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Quartz, Feldspar, and Mica Determination for Soils by Sodium Pyrosulfate Fusion¹

ABSTRACT

Soil fractions are treated with the J. Lawrence Smith Na₂S₂O₇ fusion which is shown to decompose micas and other layer silicates common in soils. The layer silicate relics are dissolved by washing with 3N HCl and hot 0.5N NaOH for 2.5 min leaving quartz and feldspar in the residue. Recovery of quartz after the treatment ranged from 99.4% in the 50 to 20µ fraction to 96.6% in the 2 to 0.2µ fraction; correction factors are applied to the respective fractions accordingly.

Recovery of K feldspar ranged from 96.5% in the 500 to 50µ fraction to 64% in the 2 to 0.2µ fraction; Na feldspar recovery varied from 95% in the 500 to 50µ fraction to 68% in the 2 to 0.2µ fraction. Highly calcic feldspars (bytownite, An 70–90, and anorthite) were completely dissolved in the finer fractions but such feldspars are uncommon in soils. Labradorite (An 50–70) is determinable in fractions coarser than 5µ. Feldspars are determined by allocation of residue K, Na and Ca to their equivalent endmember feldspars, with suitable correction factors for mineral solubility and for increase of Na from pyrosulfate in K and Ca feldspars. The remainder of the residue is allocated to quartz. Mica K₂O is obtained as the difference of total K₂O less feldspar K₂O in the residue, a differentiation important to soil fertility.

Quartz is almost invariably present with layer silicates in soil clays. As much as 15% quartz frequently is undetectable in X-ray diffractograms in the presence of appreciable mica, because of the coincidence of the 3.35A peak of quartz with the 3.33A peak of micas and because the 4.26A quartz peak is relatively weak. The quartz present can be determined by the proposed method to within ± 1% in samples low in feldspars.

¹ Contribution from the Department of Soil Science, University of Wisconsin, supported by the Research Committee of the Graduate School from funds received from the Wisconsin Alumni Research Foundation and by facilities grant G13793—Jackson from the National Science Foundation. The first author was on study leave from The Agricultural Institute, Ireland and was supported in part by the W. K. Kellogg Foundation. Presented before Div. S-2 and S-5, Soil Science Society of America, Nov. 1964, at Kansas City, Mo.

² Research Officer, The Agricultural Institute, Dublin, Ireland (formerly Research Assistant) and Professor of Soil Science, respectively.

Received for publication July 20, 1964. Accepted for publication October 26, 1964.