PHYSICAL and CHEMICAL DATA for CRANDALLITE from ALACHUA COUNTY, FLORIDA Fnrnr N

Am eric an Mineralo gi,st Vol. 57, pp. 473484 (1972)

PHYSICAL AND CHEMICAL DATA FOR CRANDALLITE FROM ALACHUA COUNTY, FLORIDA FnrNr N. Br,eNcrr,s,eo,Department ol Geol,ogyflniuersity of Florida Gai.ne su ille. F l,oridaI 260 1

Assrnecr

Crandallite in a crandallite-clay quartz sandstone was determined by scanning electron microscope and by electron probe microanalysis. Extrapolation of this information to similar rocks facilitates the distinction in thin section between crandallite and associated minerals in the phosphatic sediments from Florida. The crandallite used in this study was found to be a strontian variety intermediate between crandallite and goyazite (Cn*Gz.), and probably much of the crandallite in Florida is of a similar composition. Crandallite was formed from the elements in carbonate fluorapatite and clay minerals and during the process Sr has been enriched many times over its concentration in the source mineral (apatite). INrnooucrror .The occurrence of crandallite (pseudowavellite),CaAls (PO+)z (OH)b'HrO, in Florida (phosphaticdeposits near Bartow, Polk County) was apparently first reportedby Hill, Armiger, and Gooch (1950).Altschuler et aI. (1956)discussed the origin of crandalliteand other secondaryphosphate minerals (millisite and wavellite) which occur in the Bone Valley Formation in central Florida. Espenshade and Spencer (1963) listed crandallite as a minor constituent of weatheredphosphatic sediments in the phosphoriteunit of the Haw- thorn tr'ormation(upper Hawbhorn) of northern Florida. Except for partial X-ray data publishedby Owenset al. (1960), the literature dealing with phosphateminerals from Florida does not include a study of the physical and chemicalproperties of crandalliteitself and it is my purposeto presentthe resultsof work on crandallitefrom a specificoccurrence in Florida. Crandallite is a member of the plumbogummitegroup which in- cludesthe minerals plumbogummite,gorceixite, goyazite (Gz), crandallite (Cn), and florencite,with characteristiccations of Pb, Ba, Sr, Ca, and Ce respectively.Palache et al. (1951) include deltaite in the group, but Elberty and Greenberg(1960) have shown that deltaite is a mixture of crandalliteand hydroxyapatite.Minerals of the plumbogummitegroup are consideredto be isostructural,with an alunite- type structure,and havethe generalformula XAlg(POr)z(OH)b.H2O, - whereX Pb, Ba, Sr, or Ca, or CeAls(PO+)rOHo(florencite). Con- siderablesolid solutionexists between members of this group and be- 473 474 I'RANK N. BLANCHAND tween members of the plumbogummite, alunite, and beudandite groups. The crandallite described in this report has the approximate formula Cn7sGz36 and is thus essentially a strontian crandallite, intermediate between crandallite and goyazite.

OccunnpNcn Poorly-indurated to well-cemented sandstones and sandstone nodules occur in numerous exposures of the Hawthorn Formation from Ocala, Florida to the Florida-Georgia border. Where present at relatively shallow depth, many of these sandstones are composed dominantly of quartz grains in a matrix or a cement of clay (dominantly kaolinite) and/or wavellite. Less commonly crandallite is present and rarely small amounts of variscite (Blanchard and Denahan, 1967) can be detected. Where crandallite is present it is generally crypotocrystalline and is virtually inseparable from associatedphosphate and clay minerals. Because of small crystallite size, association with other minerals (especially phosphates), and because of a lack of detailed information on. crandallite, it is difficult to identify (with a reasonable degree of certainty) crandallite in thin section. During preliminary X-ray analysis of a large number of samples of these saldstones, one group of samples was found to consist of quartz grains in a matrix of crandallite with minor clay minerals and no other detectable phosphate minerals. This provided an opportunity to study the crandallite without the danger of confusion between cha.racteristicsand properties of crandallite and other phosphate minerals, and it is this concentration of crandallite which was used in this study. The exposure, a small road bank on U. S. 441 near Micanopy, Alachua County, Florida, is dominantly unconsolidated sand with sparse nodules of white to light-gray to light-tan poorly-indurated qua.rtz and crandallite, and with well- indurated nodules containing quartz, wavellite, and crandallite. In hand specimen the quartz-crandallite nodules have the appearance of an argillaceous sandstone and, disregarding degree of induration, they a.re megascopically indistinguisha,ble from the much more abundant wavellite-cemented sandstones of the llawthorn Formation.

Dere Crandaliite was separated from most of the associated quartz by gentle crushing and dry sieving at 325 mesh and by disaggregation with a magnetic stirrer and wet sieving at 325 mesh. Concentrates of crandallite (-325 mesh) were used for X-ray and chemical analysis, for measurement of optical properties, and for differential thermal analysis. Thin sections and polished thin sections of the nodules were prepared for petrographic study and for electron probe microanalysis, and small fragments of the nodules were coated with gold-palladium for examination with the scanning electron microscope. Chem,icqlAnalysis -325 Qualitative X-ray fluorescenceanalysis of the mesh fraction of the crandallite-bearing rock showed the presence of essential CRANDALLITEFBOM FLORIDA 475

amountsof Ca, Al, P, Sr, and Fe and minor to trace amountsof As, Y, Ce, Ti, and K. Qualitative electronprobe microanalysisof part of a polishedthin sectionof a crandallitenodule was madein orderto correctlyidentify the phasesseen in optical examinationof thin sectionsand in order to establishthe mineralogicallocation of the iron and strontium found in the X-ray fluorescenceanalysis and in the quantitative chemical analysis.Analysis for Al, Si, P, Ca, Fe, and Sr was madealong anL125 micron traverse.A specimencurrent image for the area around the traverseis shown in Figure 1, along with a photornicrographof the samepart of the polishedthin section.A scan for the Ko line of each of the elementslisted above (exceptFe) is shownin Figure I. In the microprobescans quartz showsup as high counting rates for Si and backgroundfor the other elements.Crandallite is characterizedby high countingrates for Ca, Al, P, Sr, and Fe, and low countingrates for Si. Regionsalong the traversewhich show high countingrates for Si, Al, Fe, and low countingrates or backgroundfor P, Ca, and Sr are interpretedas clay, probably (at leastin part) kaolinite and/or montmorillonite. The results of the microprobeanalysis show that (1) crandallite is the dominant isotropic or very nearly isotropic mineral visible in thin sectionand the dominantbirefringent material (excludingquartz) is clay, (2) in many placesin the thin sectioncrandallite and clay are physically separateand optically distinguishablefrom one another, but in a few placesthe two are intimately mixed,and (3) Sr is confined to the crandallite,whereas Fe is presentin both crandallite and clay. A quantitative chemicalanalysis of the -300 mesh fraction of the crandallite-bearingrock is given in Table 1. This material is dominantly crandallite,clay, and minor quartz. Becausethe composition of the clay mineral or minerals is unknown and becausethe propor- tion of the clay to quartz is unknownit is not possibleto obtain from the data a completecomposition for the crandallite.The most striking, and unexpected,aspect of the analysisis the rather high percentageof SrO. From the microprobeanalysis it is clear that essentiallyall the Sr and Ca are in the crandallite,and from the chemicalanalyses for Sr and Ca this leadsto a compositionof Cn6s.sGz31.s(wet chemical) or Cn7sGz36(X-ray fluorescence)for the crandallite.

X-RW Analysis X-ray analysesof the whole-rock sample show strong lines for quartz and crandallite and the crandallite concentrateshows addi- 476 FRANK N. BLANCHARD

, 300MlCR0ttlS ,

'rl ri

Frc. l. Electron microprobe aralysis fo,r Sr, Ca, F, Al, and Si. Scans for Ka lines of elements (top), specimen current image (middle), and thin section photo- micrograph with crossed polars (bottorn). Crandallite (Cn), clay (CI), and quartz (Q). CRANDALLITE FROM FLOhIIDA 477

TABLE I CHE}fICAI ANAIYSIS OF TIIE -325 MESH FMCTION OF A CR}NDAILITE-QUARTZ-CLAY NODIJLE

Calculated Cmposition for Crandallite (with quartz and ninor Crandallite-covazite clavs) frm FLorida

I,Iet X-Ray Cn1 CnTgGz3g Gz2 chenical !'luorescence

Ca0 IJ. )) 9.L6 5 t29 s.84 sr0 22,45 4 .5L 4.67 dzo3 36,93 35.73 33.L2 25.86

P2o5 34,29 33.13 30.77 2r.84

EzQ*3 15.23 L4,72 IJ. OO l).41 si02 L7,62

Ie203 1.53

Mgo 0.31

Kzo 0.15

Na20 0,09

EzF4 L.62 z. Jo

Cn-cz (deteroined frou SrO:Ca0 nol-. cndg.5 ctTo fraction) G"31.5 G"30

lcaer, {roO)2{On) r.u2o 2srera {roo)rton) r, uro 3Between L20o and 990' C, aBelow 120" C.

tional weak Iines corresponding to the strongest lines for kaolinite, montmorillonite and what is probably a mixed-layer clay. X-ray analyses of the partially separated and sedimented clay minerals yielded the expected strong lines for oriented kaolinite, montmorillonite and a 2l-angstrom line which may result from a mixed-layer clay. No other phases were identified although an unidentified sharp line is 478 FRANK N. BLANCHARD present at,d = 6.08.The observedand calculatedd-spacings corre- spondingto reflectionsfrom crandallite, along with similar data for crandallitefrom Fairfield, Utah, are given in Table 2. There is a close correspondencebetween the two setsof diffraction lines,however, the data doesnot correspondclosely with either of the two A.S.T.M' cards for crandallite. Lattice parametersfor the crandallite were deter"rninedfrom dif- fractometercharts using Ni-filtered Cu-radiation,a scanningspeed of 0.2o20 per min., a time constantof 4 sec.,1o beamslit, MR sollerslit, and a 0.02odetector stit. The reflectionslisted in Table 2 were used in refinementof lattice parametersusing the least squaresmethod and

]HF 2. PUIDER DIFFMCTION DATA ----

crandalli te-Il orida It te-Fairf1e1d.

Lloe hoar observed di calc.rated di2 I relative observed d? Calculated dR3 I relative

1 101 5.698 5 692 4L 5 695 5.588 34

003 Nol observed 5.4f7 Not observed 5.397

2 0r2 4.869 4.856 20 4.A66 4.A59 29

3 110 3.511 3. 508 54 3.509 3.508 35

104 Not observed 3,378 Not observed 3.368

4 02L 2.984 2.9a7 44 2.983 2.985 50

5 113 2.945 2.945 100 2.94L 2.94L 100

6 0r 5 2.862 2.866 5 2.8s8 2.857 6

2O2 Not observeal 2.846 Noc observed 2.444'

7 006 2.710 2.109 9 2.699 2.698 17

8 024 2.435 2.433 7 2,430 2.430 4

2lL Not observed Not observed 2.274

2OS Not observed

9 122 2.209 2.270 .19 2.201 2.209 16

10 LO7 2.L6A4 2.1688 28 2,1624 2.16L5 36

116 Not observed 2.1-440 Not observed 2,7387

300 Not observe;l 2.O255 Not observeil 2.0252

fl 2L4 2.0009 t.9994 3 1,9964 7.9973 2

018 Not observed 1.9364 Not observed 1.9200

12 r03 1.8972 1.8973 28 1.8960 1.8961 2'1

725 Not observed 1.8757 Noc olserved 1.8731

13 027 t.8441 .1.8448 2 1.8417 7.a402 6

lA11 possible hkl values listeil for hexagonal-rhombchedral

2cal".1at"d for a = 7.017, c = 15,252(refineil for observedl reffectlons to 5002 e, cuKo ) 3calculate

Frc. 2. Electron micrographs (scanning electron microscope). Crandallite-bearing rock (Florida) with arrows pointing to crandallite (upper a,nd lower left). Crandallite from Fairfield, Utah (lower right). Scalesin mm. the computerprogram of Applemanand Evans (1967).The resultsare: a : 7.017(+ .001) c : 16.252(+ .004) (Florida) a : 7.013(+ .0008) c : 16.196(+ .003) (Utah) The slightly larger unit cell volume of the material from Florida may result from the substitution of the larger Sr2*for the smaller Ca2*. 480 FRANK N. BLANCHARD

Eleatr,on Micro'scopy Samples of the crandallite-bearing rock were examined with the scanning electron microscope and electron micrographs were compared with those taken of a sample of well-crystallized pure crandallite from Fairfield, Utah (Fig. 2). The crandallite from Fairfield occurs as platy hexagonal crystals and as equant pseudocubic rhombohedral crystals, averaging 3 to 6 microns across (Fig. 2, lower right). This habit is similar to that described by Palache et al. (1951) for goyazite, but is somewhat different from the habit described as typical for crandallite. This may be because macrocrystalline crandallite is rare and becausethe crystal habit of crandallite has not been investigated by electron microscopy. Comparison of the electron micrographs of Fair- field crandallite wiih those of the crandallite-bearing sandstone from Florida, and consideration of the nature of the Florida crandallite as seen in thin section, suggest that the crandallite from Florida can be recognized by its crystal habit (Fig, 2, lower left). The individual crystals of Florida crandallite are platy, hexagonal, and average about 2.5 microns in maximum dimension. The crysials tend to occur (at least in part) in pockets approximately 100 microns across (Fig. 2, upper). In order to confirm the identification of this material as crandallite, the X-ray emission spectrum from a similar region of platy hexagonal crystals was examined using non-dispersive optics and a multichannel analyzer. CaKo', AlKa and PKa lines were strong, pre- cluding the possibility of the material being a clay mineral.

Li.ght Miuoscopy In thin section the crandallite from Florida is isotropic o,r very nearly isotropic, pale brown, and stands out with low to moderate re- lief (Fig.3). It constitutesthe chief matrix mineral of the rock and it is mostly physically separate from the occasional particles of darker brown anisotropic clay. The measured refractive index of crushed fragments of the crandallite is approximately 1.596 + .002. The smaller crushed fragments are colorless and isotropic or nearly so, while the larger aggregatesare brown and show very weak birefring- ence. The optical properties and general appearance in thin section are very similar to the carbonate fluorapatite (francolite) which occurs in phosphorites and phosphatic sediments of Florida, and it seems probable that erandallite may be easily mistaken as francolite.

D ifi erential T hermal Analy sis Differential thermal analyses of fract ons (100 mg) of the crandallite concentrate in silica crucibles were made from room temperature CRAN DALLITE FROM FLORI DA 481

6H O<

;gxd tr!4 oo

-a

.9 Eo

O.A

f) c6v

o' 482 FRANK N. BLANCHARD to about 1100"Cat 5o, 10o,and 25o per minute.After ttre analysisthe furnace was cooledto 500"C and the heated crandallite concentrate was replacedwith a 100mg sampleof quartz; the temperatureof the furnacewas then raisedat the sameheating rate usedin the analysis. This proceduregave a calibration check on the furnace temperature (quartz inversionat 573'C) and provided a referencefor DTA peak magnitudes. A typical thermogramfor the crandallite concentrateis shown in Figure 4. Interpretationof the reactionswas faciJitatedby comparison with the thermogramof pure crandallite (Blanchard, 1971) and by heating samplesof the crandallite concentrateto temperaturesjust aboveprominent DTA reactions(150o, 250o, 44A", 540",620o,800o, 850", 1030o,and 1090'); X-ray diffraction patterns were then made from each of these heated samplesin order to identify the phases present. The endothermicreaction at 125o results from loss o,f adsorbed water. The endothermat 180" probably results from loss of inter- layer water in montmorillonite.Below this temperaturethe mont-

Furnace Temp - - -

I I o o o O o o o o .9 cf, F\ \-E (L) o x UJ

.9 \-E c) ! -oo LrJ I I

Frc. 4. Thermogram of crandallite concentrate heated at 25'C per minute. CRANDALLITEFROM FLORIDA 483 morillonite showsa poorly developed15 A diffraction line; aftrerheat- ing to 250o the montmorillonite line has shifted to about I A. The endothermaL 435" correspondsto the disappearanceof diffraction peaksfor montmorillonite.The endothermat 525" correspondsto dehydration of crandallite and disappearanceof the crandallite structure. The endothermat 605" is assoeiatedwith the disappearanceof the kaolinite lines. The diffraction pattern between 620o and 800o shows only quartz as well-crystallized and shows a broad hump in the range of 20 to 30" 2 e (Cu-radiation) suggestingthe presenceof considerableamorphous material. At 845' a sharp exothermicreaction correspondswith the crystallization of AIPOa and possibly incipient crystallizationof apatite (both producedfrom the dehydratedcrandallite) and at 920" the exothermicreaction corresponds with crystallization of apatite. Actually between845 and 1030othere appearsto be more or lesscontinual crystallizationand recrystallizationof apatite and AlPOa. The exothermicreaction at 1085ois associatedwith the first appearanceof corundum (AlrOe from the dehydratedcrandallite residue).The chief differencebetween the thermal behavior of the crandallite-clay-quartzmixture from Florida and the thermal behavior of pure crandallite from Fairfield, Utah is that in the latter AIPOa and apatite appear immediately with the dehydration of crandallite,whereas in the material from Florida the crystallization of AIPO4 and apatite does not occur until a higher temperature is reached.

DrscussroN It is well establishedthat the crandallite in Florida phosphoriteis a secondarymineral formed during weatheringof phosphatic (apati- tic) clayey sediments.Aluminum in the crandallite is derived from clay minerals,while P and Ca are derived from apatite. In addition the apatite seemsto be the only available mineral to supply the Sr (and Ce) found in the crandallite.The concentrationof SrO in the crandallite is several percent, whereas analysesof four apatite or apatite-richsamples from Florida showonly 0.03to 0.20percent SrO'. Furthermore, the sample showing the highest percentage (up to 0.20 percent) containsappreciable crandallite associatedwith the apatite. Apparently, during the weatheringof apatite, Sr is preferentially taken up in the crandallite resulting in a twenty- to one hundred-fold en' richment of SrO. The ability of alunite-type structures to concentrate Sr has been describedby Frondel (1953), and this occurrenceof crandalliteseems to be an excellentexample. 4A FRANK N. BLANCHARD

AcraNowLEDcMENTs I am grateful to the following people for their help in pa.rt of the analytical work: B. B. I{iggins (electron microprobe), R. W. Pierce (electron micrographs), and S. A. Denahan (wet chemical analysis). The University of Florida Comput- ing Center provided time on the IBM 360 computer.

RnrnnuNcns Ar.rscHULER,Z. S., E. B. Jerro, ewl Currrrr,r (1956) The aluminum phosphate zone of the Bone Valley Formation, Florida, and its uranium deposits. U. S. Geol. Suru. Prof . Pap. 300,495-504. Anrr,nueN, D. E., elln H. R. Ev.lr.rs,Jn. (1967) Experience with computer self- indexing and unit cell refinement of powder data (a,bstr.).GeoI. Soc.Amer. Spec.Pap.115. Br,ervcrrrno,F. N. (l97l) Thermal analysis of crandallite. Qmrt, J. FIa. Acad. Sci.34,l-9. S. A. DnNener (1967) Variscite from the Ifawthorn Formation. Quart. J. Fla. Acad,. Sci. 29, 16Z-170. Er,rnnrv, W. T., aur S. S. GnnrNnnnc (1960) Deltaite is crandallite plus hydrox- ylapatite (abstr.) Geol. 9oc.Amer. BuII.7t, lffi7. Esrnrsneln, G. H., ervn C. W. Spnxcnn (1g63) Geology of phosphatedeposits of northern Florida. U.8. Geol.Suru. Eull^ tttg. FnoNnor,, C. (1958) Geochemical enrichment of strontium in minerals of the ' alunite stmcture type (abstr.) Geoi. $oc. Amer. BuIl. 69, lb67-1568. Ifrr,r,, W. L., W. H. Anrrrcon,eNn S. D. Goocrr (1950) Some properties of pseudowavellite from Florida. Min. Eng. 187,69Y7A2. Ownns, J. P., A. S. Ar,rscnur,rn,elvo R. Bnnuelr (1960) Miltisite in phosphorite from Florida. Amer. M ineral. 4s, 547-ffiL. Per,ecuo, C., H. BenueN, ervo C. FnoNlnr, (19,51) The System ol MineralogA ol . . . DanaTth Ed., vol.2, Wiley, New York.