DETERMINATION OF THE MINERAL CONTENT OF SOIL COLLOIDS

byE. TRUOG and M. DROSDOFFUniversity of Wisconsin

The nine elements—O, Si. Al, Fe, Ca, Mg,Na, K, and Ti—combined largely In the form ofsilicates and oxides, make up about 98 per centof the earth's crust. When Igneous rocksweather In humid temperate climates, the ele-ments sodium and calcium form soluble salts andare lost in large amounts by leaching. It Isvery significant that sodium and calcium do notform any resistant secondary minerals. Al-though considerable magnesium, silicon, andsome potassium also leach away, these, alongwith the other four elements, form secondaryoxides and silicates of such great stabilitythat they persist as the minerals of the inor-ganic colloid fraction of soils. According toinvestigations to date, the minerals of thiscolloid fraction (less than O.OO01 mm. dia.)usually consist of the following:

(A)Usually PresentRather Abundantly

Hydrated Iron oxide (FegOj.XHgO)

Silica (SiO2)

Base exchange material(Al2O3.4SiO2.XH2O)

(Fe2O3.4SiO2.XHgO)

Talc and Antigorite

(3MgO.2SiO2 et cetera)

Muscovite (KgO.3AlgO3.6SiO2.2H2O)

(B)Present LessAbundantlyor not at all

Aluminum oxide (AlgOj)

Titanium oxide (TlOg)

Kaolinite (AlgOj.2S10g.2HgO)

Halloysite (Alg03.2SiOg.nHgO)

Chlorites (Mg, Al, Fe silicates)

Hematite (FegOg)

Evidence Indicating the Presence 'of Muscoviteand Talc in Soil Colloids

Volk (Am. Jr. Sol. 26 (1933) 114) got adefinite X-ray pattern for muscovite in the0.002 mm. to 0.0003 mm. clay fraction of Hagers-town silt loam. Nagelschmidt (Z. Krist. 87(1934) 120) made X-ray studies of a fine clayfraction and concluded that muscovite and talc

were present. Schlünz (Chem. d. Erde 8 (1933)167), by means of petrographic studies on thesilt and coarse clay fractions of two clays,found that the Muscovite content increasedwith decreasing particle size from about 1056to several times this amount. Re also con-cluded that chlorite was present in both claysand on the basis of chemical analysis, talc inone of the clays. Quartz and feldspar de-creased markedly with decreasing particle size.

The writers ground the Vesper soil col-loid In a special tool steel ball mill forthree days and thereafter were able to extractwith ammonium acetate and sodium carbonatepractically all of the MgO and KgO and suffi-cient SiOo and AlgOg to satisfy approximatelythe formula requirements of talc and muscovite.The exchange capacity remained unchanged.Similar results were obtained with two benton-ites containing MgO but not KgO. Pure samplesof talc and Muscovite when ground and extract-ed in this way dissolved quite readily.

OUTLINE OF PROCEDURE(A) Separation of Colloid from Soil

1. Saturate soil with sodium by leaching with_N NaCl and then wash out excess NaCl withwater and finally 85% alcohol.

2. Disperse, dilute to large volume, and letsettle 24 hours.

3. Run supernatant suspension through super-centrifuge at such a rate as to give thedesired size particles.

(B) Separation and Determination of FreeSilica and Free Alumina

1. Digest in "2.% sodium carbonate solution at70°to 80°C for 8 to 10 hours to dissolvefree silica and free alumina and coagulatecolloids.

2. Wash colloids with 0.05 Jt HCl by decanta-tion, using centrifuge.

3. Free silica and alumina may be determinedin "decantate" and washings.

(C) Separation and Determination of FreeIron Oxides

1. Suspend colloid In water (0.5 g. in about250 cc. water), saturate with HoS and makeslightly alkaline with NIUOH. Shake for1/2 hour, acidity with O.I N. HCl to dis-solve FeS, and warm to drive" off H2S. Washwith 0.05 S. HC1 by decantation, usingcentrifuge.

2. Free Iron Is determined In the "decantate"and washings.

(D) Removal of Free Sulfur IntroducedUnder ((D

Wash by decantation, using centrifuge,twice with 95% alcohol and then 3 times with asolution consisting of one volume of CSg and

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Chemical and mineral composition of colloidal clays freed of uncombined silica, alumina, andiron oxides. The muscovite present was previously saturated to normal KgO content. The mineralcomposition is based on stoichiometrical allocations of chemical constituents to minerals assumed,to be present, the amount of talc being determined by the MgO, the muscovite by the K2O, the ironexchange compound by the Fe2O3, and the remaining AlgOs and SiOg being allocated to the aluminumexchange compound.

*In this case it is the S10g;Feg03 ratio.two volumes of 95$ CgHsOH; then wash 4 to 5times with alcohol to remove CSg.

(E) Determination of Exchange Capacity

Displace exchange hydrogen In colloid bydigesting at 70° to 80° C with neutral £.CH3COONH4 solution and wash twice by decanta-tion with the same solution, using centrifuge.Wash, similarly, 5 to 10 times with neutralJl_ CaClg solution, and then with &&% alcoholuntil free of chlorides. Displace exchangecalcium by washing as before 5 to 10 timeswith neutral £. CHjCOONH. and determine theexchange calcium in the washings.

(F) Saturation of Muscovite to Normal PotashContend

Recently G. Volk (unpublished data) ob-tained evidence that Muscovite Is often notfully saturated with potassium to formula re-quirements, the deficiency being probably madeup by additional hydrogen. By treating thefinely divided muscovite with a solution ofKCl and evaporating repeatedly, the muscovitecan apparently be brought to full potash con-tent. In order to make proper stoichiometrioallocations of the constituents of the colloid,it is necessary to first saturate the musco-vite to the full potash content. This may bedone as follows: Wash colloid by decantation,using centrifuge, 5 to 10 times with JL KC1 so-

lution, and then twice with water, purposelyleaving sufficient KCl with the colloid forsaturation of muscovite which is accomplishedby at least 40 alternate wettings and dryingsat 70° to 80° C, using about 5 cc. of watereach time. Remove excess KCl by washing about5 times with neutral N. CHjCOONH

(G) Total Analysis

Place colloid In weighed platinum crucible,dry, and ignite to constant weight.

1. Silica; Evaporate three times with HF,using about 5 to 8 cc. the first timealong with two drops of H2S04 and subse-quently 2 to 3 cc. of HF. Ignite and getsilica by loss.

2. Potassium; Rub up residue from silica de-termination with hot water and filter onashless paper, washing with hot water.Determine potassium in filtrate.

3. Iron. Aluminum. Titanium, and Magnesium:Ignite residue on filter paper from (2)and fuse with potassium bisulfate. Dis-solve In hot water and HC1 and determinethe constituents in the regular way.

NOTE; If the soil colloid contains organicmatter, provision should be made forits removal either before or after sep-aration of colloid from the soil.

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Chemical and mineral composition of colloidal claysPercentages of oxides by total

analysis

SiO2 Al2O3 Fe2O3 K2O MgO Total

Percentages of minerals bystoichiometrical calculation

Talc MuscoviteIron

exchangecompound

Aluminumexchangecompound bydifference

ResultingratioSiO2Al2O3aluminumexchangecompound

Baseexchangecapacityof actualexchangematerialm. e . per100 grams

Bentonite(from

Calif.)

Bentonite(from

Wyoming)

Bentonite(from

Canada)

IronBentonite(from

Calif.)

Vespercolloid

Colbycolloid