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Diaspore Recrystallized at Low Temperature

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Diaspore Recrystallized at Low Temperature American Mineralogist, Volume 63, pages 326-329, 1978 Diasporerecrystallized at low temperature W. D. Kellen Department of Geology, Uniuersity of Missouri Columbia. M issouri6 5 20 I Abstract Diaspore from clay depositsin Missouri is shown in scan electron micrographs to have recrystallizedinto relatively coarse crystals in a weathering environment and at surface temperatures.Recrystallization at such low temperaturesis unexpected,in view of synthesis temperaturesin the range of 200-300'C. Aluminum hydroxide thus is not intractably "in- soluble" under certain soil-waterconditions. Introduction ler, 1977b).Missouri diasporehas originated by desi- parent For two and a half decadesthe apparentcrystalli- licationof kaoliniticflint clay,according to all parent zation and stability of diaspore at earth-surface writerscited above. The kaolin and/or illite to ("room") temperaturehas been an intrigui4gprob- the flint clay was derived as a residual weathering product (Rob- lem. The geologichome of diasporetraditionally has from adjacentPaleozoic limestones appearedto be in an environment of metamorphism bins and Keller, 1952).The transitionfrom flint clay or in a geneticenvironment above 200oC (laboratory to diasporeusually took placelaterally or vertically synthesis).In contradistinctionto genesisat high within a rangeof a few cms to a meteror more,as in temperature and high pressure,however, diaspore the Schaefferkoetterpit. also occurs abundantly in associationwith sedi- Figure I is an SEM of representativeflint clay from (3000X). mentarydeposits of flint clay and bauxite.Despite an the Schaefferkoetterdeposit This specimen, apparent low-7',P origin for these deposits,geolo- which is typicalof Missouriflint clay,is fine-grained, gists usually have diligentlysearched for a possible has low porosity, and is very compact due to the part-time existencefor them at higher temperaturein interlockingof tiny crystalsand small books of ka- an effort to reconcilefield observationsand labora- olinite. The micrographed surface is a freshly frac- tory data. tured surface-no grinding, etching,or other process- In this paper, however,scan electron micrographs, ing which might introduce an artifact was done. SEM, will show diasporethat has recrystallizedin Other types of flint clay are shown in detail elsewhere (Keller, "surface boulder" diaspore exposedonly to the sur- 1976, II;' 1977c).A few cms inward on the pit, face weatheringenvironment (necessarilya low tem- outcrop, toward the diasporecore of the the clay "burley" perature and pressure)in central Missouri since the becomesa in character.Some oolites,or Tertiary. "burls" (a miner's term for oolites, from which the name "burley clay" originated) of diasporeare pres- ent, while the matrix also includesplates or bladesof Scan electronmicrographs diaspore,Figure 2 (3000X). The estimatedAlrO, con- Scanelectron micrographs were taken of represen- tent of this specimenis 55 to 60 percent,whereas that tative specimensfrom a typical sequenceof flint of the flint clay of Figure I is about 45 percentAlros. clay-burley-diaspore in a clay pit in the central First-gradediaspore, 68 percentAlrO, and higher, Missouri clay region (McQueen, 1943;Allen, 1935, and the interior of "burls" contain dominantly 1952; Keller et al., 1954).Although reservesof dia- blades,scales, or flakes,some of them with irregular spore in Missouri are essentiallydepleted (Keller, or "lacey" edges, as seen in Figure 3 (3000X). A 1977a), a typical deposit has been temporarily, or local, richestsmall lensof diaspore,I m long and 0.25 accidentally,spared from mining; this is the Schaef- m thick, showsincipient recrystallizationof diaspore ferkoe.tterpit, about I I km NW cif Owensville(Kel- within the massive body of the deposit, Figure 4 0003-004x/78/0304-0326$02.00 326 Fig. L Flint clay, Schaefferkoetterpit (3000x). Fig. 2. Matrix of burley clay derived from flint clay, a few cm from location of Fig I (3000X).Fig.3.First-gradediaspore,estimatedover6SVoAlO..Approximatelylmfromlocationof Figurel(3000X) Fig.4-Very rich diaspore from a small, local, relatively permeable lens in the diaspore core of Schaefferkoetterdeposit (2000X ), showing incipient recrystallization Fig. 5. Recrystallizeddiaspore from "surface boulder," near Swiss,Missouri (2000X). Fig.6 Recrystallizedsurface- boulder diaspore,Gaume pit, near Aud, Missouri (2000X). 328 KELLER: DIASPORE (2000X). Figure 4 alone may not be convincingof stone (Marmaton-Des Moinesian, Searight and incipient recrystallization.To best appreciatethat ef- Howe, l96l). The field relationshipsand identifica- fect, more striking views will be shown of "surface- tions were discussedin detail by Keller (1952, 1968; boulder" diaspore. Keller et al., 1954).Thus flint clay and diasporewere "Surface-boulder"diaspore originated, as the term depositedand formed in the Paleozoicgeologic time indicates,as residual boulders developedby weath- interval,Cheltenham to Ft. Scott! Whereasearlier ering at the surfaceof diasporedeposits exposed by hypotheses(including my own) of origin of the Mis- erosion. Probably this datesback to Pleistocene,but souri refractory clay had been primarily by down- certainly the erosion surface is no older than the ward leaching,evidence from later observationssug- EoceneWilcox formation (Bretz, 1965).A micro- gestedprominent upward leachingand flushingaway graph from a "surface boulder" from near Swiss, of metal ions (refractory fluxes)in freshrain water by Missouri, is shown in Figure 5 (2000X). Another way of the Donnan effect(Keller, 1952,1968; Keller micrograph of surface-boulder diaspore from the et al., 1954). now-depletedGaume pit near Aud, Missouri, is Early referencehas been made to a high-temper- shown in Figure 6 (2000X). The "surface-boulder" ature origin for diaspore.Ervin and Osborn (1951) diasporeis presentin well-formed,indeed euhedral, studiedphase equilibria in the systemAlrOr-HrO and relativelylarge crystals, e.9., 2 pm by 4'pm, in com- reported that "the diagram shows pressureand tem- parisonto small,thin flakes,shown at 3000X magni- peratureminima (2,000lb/in',275"C) for the stable fication for the pit-generated diaspore. Reexam- existenceof diaspore.. ." They also, however,rec- ination of Figure 4 showsincipient recrystallization ognizedproblems of applicationof their work to the within a local lens in the massivedeposit, whereas diasporein the sedimentaryrefractory clays of Penn- long exposureto weatheringsolutions developed ma- sylvania.Keller et al. (1954,p.31-34)considered the tured, largeand well-formedtabular crystals. work of Ervin and Osbornand the difficultpossibility Small boulders of "surface diaspore" consist of geologic high temperaturein the formation of mainly of coarsecrystals, whereas large boulders (a Missouri diaspore,but decidedin favor of a low- meter across)typically revert from coarsercrystals on temperatureongrn. the outsideto bladesand scales,more like those in Kennedy(1959) reexamined the phaserelations in Figure 3, in the interior. This confirmsthat surface the systemAl2O3-HrO at high temperaturesand pres- weatheringof diasporepromotes its recrystallization. sures.He seededalumina gel with a small trace of The SEM illustrationsare convincing (at leastto me) diasporecrystals, and reported (p. 569): "The lowest that recrystallizationof diasporedoes take placeun- temperatureat which the diasporehad appreciably der an oxidizing, surface-weatheringenvironment increasedin quantity and the crystalsincreased in (temperatureand pressure)as has existedin central size,in threemonths time, werethe two runs at22Oo Missouri sinceearly Eocene. at 1000and 1500bars HrO pressure."He wrote fur- ther, however(p. 563): "The approximateslope of Geologyand discussion the boehmite-diasporeboundary, computed from Diaspore in Missouri was formed from desilica- thermodynamicdata, indicates that diasporein baux- tion, as stated,of clay mineralsdeposited in karstic ite and clay depositsmay haveformed at atmospheric depressionswithin the PennsylvanianCheltenham pressuresand temperatures,a familiar conclusion formation. This was referred with slight reservation from field observations." to the Atokan Series-but clearlythe Cheltenhamis The SEM figuresin this paper are evidencethat below the Burgner formation which is post-Morro- diaspore,as it occurs in Missouri, presumablydis- wan but pre-Desmoinesian(Searight and Howe, solves and recrystallizesin a temperature,humid 1961,p. 79). The time or geologicdate when genesis weatheringenvironment. A varietyof possibleminer- of diasporewas concludedremained undetermined alogictransformations are thus available for the solu- until about 1950when the Buekerdiaspore pit, cov- tion, deposition,and recrystallizationof aluminum ered with Pennsylvanian-agerocks, was opened. At hydroxides in diverseclay and bauxite deposits.Re- this pit, the upper surfaceof otherwisefresh flint clay straint is called for in making an unreservedstate- was "rotten" and oxidized (weathered)but covered ment about blanket"insolubility" of aluminaminer- in sharp contact with overlying fresh, green, illitic, als in the zone of weathering.While it would be marine (?), thin Excelloshale. This shaleis coform- presumptous to speculate from SEM photographs ablv overlain bv thick. indubitableFt. Scott lime- the geochemistryof the surfacesolutions that mobi- KELLER: DIASPORE 329 lized the diaspore,the field occurrencesof surface- Bretz,J. H. (1965)Geomorphic history of the Ozarks of Missouri. boulder diasporedo not indicateany significantly Mi.t.touriGeol
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