z ss , is dependent upon atmospheric O 2 , total denudation rate, and other chemical and physical parameters captured in q:…”

Section: Weathering and Denudationmentioning

confidence: 99%

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

Fletcher

Buss

Brantley

2006

Earth and Planetary Science Letters

206

229

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“…According to Huang (1977), chlorite is a typical weathering product of both minerals in soil environments. Furthermore, in an aggressive weathering regime (e.g., Adirondack Spodosols) chlorite can transform further to vermiculite, usually via an intermediate chlorite/vermiculite (Stephen, 1952;Ross and Kodoma, 1976). Likewise, feldspar weathering can lead to the formation of vermiculite through an intermediate mica phase.…”

Section: Origin Of the Vermiculitementioning

confidence: 99%

The Nature of Vermiculite in Adirondack Soils and Till

April

Hluchy

Newton

1986

Clays and clay miner.

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Abstract--The clay and bulk mineralogy of soil and till from 26 Adirondack watersheds was studied. The materials consist typically of quartz, K-feldspar, plagioclase, mica, vermiculite, and kaolinite. Talc, smectite, halloysite, and hornblende are present in some samples. The clay fraction of the soils is composed predominantly of vermiculite, likely derived from the transformation of a mica precursor, and kaolinite. The soil vermiculite commonly contains hydroxy-Al interlayers which are especially prevalent in the B-horizon samples. Despite significant variation in the type of bedrock and the composition of heavy mineral assemblages in these watersheds, the clay mineralogy is remarkably uniform. This finding supports earlier suggestions that the occurrence of vermiculite in soils is more dependent on climate than on the nature of the parent material.

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“…Although this report largely concerns the formation of interlayers, studies of trioctahedral chlorite weathering are instructive. Stephen (1952) proposed that the "brucite" layer of chlorite weathered so as to leave hydroxy-Al components in the interlayer space:…”

Section: Natural Interlayeringmentioning

confidence: 99%