American Mineralogist, Volume 62, pages I0l8-1029, 1977

Metastability in serpentine-olivine equilibria

MrcHe,rl A. DuNc^c.N

Geology Branch, NASA Johnson Space CenterHouston, Texas 77058

Abstract

Phase equilibrium considerations derived from studies of progressively metamorphosedserpentinites combined with a theoretical analysis of the system MgO-SiOs-H2O by Evans elal. (1976) indicate that antigorite is the stable phase in olivine-serpentine reactions. Corre-sponding serpentine-olivine equilibria in which chrysotile is the serpentine phase are meta-stable relative to the antigorite analogs. I suggest that this conclusion can also be applied toreactions involving lizardite and olivine.

Metastable serpentine-olivine equilibria are inferred to be the result of combinations ofcrystallographic, energetic, and kinetic factors that vary for different reactions. The contrast-ing crystal structures of the serpentine minerals probably play an important role in determin-ing reaction paths where kinetics are unfavorable to the attainment of stable equilibrium; e.g.,low temperature hydration or static dehydration. The role of crystal structure is manifested inthe relative ease of nucleation of the various phases. Antigorite nucleation is hampered by itscomplex structure whereas lizardite-olivine reactions are favored by a topotactic relationship.Chrysotile is favored by crystallization from gels and oxides in synthetic systems and indilation veins in serpentinites.

Introduction

This study is an outgrowth of a broader structuraland petrologic project on ultramafic and mafic rocksin the Sultan-Darrington area, North Cascades,Washington (Dungan, 1974), These studies reveal acomplex metamorphic and structural history for theSultan-Darrington ultramafites (Vance, 1972;. Dun-gan and Vance, 1972). Yance and Dungan (1977)discuss the petrology of typical metaserpentinites,and demonstrate that prior to final tectonic emplace-ment the Sultan-Darrington primary peridotites werefirst serpentinized and then subjected to a progradeevent which reached middle amphibolite facies. Acomplete gradation from incipiently recrystallized liz-ardite-chrysotile serpentinites through antigorite ser-pentinites to second-generation peridotites is present.Subsequent to emplacement, the serpentinites andperidotites were intruded by Tertiary epizonal plu-tons which produced small contact aureoles of lim-ited extent and distinctive petrography.

New data from Sultan-Darrington metaserpenti-nites presented in this paper are petrographic descrip-tions and mineral analyses of some unusual rocks inwhich metastable deserpentinization reactions occur.

These include rocks metamorphosed in the pre-em-placement event and by Tertiary intrusions. The ser-pentinization of olivine and a number of experimen-tal studies of olivine-serpentine equilibria are alsodiscussed. The purpose of this paper is to identifysome of the causes of non-equilibrium behavior inserpentinites and in relevant experimental studies.

The following section is a brief review of the basisfor determining metastable and stable reactions inmetaserpentinites.

Phase relations

Evans and Trommsdortr (1970) recognized thatprogressive metamorphism of serpentinites results ina regular sequence of mineral assemblages whose oc-currence is governed by a small number of reversibledehydration reactions. This sequence begins in theprehnite-pumpelleyite facies with the incoming ofantigorite as a reaction product of lizardite andchrysotile. Although the reaction to antigorite tendsto be sluggish, lizardite and chrysotile are both re-crystallized to antigorite prior to the appearance ofnew-formed olivine under equilibrium conditions.Antigorite continues to be the serpentine phase which

l 0 l 8

DUNGAN: SERPENTINE-OLIVINE EOUILIBRIA l 0 l 9

participates in dehydration reactions up to the middleamphibolite facies:

antigorite * brucite = forsterite + HrO (1)

antigorite a.: forsterite * talc + HrO (2)

Under equilibrium conditions, serpentinization re-actions should be the retrograde equivalents of thedehydration reactions. In fact, there is a lack ofcorre-spondence between the serpentine mineralogy of theretrogressive pseudomorph-forming reactions andthe reactions operative in progressively metamor-phosed serpentinites. Trommsdorff and Evans (1972)suggest that several serpentine reactions (and experi-mentally-determined equilibria) in which chrysotile isthe serpentine phase are metastable with respect tothe stable antigorite analogs. An expanded theoreti-cal analysis of this premise in Evans et al. (1976)follows in part from the petrographic observationthat the reaction

chrysotilec antigorite * brucite (3)

is apparently a reversible equilibria in natural ser-pentinites. The basis for the theoretical approach pre-sented in Evans et al. (1976) is the Mg(OH), defi-ciencyl in the structural formula of antigorite ascompared to the "ideal" serpentine(s) chrysotile (andlizardite). As a result of this chemography, vapor-absent reaction (3) defines the upper stability limit ofchrysotile with respect to antigorite.

According to Wicks and Whittaker (1975), the var-ious structural types within flat-layer serpentines andcylindrical serpentines can be regarded as polytypesof lizardite and chrysotile respectively. They cite theapparently overlapping compositions and equivalentstructural formulae as criteria for regarding lizarditeand chrysotile as polymorphs. However, the mutualphase relations remain obscure because these twominerals typically occur together in serpentinites, andthey have been produced simultaneously in someexperimental studies (Chernosky, 19751' Moody,1976b). Chernosky (1971) has pointed out that thetwo-phase assemblage is metastable (except at thetransition temperature) if the coexisting phases haveidentical compositions and are indeed true poly-morphs (i.e., the reaction boundary is characterizedby a polymorphic transition). Alternatively, preferen-tial partitioning of Al into lizardite with a resultantcompositional gap between the phases would pre-

I The extent of departure of the antigorite formula fromMg,SLO.l OHl. is inversely proport ional to the a-axis dimen-sion, as the anomalous bridge structure is repeated along this axis.

clude a polymorphic transition for aluminous bulkcompositions.

The aspect of the occurrence of lizardite * chryso-ti le which is crit ical to the present study is that bothphases react to antigorite at metamorphic grades be-low those at which reaction (l) takes place. There-fore, the lizardite analog of reaction (3) defines theupper stabil ity l imit of Al-poor l izardite, although itis not known if this is a reversible reaction. Lizarditeand chrysoti le in shear planes that cut anti-gorite-olivine metaserpentinites in the Sultan Com-plex, and similar occurrences elsewhere (Coleman,1971; Mumpton and Thompson, 1975) imply real , i fpoorly understood, stabil ity f ields for both phases atlow temperatures. However, only direct replacementof antigorite, reported for chrysoti le by Evans et al.(1976), constitutes evidence for reversibil i ty of thetype (3) reaction. For the purpose of discussion, de-hydration-hydration reactions in which low-Al l iz-ardite is a (the) serpentine phase are treated as meta-stable due to the observed breakdown of this phase toantigorite prior to the first appearance of olivine byreaction (1). However, it should be emphasized thatl izardite phase relations are by no means well-de-fined.

The serpentinization of olivine andthe formation of mesh pseudomorphs

In contrast to the granoblastic-polygonal or lepi-doblastic textures characteristic of olivine-antigoriteassemblages, the textural relationship between olivineand mesh serpentine is indicative of retrogressive re-placement. The init ial step in the formation of thepseudomorphs is the growth of cross-fiber veinlets(mesh rims) along the orthogonal parting of the oli-vine. Where serpentinization continues beyond therim stage, the remaining isolated grains of olivine arereplaced by roughly equant mesh centers. Wicks andZussman (1975) have shown that the predominantserpentine phase in mesh pseudomorphs is l izardite;where chrysoti le is present, it occurs in subordinateamounts.

Temperatures for serpentinization derived fromoxygen and hydrogen isotope studies (Wenner andTaylor, 1971, 1973,1974) suggest that the alterationof olivine may be accomplished by meteoric waters at85-115"C. The composi t ion of spr ing waters issuingfrom serpentinites confirms the viabil ity of near-sur-face, low-temperature serpentinization (Barnes andO'Neil, 1969), as does a study by Campbell (1975).Thus, serpentinization appears to occur at temper-atures below the stabil ity f ield of forsterite * HrO so

1020 DUNGAN: SERPENTINE-OLIVINE EOUILIBRIA

that it is strictly retrogressive. Retrogressive anti-gorite is not common, and it has not been observed toform pseudomorphs after olivine. At one locality inthe northern Sultan Complex the assemblage anti-gorite * brucite occurs in veins and shear zones(generally annealed) cutting peridotites formed bydeserpentinization. This retrogressive antigorite is in-terpreted as an example of serpentinization of olivinenear the reaction boundary of(l), suggesting that it isindeed a reversible reaction.

Simkin and Smith (1970) recognized that MnO andNiO in olivine correlate very strongly with Mgl(Mg* Fe); manganese decreasing with increasing Mg andnickel increasing. These linear trends are present inolivines from fresh and partly serpentinized pe-ridotites (Dungan, 1974; Dungan, unpublished data).In contrast, the compositions of olivines formed bydeserpentinization exhibit marked deviations fromthe compositions of primary olivines in terms ofMel(Mg * Fe) as well as MnO and NiO. Metamor-phic olivine in incipiently dehydrated serpentinites isenriched in MnO (by as much as an order of magni-tude) and depleted in NiO relative to olivine inprimary peridotites (Vance and Dungan, 1977).These olivines may also be more magnesian (Forr_ru)than is characteristic of primary olivines. The lack ofchange in primary olivine composition in response toserpentinization indicates chemical exchange equilib-rium is not attained. These compositional character-istics imply that in serpentinization, the reaction is aboundary phenomenon in which the olivine plays anessentially passive role, the serpentine growing at itsexpense.

Metastable deserpentinization

Several examples of unusual metamorphosed ser-pentinites from the Sultan-Darrington ultramafitescontain olivine which has formed directly from oneor both of the low-temperature serpentines. Sample7l-104 was collected from a (25 X 10 m) tectonicsliver of serpentinite which occurs approximately30 m from the contact of a small (50 X 50 m) post-tectonic granodiorite pluton. Although there is nomesoscopic indication of recrystallization of the ser-pentinite, the dehydration reaction in 7l-104 is ap-parently the result of thermal metamorphism inducedby this pluton, as effects of the pre-emplacementmetamorphic event are absent from the rocks of thesouthern Sultan Complex. The thin section studied isof a serpentinite derived from an olivine-rich wehrliteparent, in which the boundaries of the original olivineand clinopyroxene grains are clearly outlined by mag-

netite segregations. The mesh pseudomorphs aredominated by the vein component (var. ribbon meshafter Francis, 1956). Microscopic brucite is inter-grown with the mesh serpentine as small veinlets, butantigorite was not recognized in thin section or in X-ray diffraction patterns of serpentine mineral sepa-r ates.

Although the mesh serpentine (see Table I forcomposition) has not been recrystall ized, it is 30-70percent replaced by small prismatic olivine grains(Fig. 1a, b). As seen in thin section, individualpseudomorphs include up to 100 olivine grains whichexhibit dimensional parallelism and nearly simultane-ous extinction, indicating that all these olivine grainsshare a single optic orientation. Nonetheless, theyappear to be discrete gains rather than the two-di-mensional expression of larger skeletal olivines. Oli-vine grains within adjoining pseudomorphs are char-acterized by a different aggregate orientation.Although the olivine has replaced all textural com-ponents of the mesh serpentine, nucleation appears tobe favored at vein margins; magnetite grains also actas nucleation sites. Bastite serpentine in this samplelacks olivine. This may be due to its more aluminouscomposition or the absence of intergrown brucite.The chemical composition of olivine in 7l-104 is dis-tinct from any possible primary olivine in terms ofboth major and minor elements (Table I ). Vance andDungan (1977) have shown that the combination oflow iron and NiO-contents and the high MnO whichcharacterizes the 7l-104 olivines is indicative of oli-vine compositions formed by deserpentinization.

Sample 04-124 is a largely serpentinized cumulusperidotite consisting of alternating pyroxene-rich andolivine-rich layers. It was collected from a small tec-tonic slice in the northwestern Sultan Complex. De-tailed study of the petrography of samples from thisbody has shown that the effects of the pre-emplace-ment metamorphism are only sporadically present,and fully recrystall ized rocks containing equil ibriummineral assemblages are absent. Antigorite is presentin many partially serpentinized wehrlites as veinletsor as a partial replacement product of mesh serpen-tine. In sample 04-124 the clinopyroxene is partiallyserpentinized, the olivine and orthopyroxene havebeen totally replaced by serpentine, and the meshserpentine has in turn been in part replaced by new-formed olivine (Fig. lc). These metamorphic olivinescrosscut mesh structures, and are texturally distinctfrom relict olivine grains in that they are concen-trated in the vein component of the mesh rather thanthe centers and typically exhibit highly irregular,

DU NGAN : SERPENTINE-OLIVINE EQUILIBRIA

Table l. New-formed olivines and coexisting serpentines

t02l

Mesh

Sampl e 7l - l 04 7l -1 04

Bast .

7t -t 04 04-124

Mat r ix Ve in Ant ig .

t 4 l 4 t 4 t 4 1 4 1 4

Chry .' 1 4 1 4

71-62

s i o z 4 1 . 7

A l z 0 : n . d .

C r z 0 : n . d .

F e O 2 . 1

M n O 0 . 5 0

M g O 5 4 . 6

C a O 0 . 0

N i o . 0 . 0 4

42.4 40.70 . 4 4 2 . 30 . 0 1 0 . 5 13 . 0 2 . 60 .04 0 .02

4 1 . 0 4 0 . 0p . d . n . d .0 .03 0 .02

4 0 . 8 4 1 . 7

n . d . n . d .

n . d . n . d .

5 . 1 2 . 5

I . 0 5 0 . 0 9

5 i . 7 5 4 . 9

0 . 0 5 0 . 0 4

0 . 0 5 0 . 0 2

41.4 41 .4

n , d . n . d .

n . d . n . d .

6 . 7 6 . 4

0 . 2 3 0 . 2 4

5 l . 3 5 l . 6

0 . 0 0 0 . 0 0

U . J f , U . J O

43.9 42 .6

U . Y Z Z . O

0 . 0 0 0 . 0 2

2 . 5 2 . 2

0 . 0 2 0 . 0 5

40.4 39 .6

n . d . n . d .

0 . 2 0 0 . 1 8

Tota l

s i 0 .998AICrFe 0.042Mn 0 .010M9 1 .949U d

N i 0 . 0 0 1

3 .979 3 .8400 .049 0 .2570 .001 0 .0380 .232 0 .2070.003 0.0025 .73 t 5 .667

0 .002 0 .002

0. 996 I .003

0 .050 0 . 1 360.002 0.005' I . 955 I . 8520 . 0 0 10 .000 0 .006

I . 0 0 2 3 . 9 3 7 3 . 9 5 8- 0 . 0 9 7 0 . 2 7 9

0. 001

0 . 1 2 9 0 . 1 9 0 0 . 1 7 2

0 . 0 0 5 0 . 0 0 2 0 . 0 0 4

1 . 8 6 2 5 . 3 9 9 5 . 4 7 3

0 . 0 0 6 0 . 0 l 4 0 . 0 1 3

8 7 . 38 7 . 986.78 6 . 9o e o

Cati ons*

' 100 .0 100 .0

0 .996

0 . I 040 .0221 . 8 8 10. 0010. 00i

To ta l 3 .000 9 .997 10 .013 3 .005 3 .008 3 .003 3 .004 9 .639 9 .900

Mg/(Mg+Fe) 97 .9 96 .1 9 7 . 5 93. I 0 7 n9 6 . 69 3 . 59 4 . 8

*OLiuines ealeulated on the basts of 4.0 oaggens" 7414 antigoz"ite bn the basis of 13.026,otygents and rematning setTtentinee on 74.000.

n.d. = not arnlysed,

branching shapes. As in 7l-104, new-formed olivineis lacking in bastites. They are similar to the 7l-104olivines in that they follow the compositional patternof low FeO and anomalously high MnO (Table l).Mesh and bastite serpentines in 04-124 also exhibitcompositional characteristics similar to those in 7l-104 (i.e., mesh : 0.75 percent Al2Os, 0.12 percentCrzOs us. bastite = 0.6 to 1.0 Al2Oe, 0.2-0.8 percentCrrO'). In addition, all the olivine grains which occurwithin an individual mesh pseudomorph share thesame optic orientation, whereas those in adjacentpseudomorphs are oriented differently. However, thissample contrasts with 7l-104 in that fine-grainedantigorite is present as rims around the new-formedolivines, which complicates interpretation of theparagenetic relationships in this sample relative to 71-to4.

Tectonic slices of ultramafic rock in the northern

Sultan Complex exhibit progressively higher degreesof recrystallization and grade of metamorphism tothe east. Metaserpentinites in the northern SultanComplex (all metamorphosed prior to middle orearly Tertiary emplacement) include antigorite ser-pent in i tes , par t ia l l y deserpent in ized o l i v ine-antigorite rocks, and fully dehydrated metamorphicperidotites. The Darrington peridotites, farther to theeast, record the highest grade of metamorphism.They are generally completely deserpentinized andare characterized by the assemblage forster-ite-talc-tremolite. Some high-grade metaserpenti-nites in both the Sultan and Darrington units exhibittextures which may indicate that the early stages ofdeserpentinization may have resulted in olivine form-ing directly from mesh serpentine as in 04-124. Incontrast to many of the Sultan-Darrington metaser-pentinites, in which deserpentinization and recrys-

t022 DUNGAN: SERPENTINE-OLIVINE EOUILIBRIA

Fig l . Three examples of new-formed ol iv ine f rom Si l tan-Darr ington metaserpent in i tes. Al l microphotographs taken wi th part ia l lycrossed polars

a. Sample 7l'104. Cross-fiber veins ofmesh serpentine oriented vertically, roughly parallel to magnetite segregations, are partiallyreplaced by pr ismat ic grains of new-formed ol iv ine or iented normal to vein d i rect ion (paral le l to f iber d i rect ion of the veins)

b. sample 7l-104. Prismatic olivine grains replacing vein and mesh center components.c Sample 04-124 New-formed olivine and antigorite have partially replaced mesh serpentine. Unrecrystallized mesh serpentine in

lower lef t -center ; o l iv ine is l ight colored, and ant igor i te forms a f ine-grained f ibrous mat around the ol iv ine. Note the unusual rectangulargrain of olivine in right center which surrounds an unrecrystallized mesh center

d. Sample l4l4 Bifurcated chrysotile-olivine veinlet in antigorite-olivine-magnetite matrix. Linear segregation of magnetite in lowercenter is refracted at vein margin by di lat ion so that i t is of fset normal to the growth di rect ion of the chrysot i le-ol iv ine assemblage

e- Sample 1414 Bifwcalion in chrysotile-olivine veinlet Note curvature in olivine grains caused by rotation during growth Olivinecurvature is parallel to chrysotile fibers

f Samp le l 4 l 4 De ta l l o f o l i v i neg ra i nsa tmarg ino f ve in l e t .Theg ra inon the le f t i sa ra reexamp leo fo l i v i nenuc lea t i ononap reex i s t i ngmatr ix o l iv ine erain.

tall ization were synkinematic, relict textures such asrelict unrecrystallized lizardite bastites in associationwith antigorite plus new-formed olivine (see composi-tion of 7I-62 in Table I ) provide strong evidence forstatic metamorphism (Vance and Dungan, 1977).The metamorphic olivine in these statically metamor-phosed rocks occurs as large mosaic grains comprisedof many small individuals which have nearly the sameoptic orientation (Fig. 2a, b). In some examples theboundaries of these composite grains coincide withmagnetite segregations marking relict mesh pseudo-morph outl ines as well as the boundaries of the reli"rtbastites. Thus, they appear to be higher grade equiva-lents of metaserpentinites (e.g., 04-124 or 7l-104) inwhich new-formed olivine grew directly from liz-ardite in mesh serpentine rather than from antigorite* bruc i te ( react ion I ) .

Sample 14l4 is an antigorite-forsterite metaser-pentinite from the Darrington peridotite that is pet-rographically distinct from the samples discussedabove. It consists primarily of a fine-grained aggre-gate of antigorite intimately intergrown with smallerneedles of new-formed olivine (0.05-0.1 mm inlength). The features of interest are several sub-parallel veinlets (l-0.5 mm in width) consisting ofintergrowths of chrysoti le f ibers and olivine (identi-f ication of chrysoti le was made by SEM). The olivineoccurs as tablets and needles otiented normal to themargins of the veins. The long-dimensional axes of

DUNGAN: SERPENTINE-OLIVINE EQUILIBRIA 1023

the olivines are oriented parallel to the chrysotilefiber directiolr, and where the olivine occurs as smallneedles, the two phases are intimately intergrown.This textural relationship is the result of the simulta-neous growth of the two phases at the time of forma-tion of the veins. Although olivine has nucleated onpreexisting matrix olivine grains where the vein mar-gins transect them (Fig. 1f), the scarcity of olivine inthe matrix antigorite indicates that most of the oli-vine in the veins did not form in this manner. Linearsegregations of magnetite which intersect the veins atoblique angles are offset parallel to the chrysoti lefiber direction (Fig. ld), indicating that the veinsformed by dilation rather than in sila replacement ofthe preexisting antigorite-olivine host. Microprobeanalyses (Table l) of serpentine-olivine pairs in theantigorite matrix and in the veins show that the oli-vines have similar Mg/(Mg * Fe), as do the twoserpentines. This relationship suggests that both oli-vine-serpentine pairs achieved at least an approachto Mg-Fe exchange equil ibrium under similar condi-t lons.

The olivine in the three metaserpentinites describedabove is interpreted to have formed by metastabledeserpentinization reactions within the stability fieldof antigorite or antigorite * forsterite. Differences inserpentine mineralogy among the samples indicatethat the olivine formed via various reaction paths. Insample 7l-104 the reaction is

Fig 2a Sample 7l-62 Relict lizardite bastites in a new-formed peridotite. Bastites include magnetite segregations parallel to pyroxenecleavage Note re l ic t chromite grains in r ight center of photograph. Matr ix consists of granular o l iv ine (Forr) p lus minor ant igor i te andback-react ion serpent ine. Plane polar ized l ight

b. Sample 71-62 Detai l under part ia l ly crossed nicols showing mosaic of o l iv ine grains which have replaced indiv idual meshpseudomorphs. Al l the smal l grains wi th in the boundar ies of s ingle mesh pseudomorph have near ly the same opt ic or ientat ion. Grains atupper lef t at ext inct lon contrast wi lh grains in lower lef t and center r ight . Note the sharp boundar ies between serpent ine in bast i tes andthe new-formed ol iv ine.

t024 DUNGAN: SERPENTINE-OLIVINE EQUILIBRIA

l izardite (t chrysotile) * brucite

e forsterite + HrO.

Sample 04-124lacks brucite, but antigorite is present.Olivine may have formed as in 71-104 until brucite inthe rock was exhausted, with the antigorite formingindependently from lizardite and/or chrysotile. Al-ternatively, olivine formation may have proceeded bythe lower-temperature metastable reaction

lizardite or chrysotile e antigorite

* forsterite + HrO (4)

The close association of the antigorite with the new-formed olivine suggests that (4) was operative.

The reaction which produced the vein assemblagein l4l4 is impossible to discern because the vein didnot form by direct replacement of a preexisting as-semblage. Textural relationships are equivocal, al-though the mineral compositions suggest that theveins formed at the same temperature as the anti-gorite-olivine matrix.

Stability and metastability in experimental studies' of serpentine phase relations

Johannes (1975; see also in Evans e, al., 1976) hasproduced experimental brackets for (2) at waterpressures from 2 to 15 kbar. This study is the firstexperimental reversal of an antigorite-dehydrationreaction. As noted in Evans et al. (1976), the temper-atures at which reversals were determined are com-patible with temperatures for this reaction inferredfrom field-petrologic studies (Trommsdorff andEvans, 1972).ln previous experimental studies, anti-gorite was not synthesized because serpentine miner-alogy is strongly dependent on the nature of thestarting materials. Salient points of previous experi-mental studies are briefly summarized below.

(l ) Most early experimenters employed mixtures ofoxides or gels as starting materials with the result thatchrysotile was formed rapidly and generally to theexclusion of other phases (Bowen and Tuttle, 1949;and others). Due to the apparently fast reaction rates,short run-times were considered adequate for attain-ment of equi l ibr ium.

(2) Johannes (1968) employed crystalline startingmaterials (serpentine phase : synthetic chrysotile)and discovered that the previous attempts to bracketreaction boundaries were hampered by disequilib-rium crystallization of chrysotile in the stability fieldsof forsterite + HrO. These synthesis experiments re-sulted in estimations of reaction temperatures which

were 50-l00oC too high (see Fig. 2 of Moody,1976a). Similar results were obtained by Scarfe andWyllie (1967) and Chernosky (1973) for the reactionchr : fo * ta * H2O. Chrysotile was the syntheticserpentine phase present in the reactants and prod-ucts of experiments conducted by Johannes andChernosky as well as Scarfe and Wyllie.

(3) Dietrich and Peters (1971) attempted to definethe phase relationships between chrysotile and anti-gorite, using natural serpentines as starting materials.Despite extremely long run times (up to 270 days), nochange occurred in the proportions of the mineralsafter hydrothermal treatment at temperatures below490"C. At higher temperatures and with run times>60 days, chrysotile disappeared, and forsterite andtalc appeared. Antigorite may have grown at theexpense of chrysotile, but it was only present in theproducts of runs in which it was also included in thestarting mix. Thus, there is no direct confirmationthat antigorite formed from chrysotile, even thoughexperiments were conducted within the stability fieldof antigorite.

(4) Syntheses ofantigorite by Iishi and Saito (1973)were achieved by using starting gels which weredeficient in MgO and HrO relative to Mg.Si4Olo(OH)8. Experiments were conducted within thestability field of antigorite (300-500"C). Antigoritewas formed in subordinate amounts relative tochrysotile, its abundance increasing with increasingdegree of MgO and H2O deficiency and increasing Pand T. Apparently antigorite grew from theMg(OH)z-deficient residuum which remained afterMgO, SiOr, and HrO had been exhausted in the pro-portions necessary for the stoichiometry of ideal ser-pentine by the formation of abundant chrysotile(tminor l izardi te).

(5) Moody (1976b) performed olivine-hydrationexperiments in which natural and synthetic olivinesplus HzO were reacted under conditions of bufferedoxygen fugacity (iron-magnetite at 300-400"C and0.5-2.0 kbar). These runs resulted in the formation oflizardite * brucite * magnetite by direct replacementof olivine. Examination of the run products by scan-ning electron microscope showed that with increasedrun times chrysotile fibers replaced lizardite platelets.Synthetic olivine plus serpentine and brucite formedby the hydration of olivine were used as reactants inattempts to reverse the reaction, with the result thatolivine grew from the hydrated phases.

(6) The occurrcnce of lizardite in natural and syn-thetic systems may also be in part dependent oncomposition. Roy and Roy (1955), Gillery (1959),

DUNGAN : SERPENTINE-OLIVINE EOUILIBRIA

and Chernosky (1975) have synthesized chryso-t i le - l i zard i te mix tu res in the sys tem MgO-AlrOs-SiOr-HrO, and noted the tendency for in-creased Al-content to favor formation of greater pro-portions of lizardite. The crystallographic rationalefor this behavior was first recognized by Radoslovich(1963), who suggested that substitution of Al in liz-ardite reduced the mismatch in size between the oc-tahedral and tetrahedral sheets, thereby reducingintralattice strain and enhancing lizardite stability(see also Chernosky, 1975 and Wicks and Whittaker,1975). The tendency for greater thermal stability inAl-lizardite is expressed by the persistence of alu-minous lizardite bastites in metaserpentinites of theSultan-Darrington area (e.g., Fig. 2a, b). The restric-tion of these relict bastites to statically recrystallizedmetaserpentinites suggests that this mode of occur-rence could reflect metastable persistence. However,the role of compositional effects on serpentine phaserelations is a subject which requires study, as it maybe one of several critical factors which determinetheir mutual stability fields.

Discussion

Given the assumption that reactions ( I ) and (2) arestable equilibria and that the analogous reactions inwhich the serpentine phase is lizardite or chrysotileare metastable, causes for the metastability have beenexamined for both natural and synthetic occurrences.Four modes of occurrence are discussed: (l) per-sistence of lizardite and chrysotile into the stabilityfield of antigorite in progressively metamorphosedserpentinites; (2) formation of the low-temperaturephases in experimental studies within the antigoritestability field; (3) the hydration of olivine to formmesh pseudomorphs; and (4) the dehydration of liz-ardite and chrysotile to form olivine. The widespreadmetastability associated with serpentine equilibria isinferred to be at least in part the result of a combina-tion of crystallographic, energetic, and kinetic fac-tors.

Although antigorite appears in metaserpentinitesat low metamorphic grade, lizardite and chrysotilecommonly persist as metastable relics to muchhigher metamorphic grade than the first appearanceof antigorite (upper prehnite-pumpelleyite facies orlowest greenschist facies). Evans e/ al. (1976) suggestthat this sluggishness is largely attributable to thesmall AS associated with the water-absent antigorite-forming reactions.

The low AS inherent in the antigorite-forming re-action apparently promotes thermal overstepping in

experimental studies as well. Here the short run timesand lack of strain energy allow chrysotile or lizarditedehydration reactions to proceed by virtue of therelatively large AS associated with the liberation ofHrO-vapor. The study of Dietrich and Peters (1971)in which antigorite failed to form from a chrysotile-only starting mixture, despite relatively long runtimes ()60 days), il lustrates the problem of formingantigorite from another serpentine phase.

The thermal regime associated with the formationof olivine in sample 7l-104 is a natural analog of theexperimental studies which record serpentine dehy-dration without antigorite-formation. In this ser-pentinite, olivine was formed by purely thermal meta-morphism induced by a small pluton, which would beunable to maintain high temperatures in its aureolefor a prolonged period. The relatively rapid rise intemperature produced by an igneous intrusion intocold country rocks more nearly simulates raising thetemperature of an experimental charge several hun-dred degrees from room temperature than does thethermal regime associated with regional metamor-phism. Apparently the serpentinite at this locality washeated so rapidly above the temperature required forthe reaction lizardite (1 chrysotile) * brucite s for-sterite + HrO that the dehydration reaction occurredbefore antigorite was able to nucleate and grow.

Thermal overstepping of reaction boundaries oc-curs in all metamorphic equilibria because the activa-tion energy associated with the nucleation of the newphase(s) must be overcome before crystallization cancommence. Consequently the magnitude of the acti-vation energy is important in low AS reactions inwhich one serpentine phase forms from another. Inthe case of the less kinetically favorable retrogradereactions, its importance may be even greater. I pro-pose that the relative ease of nucleation of the variousserpentine phases plays a very important role in pro-ducing metastability in serpentine-olivine equilibria.The effect is pronounced because the complex crystalstructure of antigorite acts as a nucleation barrier(i.e., large activation energy), while the topotacticnature of intergrowths between chrysotile and olivineand especially lizardite and olivine result in a loweractivation energy and greater relative ease of nucle-at ion.

The distinctive antigorite crystal structure is char-acterized by (l) an unusually long a dimension (20-l20A) and (2) a curved "alternating half-wave"structure (Kunze, 196l ). The half-waves are joined atan inflection point by a unique bridge structure whichis the cause of Mg(OH), deficiency in antigorite. The

1026 DU NGAN: SERPENTINE-OLIVI N E EQUILIBR]A

tetrahedral sheets on either side ofthe inflection haveopposite polarity. Kunze (1961) recognized that thebridge structure represents a barrier to the nucleationof antigorite, because the nuclei must consist specifi-cally of the unique bridge structure from which thehalf-waves spontaneously radiate outward. Nuclei inwhich a single tetrahedral polarity occurred would beforced to propagate as a stoichiometric serpentine,either as a cylindrical (chrysoti le) or as a flat-layerstructure (l izardite).

Olivine-hydration experiments by Moody (1976b)demonstrate preferential replacement by l izardite.This tendency is also characteristic of mesh pseu-domorphs. Although mesh pseudomorphs are multi-phase assemblages (l izardite t chrysoti le t brucite tmagnetite) and are texturally composite as well, theircoherent pattern ofextinction suggests that some pre-ferred orientation is inherited by the serpentine fromolivine. Wicks (1969) determined that intergrown liz-ardite and brucite in a mesh serpentine veinlet weremutually oriented with the l izardite (001) parallel tothe brucite basal plane. He inferred that this corre-spondence was controlled by the nature of the olivinereplacement mechanism; i.e., the reaction was topo-tactic. The hydration of olivine in basalts (iddingsiti-zation) results in pseudomorphs composed of sheets i l icates and i ron ox ides. Brown and Stephen (1959)and others have shown that olivine exerts a topotacticcontrol over the rdplacement products.

Raleigh and Paterson (1965) performed non-equi-l ibrium dehydration experiments on natural serpenti-nites. Under certain run conditions, several of thelizardite-chrysoti le serpentinites contained fine-grained new-formed olivine produced by deserpenti-nization of mesh pseudomorphs. This olivine is moremagnesian2 than the olivine (=Foro) which character-izes the parent sample, and the elongate grains withinan individual pseudomorph share a single optic ori-entation. A significant point is that where relict oli-vine is present in the serpentinites, the new-formedolivines are in optical continuity with the adjacentpreexisting grains. Thus, secondary olivine formedfrom serpentine duplicates the orientation of theprimary olivine from which the serpentine formed.These textural relationships imply that there is a

'? Raleigh and Paterson determined ol iv ine composi t ions by theX-ray method of Hotz and Jackson (1963). They recorded compo-si t ions of Fo"u and Fo,or , the lat ter obviously in error . They suggestthat the anomalous composi t ions resul t in part f rom subst i tut ionsof addi t ional cat ions not present in large quant i t ies in the ol iv inesused to cal ibrate the curves. Microprobe analyses of new-formedol iv ines suggest that the extraneous cat ion is probably Mn.

single preferred structural correspondence betweenolivine and serpentine and,/or intergrown brucite.The textures and olivine compositions produced byRaleigh and Paterson bear a striking resemblance tothe new-formed olivine in 7I-104 and to a lesserextent 04-124. Although there is no relict olivine inthe latter samples with which to compare optic orien-tations, the origin of the preferred orientations ofolivines in the deserpentinized rocks is here inferredto be inherited from preexisting serpentine by virtueof some topotaxial intergrowth. A possibly importantimplication of Raleigh and Paterson's experimentalresults, exemplified by the olivine-serpentine rela-tionships exhibited in 7l-104, 04-124 and 7l-62, isthat peridotites formed by deserpentinization couldbe characterized by topotactically induced preferredolivine orientations identical to those present in theparental peridotites. Miers (1970) has measured fab-ric orientations in some statically deserpentinizedDarrington peridotites which are similar to those ob-served in syntectonically recryslallized primary per-idot i tes.

Experimental dehydration studies of serpentines byAruja (1943) and Hey and Bannister (1948) demon-strated that the resultant forsterite was oriented withrespect to the serpentine. Brindley and Zussman(1957) and Bal l and Taylor (1963) performed addi-tional experiments in order to better define the natureof the topotaxy and to model the reaction mecha-nism. While this work provides support for the topo-tactic nature of the olivine-serpentine reactions, theexperimental techniques indicate that their resultsshould be extrapolated with reservation to naturaland synthetic equilibria. The starting mixtures forthese experiments were serpentine powders or in afew cases bundles of chrysotile fibers. Brucite was notincluded.

Brindley and Zussman (1957) examined the ther-mal decomposition of chrysotile, 6-layer serpentine.lizardite, and antigorite in air at one-atmosphere inthe temperature range 500-800'C. Talc was not pro-duced in any experiments. X-ray determinations ofthe mutual intergrowths indicated that all serpentinephases yielded the same rather strong preferred oli-vine orientation with respect to the parent serpentine,except for chrysotile, which showed a second weaklydeveloped orientation as well. The results of these X-ray measurements are as follows: (010)1 and (013)1paral lel to (100)", (001)1 and (01 1)1 paral lel to (010)",and by implication (100)1 is parallel to c".

Brindley (1963) noted that the replacement rela-tionship for the orthoserpentine lizardite could be

DUNGAN: SERPENTINE-OLIVINE EQUILIBRIA 1027

recast in the following notation in terms of crystal-lographic axes and unit-cell parameters

bto // arr, bh(10.2{) : 2ay,(10.6A)

cro / / bn, 3cr.(18.0A) : 2b11"Q8.4A)

aro // crz 3a1"(14.4A) : cy,(14,6A)

As Brindley (1963) and Wicks (1969) have noted,this type of replacement in which unit-cell dimensionsare inherited implies that the close-packed oxygenframework of the parent phase remains relativelyundisturbed and cations are free to migrate. Indeed,if forsterite were to replace lizardite as prescribed bythis volume relationship, substantial addition of mag-nesium from an external source would be required.

Ball and Taylor (1963) performed dehydration ex-periments on chrysotile fibers under hydrothermalconditions (400-830"C and 0-1.4 kbar P"-6). Above500'C forsterite was formed with a strong preferredorientation, and the talc produced in some runs like-wise exhibited a strong orientation with respect to theserpentine. Several orientations for olivine which ap-parently were dependent on the presence or absenceof HrO and the specific P,I conditions were found,although several runs contained mixtures of severalorientations. A tendency for a decrease in thestrength of the preferred orientation was recorded inhydrothermal runs with Pr"o ) 0.6 kbar, suggesting adecreasingly important role for oriented nucleationas reaction rates increase with increasing pressure. Inall types of preferred orientations between chrysotileand olivine, the forsterite a-axis was normal to (001)of chrysotile. This indicates that the planes of close-packed oxygens wjthin the mutually intergrownphases are parallel. Ball and Taylor (1963) also repro-duced the lizardite-olivine relationship recognizedbyBrindley and Zussman (1957) under hydrothermalcondit ions (520'C and 0.5 kbar). Br indley and Al i(1950) found a similar relationship between chloriteand ol iv ine.

Martin and Fyfe (1970) attempted to evaluate thekinetics of forsterite hydration by means of experi-mental reaction-rate studies, using synthetic and nat-ural olivines as starting materials. The identity of theserpentine mineral was not established, but the re-sults of Moody (1976b) suggest that it was lizardite.Initially, very fast reaction rates were recorded. Thiswas interpreted as evidence for a heterogeneous reac-tion characterized by a small activation energy. To-potaxy between serpentine and brucite was suggestedas the factor promoting serpentine nucleation but

topotaxy among all three phases appears equally pos-sible. After a portion of the olivine was hydrated toserpentine * brucite the reaction rates decreased,indicating that the diffusion rate of HrO to the reac-tion interface may have become the rate-controllingstep.

Processes relating to diffusion and nucleation mayoperate differently in powder aggregates versus rocks,due to the greater pore space and surface area presentin the former, Consequently, where reaction kineticsare a function of diffusion and nucleation, and nucle-ation and growth rates are strongly dependent on thecharacter of topotaxial intergrowths, comparisonsbetween experimental studies and natural rock sys-tems such as those made in this paper are somewhattenuous. Additional degrees of complexity not con-sidered in detail in this paper (or in some experimen-tal studies cited above) are the roles ofother phasessuch as brucite, talc, and magnetite in promotingoriented transformations in specific reactions. Al-though experimental results corroborate the pet-rographic interpretation that some of the reactionsregarded as metastable proceed as oriented transfor-mations, more data are needed concerning the mu-tual orientation relationships among the constituentphases in natural and synthetic serpentine-olivineequilibria. Topotaxy and the ease of nucleation inher-ent in this type of reaction is inferred to be an impor-tant factor in determining the mineralogy of serpen-tine-olivine equilibria under conditions in whichactivation energy becomes a proportionally signifi-cant amount of the energy budget.

As was noted above, the mesh-forming process cantake place at temperatures as low as 25-1l5oC. Un-der these conditions, nucleation effects may play animportant role in the replacement mechanism. Theexperiments conducted by Martin and Fyfe (1970)may provide insight into the consistent two-stage de-velopment of the mesh texture. In the incipient stagesof serpentinizalion, fluids find easy access to reactionsurfaces on olivine along its parting surfaces, and theorthogonal pattern of cross-fiber veinlets is formed.When the fractures are sealed by serpentine (* bru-cite) the reaction rate (and possibly the mechanism ofnucleation and/or growth) changes, as diffusion ofHrO to the reaction interface is impeded by the veins.This second stage coincides with the formation of themesh centers. In the reverse case, where olivine formsby deserpentinization of mesh pseudomorphs (e.g.,7l-104), nucleation of olivine is probably enhancedby topotactic relationships between the olivine andphases of the pseudomorph. A similar mechanism

DUNGAN: SERPENTINE-OLIVINE EQUILIBRIA

may account for reversals of metastable equilibria inanalogous experimental systems.

The chrysotile-olivine assemblage in the l4l4 veinscontrasts with these other examples in that it did notform by replacement of a preexisting substrate. Thetextural evidence for dilation indicates that the two-phase vein assemblage grew by precipitation from avapor phase. The nature of the intergrowth betweenolivine and chrysotile in these veins is unknown, butthe olivine is apparently not constrained by a rigidthree dimensional orientation with respect to thechrysotile. The presence of chrysotile rather than an-tigorite is remarkable in that the chrysotile has nucle-ated on antigorite. The exclusion of antigorite fromthe vein in favor ofchrysotile suggests that this occur-rence is similar to synthetic systems in which chryso-tile is produced by crystallization of gels or oxidemixes as opposed to a starting mixture composed ofsilicate minerals.

Conclusions

Metastability in serpentine dehydration reactionsoccurs under conditions of thermal upgrading wheresynkinematic recrystallization is lacking. Metastabil-ity is also characteristic of olivine hydration reactionswhich proceed under static conditions; most notablythe formation of mesh pseudomorphs which also oc-curs at relatively low temperatures. Under these con-ditions, nucleation effects are a potentially significantfactor in promoting the occurrence of metastablephases and react ions (e.9., Spry, 1969, p.86-95). I t ispostulated that the peculiarities and large superlatticedimensions of the antigorite crystal structure act toretard the formation of this phase, particularly fromother serpentines due to the low AS associated withthe reaction. The difficulty with which antigorite nu-clei form is compounded by the ease with which theother serpentine phases, lizardite and chrysotile, nu-cleate from appropriate starting materials. The roleof oriented transformations between olivine and liz-ardite and to a lesser extent olivine and chrysotile hasbeen advanced here as a significant factor in pro-moting metastable hydration and dehydration in nat-ural and synthetic equilibria. Both the petrography ofsome metastable reactions in serpentinites and theresults of a variety of experimental studies supportthe existence of topotactic intergrowths between oli-vine and lizardite-chrysotile. Although a general ten-dency for the development of oriented intergrowthsin olivine-serpentine reactions and their potential forinducing formation of metastable mineral assem-blages is supported by the available data, it is not

suggested that one simple reaction model accountsfor all observed metastable behavior. In fact, theactual reaction mechanisms involved in the severaldifferent equil ibria discussed above are not known indetail, particularly with regard to the potential rolesof additional phases such as brucite, magnetite, andtalc. Although topotactic relationships between ser-pentines or olivine and these phases would not mate-rially alter the concept of nucleation controlled reac-tion paths, they obviously modify the detailedinterpretation of the reaction mechanism. The firststep in testing the general model proposed here is todetermine the nature of multiphase intergrowths pro-duced in metastable serpentine equil ibria by X-ray orelectron microscope techniques.

AcknowledgmentsI thank Bernard W. Evans for advice, encouragement and finan-

c ia l support (NSF Crant I l -2589) dur ing the course of th is study.

J udith Moody provided SEM data on the mineralogy of the vein in

1414, a thoughtfu l review of the manuscr ipt , and st imulat ing dis-

cussions of the problems deal t wi th above. The manuscr ipt was

also reviewed by J. V. Chernosky and C. H. Simonds, whose

suggestions greatly improved the paper.

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Manuscript receiued, August i,8, 1976; accepted

for publication, February II,1977.