Wrtrrir T. Hor.Sor<

THE AMERICA\ MI\_ERALOGIST, VOL 51, JA\UARY FEBRUARY, 1966

DIAGENETIC POLYHALITE IN RECENT SALT FROM BATA CALIFORNIA wrtrrir T. Hor.sor<, Californ'ia ResearchCorporation, La Habra, Calif ornia.

ABSTRACT

Halite and gypsum norv being deposited on extensive tidal flats at Laguna Ojo de Liebre, Baja California, are underlain by up to two meters of bedded evaporites.The porous evaporite sediments are permeated by bitterns whose stage of evaporation increasesrvith distance from the lagoon. Beyond 4 to 8 km from the lagoon, gypsum crystals in the sediments have been altered to fine-grained polyhalite, by a bittern nearly in the potash- magnesia{acies (concentration 60X seawater). Although older polyhalite has occasionallybeen describedas a primary precipitate, this modern occurrenceconfirms other geologicaland experimental evidencethat polyhalite can replacecalcium sulfate and that this can occur at an early diagenetic stage.

INrnolucrroN Polyhalite, KzMgCaz(SOa)t.2HrQ,is a common minerai in evaporite rocks of the potash-magnesiafacies, but has not been describedin evaporites forming from seawater at the presenttime. Although somepoly- halite is consideredto be a primary mineral cr1'stallizeddirectly from brine, most polyhalite is evidently a replacementof other sulfate minerals.The timing of this replacementis generallynot known in the rocks, so this first discovery of polyhalite replacing Recent g)rpsum sediments mav be useful in the interpretation of the earliergeological occurrences. The polyhalite occurs in thin evaporites along the inner shore of Laguna Ojo de Liebre, about half way down the west coast of Baja California, Mexico. Field work was done in cooperation with F. B. Phlegerand J. R. Bradshaw of the ScrippsInstitution of Oceanography. The courtesiesextended by Mr. Charles McClaughry, Nlanager, Ex- portadora de Sal, Guerrero Negro, B.C.N., and by Mr. Francisco \tlufr.oz,charter pilot at Tijuana, B.C.N., greatly facilitated the field work. Material was first collectedin Mav 1960,and in August 1961we re- turned to core the evaporitesediments. Although studv of this material is incomplete,the occurrenceof polyhalite seemsof sufhcientinterest to warrant a separatepreliminary description.

S,trr Flers oF LAGUNAO1o nr Lrnnno These salt depositswere first describedin detail b-v*Wittich (1916). Recently Phlegerand Ewing (1962)described the generalsedimentology and oceanographyof the coastal lagoonsin Baja California, with par- ticular attention to Laguna Ojo de Liebre. Sincethen, the sedimentsof 1OO WILLIAM T. I]OLSER the beachesand salt flats have beencored, and we will pubiish the details when the new material has been thoroughly studied. Here I will onll- outline thosefeatures of the depositsthat are pertinent to the occurrence of poll'halite. Figure 1 showsLaguna Ojo de Liebre (also knorvn as ScammonLa- goon) extending from the Pacific Ocean nearly 40 kilometers into the coastal plain of Baja California. The detailed study by Phleger and Ewing (1962)shows that the lagoon has a U-shapedtidal channelabout

Frc. 1. Index map of part of Baja California.

50 ft deep,bordered bv a verv shallowshelf. Surfacetemperatures of up to 25o C. combined with a light but steady northwest wind, Iead to an evaporation rate of nearlv two meters per year. Consequently,the sa- Iinity increasesto about 45'/.. (parts per thousand by weight) by the time the rvater has reachedthe head of the lagoon (Phlegerand Ewing, 1962).However, the lagoon is localll'mixed, with no salinit-vla-vering. At the upper end of the lagoon, the lagoon shallowsare borderedb1- a low sand beach, cut in a few places where tidal channelsflood and drain the flats behind the beach (Figs.2,3). The flats are coveredintermit- tentll- with a few inchesof brine with salinity over 70o/oo,driven shore- ward by wind as weil as tide. It supportsa considerablegrowth of both DI AGI':, N ET I C POLY II A LI TE 101

algae and salt grass,and in the landward part of this zone gvpsum is crystallizing.Be.vond this, the flats are white with both salt and gypsum crystals, and are apparentll. coveredwith brine only at irregular inter- vals, probabll'driven b,v-high winds or storm tides.

O KILOMETERS

+ POLYHALITE PRESENT

O POLYHALITE ABSENT

Frc. 2. Polyhalite at Laguna Ojo de Liebre.

The flats are underlain by thinly bedded sediments to a depth oI l-2 meters,mainly gypsum and detrital (qtartz and calcite) sandsalternat- ing with dark mats of algae and grass.These beds lie on massiveand compact gray-green silts and sands that possibly represent an earlier beach deposit. A wide variety of earlier strand lines have been noted along this coast (Wittich, I9l2), and 15 km east of the salt flats wells IO2 WILLIAM T HOLSER have penetrated50 m or more of Pleistoceneand Recent sands (Mina, 1957). Beneath the salt flats halite is in the upper part of the bedded sediments, onl.v a centimeter or two thick at the outer edge of the flats, increasingto 1.3m in the centersof the two "ba1,'s"that extendeastward and southeastwardfrom the main flat (Figs.2,3). Gypsum is wide- spread:mixed with the halite, as separatelayers, as large crt'stalsin the underlying detrital silts, and as windblown surfacesand. Brine lies onll'a few cm below the surfaceof the salt flats. It is not onlv saturated with sodium chioride, but shows the high potassium,

Frc. 3. Salt flats viewed eastu,ard from above the lagoon, with tidal rivers cutting through the beach ridge into the lagoon in the foreground. Tidal flats are darkened by algae and grass. The remains of a causervayand loading dock runs for 3 kilometers across 'I'he the flats. thickest deposits of salt, permeated l.ith highll. evapotuted brines, fill the r,vhite"ba1'5" in the background. magnesium, and bromide indicative of a high degree of evaporation (Table 1). A tow rainfall of only 3 cm per year has never resultedin an1' surface runoff within the memorl' of local inhabitants. Consequentlr-, the brine has not been diiuted by fresh water in recent times. However' Phlegerand Ewing (1962)believe that the detritus on the beachand the Iagoonhad their ultimate origin in floodsfrom the coastalplain in a wet cycleseveral hundred years ago. The detrital componentsof the evaporite sediments, principalll' composed of quartz, hornblende, biotite and plagioclase,probably representsa reworking by tidal currents of these older sediments.The evaporitesmust be more recent than the wet cycle. Wittich (1916)emphasizes the importance of shorelineemergence in the formation of the evaporites,but he alsopoints out that much of the salt may evenhave beenreformed in the few vearssince a large but uncertain amount rvasmined during the last haif of the nineteenthcentury. DIAGL,NETICPOLVITALITI' 103

Phleger and Ewing (1962) suggesta mechanism for supply of the partially concentratedlagoon water onto the tidal flats, by an asvm- metric tidal flow. I should also like to suggestthe importance of the on-shorewind, which can be seento drive a surfacelayer of brine landward, even as the ebbing tide is flowing outward in the lower part of the brine. fn summary, the present evaporite regine on the tidal flats seemsto

Telr.r 1. BrrNes nou LecuNe O;o nr Lrnlna, B.C.S., Mrxrco

Na 10.77 tJo 42.5 47.4 K 0.39 045 1.05 112 Mg 1.33 7 826 nd Ca 0.42 0 528 nd. 01 CI 19.8 26 48 61.0 183.1 Br 0 066 0.088 0 188 2.+4 SOa t. to n.d. n.d. n.d. Total Solids 35 54 42.97 29r.5 K/Cl 0 020 0 017 0 017 0.061 Mg/Cl 0.067 0.069 0.258 Br/Cl 0.0033 0 0033 0.0031 0.0133 Ms/K 3.4 +.1 4.2 Br/K 0.r7 0.19 0.18 0.22

1. Normal seawater (Harvey, 1955,p. ). 2. CRC 17,0083 Fromlagoonshallor.snearbeach.l 3. CRC 17,008-7. Tidal flat, with gypsum and algaemats, I km eastof beach.r 4. CRC 17,008-20.Brineunder surfaceof salt flat, 4km east oI beach.l I Analyses for K and Na by Smith-Emery Company, for all other elements by Gale Baker. have resulted from a fortuitous combination of low runoff and hish evaporation.

PotvnlurB Figure 2 showswhere the polyhaiite was found, in two large embay- ments on the salt flats, 4-8 km from the edgeof the lagoon. The polyhalite occursas a 1 cm thick lar.er of white hardpan underlying about 5 cm ofcoarse halite-gypsumsand, interbeddedwith someorganic material (Fig. a). At the northwesternmost locality (Fig. 2) coring showed scattered polyhalite to a depth 1.2 meters in halite and gypsum-rich silts. This mav be the "diinnen Calichebrindern', identified by Wittich (1916,p. 26,28) as calcite. An *-ray diffraction pattern of the white hardpan material showed 104 WILLIA,II T. IIOLSI]R many lines of polvhalite coincident with the new standard pattern (XRDF 10-355) for polyhalite. A minor amount of polyhalite was also seen in r-ray diffraction patterns of an underlying black sand. A thin section of the hardpan confirmed the presenceof polvhalite.

Frc. 4. Evaporite sediments in the polyhalite area. Brine, which normally levels near the top of the pit, was scooped out to display the sedimentary stlucture. White layers are halite and gypsum, dark layers are mainly decomposed grass, and the dead rvhite layer near the bottom of the pit is polyhalite.

As shown in Fig. 5, the outlines of gypsum crystals are easily seenin plain polarized iight. IIowever, examination under crossed polarizers shows that only a small central part of each crystal is really gypsum' This remaining gypsum is surrounded by a narrow band of radially oriented, fibrous polyhalite, and the remainder of the crystal is an aggregate of 0.005 mm crystals of polyhalite. Polyhalite also occursbetween evident gypsum pseudomorphs,where it ma1' represent either a cement or replacement of a gypsum cement. The extremely fine size of the polyhalite crystals precluded detailed optical measurements,but the averagerefractive index is normal for the DIAGI,NA,TICPOLYIIALIT]J 105 mineral. The fibrous polyhalite rims on the gypsum are length-slow. The individr"ralpolyhalite crvstals have a ver1.irregular extinction, with a suggestionof spherulitic structure. Gypsum crystals in all stagesof replacementsare found in this layer, although most are nearly com- pletely replacedby polyhalite. The texture is very similar to the mosaic and fibrous poll'halite crystals describedfrom evaporite rocks of various agesand localities(Braitsch, 1961; Celet, 1953;Ladame, 1942; Schaller and Henderson, 1932;Stewart, 1949). Somecrystals are not onlv replacedby polyhalite, but have rims of an

Frc. 5. Photomicrographs of polyhalite replacing euhedral gypsum crystals; interstitial material is also polyhalite. X40, plane polarized light on left, crossed polarizers on right. opaquewhite mineral that also appearsas a cement betweenthe grains along with polyhalite. At high magnification this material is found to be composedof very small (lessthan 0.ip) equant grains of high refractive index and high birefringence,but it has not been identified. The occasionaldetrital grains of plagioclase,hornblende, and biotite are unaltered.

BnrNB eno Cnysrar, Eeurr,rBRrA The question arises as to why calcium sulfate was deposited first as gypsum and then altered to polyhalite. Can it be explained on the basis of equilibrium with a brine of changing composition, or does one have to assumethat the gypsum was crystallized metastably and then converted to a more stable polyhalite? Considerableinformation is available on the equilibrium relations of calcium sulfate minerals in the sea water system. Although at 25o C. 106 WILLIAM T HOLSER gypsum is the stable cr1'stalin equilibirum with sea water concentrated by a factor of up to 5.5X, anhydrite is the stable mineral at higher sea salt concentrations,at least up to 11.5X, where halite starts to precipitate (e.g.,D'Ans et al., 1955).On the other hand, Iaboratory experiments and field observations indicate that the cr1'stal actuall.v growing from thesebrines is usually metastablegypsum, even along with halite. The experimentsby Van't Hoff and D'Ans (D'Ans, 1933, see also, Autenreith, 1958; Braitsch, 1962) show that pol1'haliteis the form of calcium sulfate at equilibrium by the time the sea salt is concentrated enough to precipitate potash-magnesiasalts at about 63X. In a much- quoted calculation, Janecke (1912) placed the transition anhvdrite- polyhalite at abor-rt22X seasalt concentrationbut there seemsto be no good data on which to basesuch a calculation.IIowever, it is still rather certain that the transition takes place somewherein the halite facies. After the simple evaporation of sea water to this stage of evaporation, there is not much sulfate availableto form polyhalite, it all having been precipitated previously as gypsum or anhydrite. However, if additional sea water or sulfatic meteoric water is added to the system, sulfate should precipitate. Experiments b-"-Lepeshkov and Novikova (1958) indicate that, unlike anhydrite, polyhalite is easily cr1'stallized when such an addition of sulfateis made to the concentratedsea water. There is, therefore, Iess chance for the metastable crystallization of gypsum from these highly concentratedbrines. This suggeststhat at Laguna Ojo de Liebre the brine has becomemuch more concentratedsince the original crystallizationof the gypsum, lead- ing finally to replacement by poli'halite. The conversion of gypsum or other sulfatesinto polyhalite,within the stability rangeof the latter mineral,has been shown experimentally by Van't Hoff (1912). It is interesting to note that no trace of anhydrite was found in any sedimentsat Laguna Ojo de Liebre, despitea diligent searchfor it. Evidence for a change in concentration of the brine since crystallization of the main part of the evaporite is also shown by studies of bromide geochemistry.The pioneeringstudies of D'Ans and Kuhn (1940) and much subsequentwork in Germany, the Soviet Union, and elsewhere, have proved that the bromide in solid solution in halite is a direct mea- sure of the stage of sea salt concentrationin the brine from which the halite cry-stallized. The distribution coefficient for bromide between crystal and brine, based on measurement of both artificial and natural solutions,is such that the first halite crystalsto grow from the present- day sea water should have a bromide content of about 70 ppm. As evaporation proceeds,with or without additions of new sea water, the major part of the bromide, rejected from crystallization with the salt, DIAGENETIC POLYIIALITE 107 builds up in concentration.Consequently, the bromide content of new salt crystalsis higher; and at the point wherepotash-magnesia minerals begin to crystallizeit is about 250 ppm. At Laguna Ojo de Liebre, sam- ples of halite near the polyhalite locality analyzed 51 and 92 ppm. Brine that flowed from the salt into the hole from which these sampleswere taken gave the analysisshown in Tabie 1, column 4. Not only the bromide, but alsothe mzrgnesiumand potassiumcontents of the brine (rela- tive to chloride) indicate a sea salt concentrationof 60X, nearly to the potash-magnesiafacies. A brine o{ this compositionshould be in equilibrium rvith halite containingabout 270 ppm Br. In a seriesof pubiications,Valiashko (1951; 1959)has emphasizedthe change of ph1'sical conditions that occurs when an evaporite is far enough along in its courseof crystallizationto form a solid but porous crystalline bodi.. He suggeststhat the processstabilizes at this point, with Iittle further evaporation, awaiting tectonic or other changes. Valiashko has calculated that such a stage is reached with about 50 per cent porositv, near the beginningof the potash-magnesiafacies, and I suggest that lossof brine bv reflux (Scruton, 1953)may bring on consolidationof the evaporitesat an earlier stage.AIso evaporation may stili be important from this aggregate,with the assistanceof capillary movement of brine upward through the pores.This may result in a higher stageof evaporation of the brine, with the depositionof only a small quantity of late- stagehalite around the pores.At Laguna Ojo de Liebre, the occurrenceof pol.vhalite in a band cementing the loose sediments just under the surface of the salt flat suggestssuch a caliche-typedeposit. The high-Br halite that shouldbe associatedwith the brine might be presentin only a small amount that was overlooked in our sampling. Similar apparent disequilibriaof bromide between crystal and brine have beenfound b1' analysesof brine inclusionsin halite, of Permian age from Kansas and Silurian ase from Ontario. These mav have a similar origin (Holser, 1962).

GBor,ocrcar,SrcnrlrceNcB Much of the polvhalite in the geologicalsection has long been recog- nized as a replacementof gypsum or anhydrite, becauseof pseudomorphic or other replacement textures. The early experiments of Van't Hoff sup- ported the feasibility of the replacement reaction. The replacement origin has been well documented in many occurrences,from the Permian of New Mexico (Schaller and Henderson, 1932), England (Stewart, 1949),and Germany (Schulze,1960), and in the Miocene of Ukrainian SSSR(YArzhemskii, 1949).In all of theselocalities the rocks have been deeply buried since deposition, so that calcium sulfate is also present as WILLIAM T. IIOLSER anhydrite rather than g1'psum.Pseudomorphs after ear'lv gypsum are common, having been replacedb1' anhl'drite, halite or poll'halite. Poly- halite has aisobeen found in lake bedsin Central Asia, of Tertiary (Voro- nova, 1954)and Pleistocene(Norin, 1942) age,but detailsare lacking. That the replacementof gvpsum or anhydrite b1.poil'halite may have beendiagenetic was suggested by Stewart (19+9),Schaller and Henderson (1932), and Schulze(1960), as well as in the reviews bv Lotze (1957), and Borchert (1959). However, all of these r,vriters,except Schulze,still have somedoubt as to whether the replacementby polyhalite ma1'have taken placeat a later time, after consolidation.Poll'halite diagenetically replacinggypsum at Laguna del Ojo de Liebre lends credenceto a dia- geneticorigin for polvhalite in theseearlier rocks. It aiso minimizes the necessitvfor an intermediatestage of anhl.drite or bassanite,which was suggestedby Dunham (7919), Schaller and Henderson (1932) and Braitsch (1962),without anv direct evidencefor either mineral. Such an intermediatestage, for which Stewart has considerabieevidence in other transformations,is apparently not alwal's necessar)/. The relations between brines and evaporites at Laguna del Ojo de Liebre also suggest that some alterations to poll-halite, and perhaps other replacements,may be causedby a ver1,'localized change in brine composition,similar to calicheformation, without requiring the devel- opment of an extensivebody of concentratedbrine. Therefore,an occurrence of polyhalite in evaporite rocks has less significanceas a lead to possiblenear depositsof solublepotash minerals,than might otherwise have beensupposed.

Rprrntncps

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M anuscr'i.pl,receiaed, A pril, 1, 1965; accepted,for Publieation, October6, 1965.