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Pressure Solution: Possible Mechanism for Silicate Grain Dissolution in a Petrocalcic Horizon

Dep. of Agronomy and Horticulture, Box 30003, New Mexico State Univ., Las Cruces, NM 88003

*Corresponding author.

ABSTRACT

In the petrocalcic horizon of a middle Pleistocene soil in southern New Mexico, up to 58% of the silicate grains have dissolution features, such as embayed and serrated edges. The purpose of this study was to determine if pressure solution produced the dissolution features. Samples were analyzed in thin section using light and electron microscopy, and by measuring pH and Si concentration of saturated paste extracts. Four lines of evidence support the pressure-solution mechanism in the petrocalcic horizon: (i) silicate grains are smooth where they contact pores, but serrated where they contact calcite; (ii) dissolution pits on silicate grains generally match the shapes of impacted calcite crystals; (iii) silicate grains are concentrated into horizonal layers in the upper petrocalcic horizon, which testifies to the pressure generated by crystallizing calcite; and (iv) outside the petrocalcic horizon where crystallization pressure is less, silicate grains coated with calcite crystals have fewer dissolution features than grains in the petrocalcic horizon. No relationship was apparent between silicate-grain dissolution and petrocalcic-horizon pH or Si concentration. The pH (7.6) was below, and the Si concentration (8.6 mg L^–1) was above the levels required for dissolution. Thus, pressure generated by crystallizing calcite appears to have caused dissolution features on the silicate grains in the petrocalcic horizon.

Contribution from New Mexico State Agric. Exp. Stn., Journal Article no. 1551.

Received for publication September 20, 1990.