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Soil Science Society of America Journal Abstract -

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


This article in SSSAJ

  1. Vol. 29 No. 2, p. 159-163
    Received: July 20, 1964
    Accepted: Oct 26, 1964

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  1. P. V. Kiely and
  2. M. L. Jackson2



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.

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