192 Revisla Brasileira de Oeocitncias Volu me 3, 1 ~ 7 3

THE COMPOS ITION OF SAPPHIRINE FROM SALVADOR, BA HIA A ND CONDITIONS OF ITS FORMATION

JOHN C. ST O RME R JR .'

RESUMO Sa firi na cum min eral raro d e roc he s mct a morficas de alto grall. Duas amostras de Sa lvador, Ba hia , foram a ualisadas p Ol' meio de microsson d a elctron ica j un tamente co m os min erals associados. Essas amostras sao semelha ntes a ou tras de scritas na Iitera tura, mas possucm urn teor em Fe m ui to alto. Q uase mct ad c de sse Fe deve substituir 0 AI. Evidencias qu imicas, pctro­ graficas e de ca mpo in dica m q ue a safirina formou-se pela rcacao de silica m etassom a tica com espi nelio num corpo de bron aitito [roch a form ada principal mente de ortopi ro xeni o). M aior silica­ tizacao tra nsfon no u a safirina cm cordicri ta , em a lg- umas pa rte s. A tem per a tur a indi cad a pelo teor {~ em ALlOJ des ort opi ro xenios de a proxi madamente I DOOoe , e a prcssac ccstim ada entre 5 e 8 kb.

ABSTRA CT Ana lyses of two sa pphirine specim ens from Salva dor, Ba hia, Bra zil along wit h partial analyses of some associa ted mine rals were ma de by electron micropro be. These sa pphirines arc simila r to other analysed spccimens, but arc a mo ng the most iro n rich (9.2 an d 9.8 w t % to ta l iron as FeO). Nearly half of th is iro n must be presen t as Fe +:1 substituting fIJI' AI. Field petrogra phic a nd che mical ev ide nce suggests that the sa pphirinc developed by adition of silica to a spinel beari ng bron zit ite bod y. Further int rod uction of silica appar ently produced re ac tion of sa pphiri ne

to cord ieritc in parts of th e bo dy. T he temperature of {orm a tion given by AI2 0 3 content of the or thop yro xene was nca r I ooooe ; thc pressure is estima ted to be in the ra nge 5 to B kiloba rs.

INTRODUCTION Sa pphirinc is an un usua l mineral of high grade metamorphic rocks (granu lite facies). It character istically occu rs in assemblages poor in silica but rich in both aluminum a nd magnesium . Sapphirine is generally considered to be a rare min eral but several new occ urences have been discovered in recent years among them the occur ence at Salvado r, Bahia (Allard and Fujimori, 1966). It is probab ly more common than pre­ viously though t, a nd should be expec ted in rocks of favorable composition in any granulite terrain.

T he simplest form ula unit can be writte n M g2A1 4SiO l o ' The silicon and two aluminum ions in this unit would be found in tetra hedral sites, and the remaining two aluminum and magnesium ion s in octahedral sites according to the struc ture analysis made by Moore (1969). T he sa mple ana lysed by Moor e and most other natural samples have a grea ter number of Al ions substituting for Mg and Si, than this simple for mula, (Deer , et al., 1963, Schreye r a nd Seifert , 1969) and more complex formula units have been proposed to reflect this common substitu tion. T he sapphirine described by Allard a nd Fujimori (I 966) occurs in a roughly spherical mass of phlogopi te and spinel bearing bronzitite. T his mass, a bout 5 m in dia meter, is found in an outcrop of hypersthene gra nulite nea r the shore, 150 m east of the Bacia das Mocas, Salvador, Brazil. Similar masses of bronzitite enclosed in hypersthene gra nulite occur throughout the Salvador a rea and are believed to be of sed imentary origin. Q uartz­ -feldspathic gra nulite, gra nite and associated aplite a nd pegma tite dikes also occur in the immedia te area.

"Dept. of Geology, U nive rsity of Georgia, At hen s, Geo rgia 30 602, E VA Rtvista Brasileim de Geocimcias Volu me 3, 1973 193

Sample description The different samples were selected for a nalysis, each seemed to be represent ati ve of the two most distin ctive sa pphirine assemblages. Num ber 100, from near the cen ter of the mass, consists predominantly of coa rse-grained bronzite and phlogopite with min or amount of spinel. The grains of spinel have rims of sapphirine surrounding them. Number 101 is from nearer the outside of the mass. Here spinel is com pletely absent, ort hopyroxene and phlogopite still predominate but sapphirine and cord ierite occur as symplectitic intergrowths within large gra ins of hypersth ene. The int ergrowths have the a ppea ra nce of fine bands of cordierite or sapphirine 10·50,u in wid th in generally linear or plumose patterns. In pla ces' the proportions of sapphiri nc, (or cordierite) in the se inter­ growths reaches 50 %. Althou gh the appearance suggests some structural control, solution or reac tion along cleavage plane s is not obvious. The composition of the symplectite varies systema tically ,\(T O,\S the sect ion (2 r ill) the hvpcrstbcuc host crysta ls remain the same bu t on one side the intcrgrowth is composed entirely of sa pphirine; on the ot her side the in tergrowth is composed of cordierite ; while in the interm edia te a rea cordierite appears to be rep lacin g sapphirine. Some of the bands or stringers of sapphirine arc completely replaced by cordierite, some still contain isolat ed sections or islands of sapphirine . Although no atte mpt was mad e in this study to accurately determine the optical properties, the two sa mples of sapphirine were a pparently identical and within the range of commonly repo rted properties (Deer et al, 1963). T he only apparent differences between the two sapphirine samples were the textu res a nd the associated min erals. A paper des­ cr ibing the field rela tion ships and microscopic observat ions in more detail is being prep ared by Dr. G. O. Allard and myself.

Mineral compositions Sa pphirine, orthopyroxene, spinel and cordierite were a na lysed for the elem en ts listed in Tab. I, using a Material Anaryses Co. Model 400S electron micro­ probe. Although these are par tial ana lyses, the totals, being ncar 100%, indicate that an y other elements may be present only in very min or amounts. An alysis conditions were : acce lera ting volta ge 15 kV ; sample current 0.051' A on brass; RA P, PET , and Lif crysta l spectrometers ; sealed and thin window flow pr oportion al counters as appropriate to detect K I1. radiat ion ; counting tim e 20 sec. using beam curre n t stabilizer. Various a na lysed silicate min erals were used as standards. All a na lyses were correc ted for background and drift using a computer progr am written by myself. The empirical correction factors of Albes and Ray (1970) were applied for the sapphirine and spinel where absorbtion and ot her effects were significant. In other cases standards were sufficiently close in composition to the analysed mat erial. All calcula tions were carried ou t to four significant figures and subseq uently rounded in Tab. I to values approximating the estima ted accuracy:

SAPPHIR INE Altho ngh there are slight differences all tb ree a nalyses of sapphirine from these samples are essentia lly the same. In general the analyses are very similar to those of other natural sapphirines. T he iron content I however, is near the highest previously report ed values. The analyses in T ab. I show a deficien cy of silica as compared to the simple formul a (M gFelzAI4Si0 1o • This deficiency is also common in other analysed natural sapphirines (Deer et al., 1963 ; Wilson and Hudson, 196 7). Substitution of Al for Si in tetrahedral sites and concurre nt substitution of Al and other trivalent ca tions for Mg and Fe + 2 on other sites has been considered to account for most of the compositional variation of both nat ural and synthetic sapphirine (Deer et al., 1963., Scbreyer and Seyfert, 1969). If the deficiency of silica in these ana lyses is made up by Al in tetrah edral sites as shown in T ab. I, then a conside ra ble portion of the iron mu st be presen t as Fe + 3 to ma inta in 194 Rw iJta Bmsileira de (;rocilncias Volume 3, 1973

Table I - Mineral analy ses

Sampl<: 100 Sam ple 101 Sapph. Sapph . Sapph irine Bron zitc Spinel Hypersthene Cordicrite w/hyp." w/co rd.""

Sial 13.1 51.0 L.05 12.4 12.4 50.2 49.3

A1l 0 J 60.2 6.9 6 1.5 60.7 6 1.8 7.6 33.9 EFeO·" 9.2 14.3 23.7 9.S 10.0 20.9 3.3 MgO 18.7 26.6 15.S 17.5 16.8 22.3 12.3 CaD L.02 .05 < .05 <.02 <.02 < .05 <.02 r ro, .05 .1 < .05 < .£)5 < .05 .00 Total 101.2 lJl\.9 100.0 100.4 IUl.U 101.0 98.8

numbers of ions on the basis of: O xygen = 10 6 10 to 6 18

mok pcr('("/II 4.94-Si .7H-Si } 1.0 .92·Si } I 0 .61 .74·Si } 10 .74-Si } 1.0 .92-Si } 1.0 .22-AI .08-AI . .26-AI . .26-N .OS-A 4.00-AI 4.00·AI: 4.0 .o7-AI } .33 4.00-AI} 4.0 4.00-N} 4.0 .OS-AI } .28-Fe .46·J-"c } , I I .22· Fe 1.0 .05-AI} .09-AI .32-Fc 1.01 I.H3· Mg 1.65-Mg • . .72-Mg ,49· Fe 2.10 .50-Fe } 2.08 .61-Ml! . 7.4 mol ~{, 1.56-M g 1.50-Mg 7.8 mol "..

A1Z0J AIZOJ "Sapphirine associated with hypersthene < .05 = no t found 011 the limit of detection 0.05% "Sapphirine associated with cordicr ite < .05 = not found at the limit o f detection 0.03% ·"To tal iron content repo rted as FeO - = not analysed

the charge balan ce (nearl y 50 % in sample 100).Unfort unatly Fc+2 a nd Fe +3 cannot be determined separately by electron microprobe techniques, bu t other reported a nalysis

com monly show significa nt Fe203 contents .In several cases Fe + 3 accounts for a bout 50;;, of the tota l iron content (compare Meng and Moore, 1972 T ab. l II ).

HYP ER STHEN ET he Mg-Fe ra tios of the ort ho pyroxe ne from the two sam ples is quite different. This is especially striking since the sa pphirine is essentially of constant composition. T his can easily be seen in the lower portion of Fig. 1. The orthopyroxene composition a ppears to cha nge as the assem blage cha nges its min eralogical composition. There was no noticeab le zoning or compositional grad ients within grains of ort hopyroxene but there was some variabi lity between grains a nd the averages in T ab. I a-nd Fig. 1 represent a spread of values in Mg and Fe. The ra nges of values from the two sa mples, however, do not overla p and an overw hel ming maj ority of points lie 'close to the average value. Ano ther interesting feature of these a na lyses is the high alumina content, which remain s nearl y the same in both samples.

SPINEL The mole proportions of ca tions in the spinel analysis were recalculated to the spinel and hercyni te molecules. T he small amount of Fe remaining was calculated as magnetite. The results of this procedure show that the spinel phase in sample 100 is predominantly a spinel-hcrcynite solid solution with an almost negligible amo unt of magnetit e.

CORDfER ITE The total or the analyses in Tab. I would permit a small amouot or

water in the com position of this cordierite. 1.5-2.5 % H 20 is com mon in na tural cordierites Heuuta Brasiu ira de Geccdncias Volume 3, 1973 195

~c d

,, ,, \,", ,, ,, ,, '., sa ,, , : ,,,, ,, ,, ) O L-~~~~_~_...:S~P"d-6_~_~_~_~_----' (Fe,Mg AI203 MgO AI20 3 cd sa --- + --- ,, ------, "------6" 5 p

Ck··----0 ------:.-:D Sample 100

+-.::::::::+:::=== + Sample 101

6 Stoic hiome tric compositions

Figure I - Co m positions of the ana lysed minerals. Ideal mi ne ra l compositions ind ica ted by small triangle. Assu med ini tia l com position is show n by a star wit h path of silica me tasom ati sm indica ted by a n arrow. cd, cordie rite ; hy , hypersthene (hronzite); sa, sa pphirine; sp; spinel 196 Reoista Brasilara de Geociincius Volume 3, 1973

(Deer et al., 1963). In all other respects the cordierite is entirely no rm al. T he common preference of cordierite for Mg vs. Fe relati ve to coexisting min erals is easily seen.

PETROGEN ESI S The field relations of the samples and the textural relati on ship of the miner als within each sa mple suggest that sapphirine was formed by the reaction of spinel with silica:

2MgAI, 04 + SiO, = Mg ,AI4SiO,o (I) spinel sapphirine With the introduction of more silica sapphirine was converted to cordieritc :

Mg,Al4SiO ,o + 4SiO, Mg,Al4Si, O' 8 (2) sapphirine cordierite

T he assemblage in sam ple 100 with sapphirine rim s surrounding the spinel cor­ responds to reaction I. And the repl acement of sa pphirine by cordierite in sample 10 1 corresponds to reaction 2. T he course of the se reactions is also shown in Fig. I. T he ap proximat e composition of sa mple 100 is indica ted by a star and the effect of adding silica by an a rrow toward the silica apex. An interesting fea tu re of the ana lyses as shown in the lower portion of Fig. I is that the orthopy roxene also changes its composition with respect to Mg and Fe as reactions I and 2 pr ogress. Note that the hypersth ene in sample 101 is more iron rich tha n in sample 100. Magnesium is a ppa rently pr efer enti ally distri ­ buted int o sapphirine with respect to spinel and into cordierite with respect to sapphirine . As the spinel reacts to sa pphirine the hercynite component of the spinel reacts with the ensta tite component of the orthopyroxene and silica to produce mor e magn esian sapphirine..

' 2FeAI, 0 4 + 2MgSiO , + SiO , = Mg,AlSiO ,o + 2FeSiO, (3) hrrcmite enstatite sapphiritu ferrosilite And, in the same way, the iron sa pphirine compo nent reacts with the ensta tite to pr oduce cordierite and a more iron-rich orthopyroxene.

Fe, Al4SiO ,o + 2MgSiO, + 4SiO, = Mg ,Al4Si,Ol8 + 2FeSiO, (4) Fe-sapphirine enstatite cordierite ferro silite

T he effect of ad di ng silica to the bronzitite was to affect a progressive silicifica tion of the Mg-AI bea ring minerals, and, as a conseq uence of the increasing pr eference for Mg with respect to Fe, the ort hopyroxene became more iron rich . . The ori ginal bronzit e-phlogopite-spinel assemblage was quite silica poor. But abundant silica was present in the surrounding rocks as demonstrated by the granites, pegmatites, and aplite di kes. T he bronzitit e must have been rela tively impermeabl e to this silica since the assem blage characteristic of the first reaction is still pr esent in sample 100 from near the center while the assem blage character istic of the second, more sitica rich, reaction is present in sample 101 from near er the outside . T he origin of sa pphirine by introduction of silica into an aluminum rich rock of ultrabasic composition is a pro cess which has been suggested for many of the other sapphi­ rine occure nces such as those of West Gr eenland (H eard, et al., 1969) and Australia (Wilson and H udson, 1967). This type of paragenesis. must be contrasted with occurrences where sap phirine and q uartz occur together in sta ble association, appare ntly, as the high pressure eq uiva lent of cordieri re. T hese have been described by Da lwitz (1968), Morse and Talley (1971), and Cha tte rjee and Schreyer (1972), Reoista Brasileira de Geociinc;fll Volu me 3,1973 197

The occurrence of sa pphirine in Bahi a a ppears to be a result of silica metasom atism of a n originaly silica poor assemblage of bronzite-phl ogopite-spin el. All the minerals of this assemblage, a nd the metasomatic products (except phl ogopite} , correspond to phases in the system MgO -AI20 3Si0 2 (all owing for FeO in the na tural equivalents). T here is a considera ble amo unt of experime ntal data available on this system, some of which may be used to obtain some indication of the pr essur es and temperatures which produced these rocks. A very good estimate of tempera tu re can be mad e using the data of Anastasiou and

Seifert (1972) on the solubility of AI203 in or thop yro xen e. The two analyses of orthopy­ roxene in T ab. I show approxima tely 7 wt % A1 203 • If this value is compa red with the experimental results diagram ed in Figs. 4, 6, and B of An astasiou a nd Seifert it indica tes a temper ature about I ooooe. The temperature indicat ed by Anastasiou and Seifer t data and its varia tion wit h pressur e are shown in Fig. 2. Because their experime nts were done under condition where P(H 20) = Pttotal], their sapphirine reacted to cordierite + + spinel below 3.5 kb and the upper limit of their data is 5 kb. But, since the water pressure should not directly affect th e orthopyroxene or the sapphirine, this temperat ure curve can probabl y be extrapolated for pressur es betw een I and to kb, so long as the coexisting aluminous phase is sapphirine. T he effect of pr essure in any case is very small. (We would, however, expect the AI20 3 contents of orthop yroxene to be different with different coexis­ ting aluminous assemblages). T he most serious problem in applying this geothermometer to natural assemblages is the effect of the iron content of the pyroxene on the solubility of alumina. Anas tasiou a nd Seifert discuss the problem but give no quantitati ve solution. T here is no firm theoreti cal or empirical evidence to suggest j ust wha t the effect would be for the assemblages and pressur e ranges with which we are concerned . If we assume tha t the difference in alumina content of the two ana lysed pyroxenes is du e only to enha n­ cement of the AI 0 solubility by Fe for M g substitution , th e AI 0 content can then 2 3 2 3 be linearly extra polated back to pure ensta tite. T here it has a value of 5.1 wt % which is eq uivalent to a temp erat ur e or about 950o e. It is prob ab ly safe to estimate a temperature of I 000 ± 50°C for this sapphirine bearing bronzitite. Estimat es of pressure for this assemblage can be mad e by considering the condi tions under which cordierite and sapphirine can coexist. At 900 to I OOO°C cordierite brea ks down at pr essur es of 8 to 11 kb to assemblages of sapphirine enstatite + quartz or ensta­ tite + sillimanite + quartz (Newton, 1972) and sapphirine will break down to cordierite and spinel a t low pressures. The problem with this approach is that cordierite stability is greatl y affected by the amount of water which ent ers its structure (much like the water in zeolit es). For instan ce anhydrous cordierite breaks down to sapphirine and q uart z (reaction 2 in reverse) at pressures of about 8.5 kb a t I ooooe whereas hydrous cordierire is apparently stable up to 10 or II kb (Newton, 1972). U nder conditions of P(H zO) = P(total) sapphirine brea ks down to spinel + hydrous cordierite at pressures near 3.5 kb (Fawcett and Yoder, 1966). However, it is a pparently stable down to atmospheric pressur e under dry conditions (relative to an anhydrous cordierite). Using the free energy dat a of Bird and And erson (1972) for hydrous cordierite, an d a sorbtion energy of about 2.5 kcal per mole of wat er, a very rough free energy value for sapphirine can be obtained from Newton's (1972) curve for reaction (1). Further ca lculation then shows that the reaction:

5Mg2AI.SiO,o = Mg 2AI.Si,O ' B + 8MgAI20 . (5) sapphirine cordierite spinel 198 Reouta Brasileira de Geocitncias Volum e 3,1973

15

...... '...... '.• .. en+~ il: · : :

10

W .. 0:: ::J (f) (f) W 0:: ph-limit 0...

5

sa(+en) cd +sp

\ \ \ '. \ . \ .. .. \ .. . \ . .. \ . . \ . \ .. \ . \

800 900 10 0 0 11 00 120 0 TEMPERATURE °C Figure 2 - Stability relationships of sapp hir ine, cordierite a nd associated mineral s. cd : h, hydro us cordierite; cd, an hydro us cordicr itc; ph, phlogo pite ; en, ensta tite ; sa, sapphi rine ; sp, spinel; qz, q ua rtz . (Afte r New ton, 1972 ; Faw cett and Yod er, 1966 ; Yod er, and Eugster, 1954 ; Anastasiou and Seifert, 1972) Reusta Brosileira de Oeoc itncias Volume 3, 1973 199

should have a strongly posm ve free energy, i.e., sa pphirine sho uld be sta ble at 1 OOGoe and a tmospheric pressure. This is also confirmed by the fact that sa pphirine can be synthesized at 1 atm. from dr y mixtures of the oxides (Deer, et ai., 1963). The prob lem of determining the stability field of a natural cordierite is compounded by the fact that cordierites can apparent ly take on or discha rge wat er relati vely rapidly (Schreyer and Yoder, 1964). T he water in the analysis of a natural cordicritc probably does not represent the amount present a t the high temperature equilibrium. But, in the assemblage we a re considering here, there is some evidence to suggest that the environment

was not entirely dry, although P(H 20) did not necessarily equal tot al pr essur e. T he primary evidence is the :presence of abundant phlogopite without an y evidence of its breaking do~n. T he P-T curve for phlogopite break down [P(H,O) = P(tot all] is shown in Fig. '2 (Yoder and Eugster, 1954). The presence of iron, as in these sam ples, would tend to lower the tempera tures somewha t, depending upon the oxygen fugacity (Wanes a nd Eugster, 1965). Estim at es based on this work suggest that the water pressur e would have to be well over 2 kb a t I ooooe. The granite , pegma tite and aplite dikes, as well as amphi bole and biotite in surrounding rocks, also suggest tha t some wa ter was pr esent. H ydrous cordierire

ca n a ppa rently retain its wa ter even under conditions where P(H 20) docs not eq ua l tota l pressure (Newton , 1972, p. 412). It seems reasona ble, then, to assume that sta bility fields based on hydrous cordierite are applica ble to the sapphirine-bcaring assemblage discussed here. This would place the minimum pr essure ncar 4 kb a nd the maximum ncar to kb. The beginn ing of melting would also place a constraint on temperature and pr essure as shown in Fig. 2. At 1 OOQoC, we should expec t to see some melting in hyd rous cordieri te­ -sapphirine assemblages at pres sures above 7 kb. T he a bsence of a ny melting suggests that pre ssures were not mu ch, if any, high er.

CO NCLUSIONS The sa pphirine bearing assemblage from Salvador , Bahia apparently formed at tem pera tu res ncar I OOO°C and pressures between 5 and 8 kb (depths of 15 to 24 km]. T he formation of sapphirine from spinel and the subsequent reaction of sa pphirine to cordierite were due to the addition of silica from surrounding quartzfeldspathi c rocks to an aluminum-rich ultrabasic rock, a type of paragenesis which is common for sa pphirinc. . Many new occurences sho uld be found if regions of gra nu litic rocks (such as those of Brazil) are examined for loca tions where Mg and Al rich rocks cou ld have been affected by silica metasomati sm. Such sapphirine-bea ring rocks can be useful in esta blishing temperatures of metamorphism, using the aluminum content of the ortho pyroxene coexisting with the sap phirine as a geothermometer.

Acknowledgements I am very mu ch ind ebted to Dr. Gilles Allard, who sugge sted this problem , provided the samples, and gave all the informa tion on the field relationship. T his work was prep ar ed while I enjoyed th e position of visiting professor in the In stitute of Geocie ncias, U niversity of S1\o Paulo and used the facilities of their microprobe laboratory. Financial suppor t was also pr ovided by the Na tiona l Science Fou nda tion (U.S.) through a grant to the U niversity of Georgia.

REFERENCES ALBEE, A. L. and RAY, L. - 1970 - Correction fact ors for electron probe micro analysis of silicates, oxides, ca rbonates, ph osphat es, and sulfates. Anal. Chern., 42 : 382-403 ALLARD, G.O. and FUJIMORI, K. - 1966 '- A new occurence of sapphirine in Salvador, Bahia, Brazil. (Abs.) Can . Mineral., 8 : 660 ANASTASIOU, P. and SE IFERT, F. - 1972 - Solid solubility of AL.l0 3 in enst atite at high temperature an d 1-5 kb wa ter pressure. Contr. Mi neral. and Petrol., 34 : 272-28 1 200 Revuta Brasileira de Geocimcias Volume 3,1973

BIRD, G, W. and ANDERSON,G. M . - 1973 - T he free energy of form ation of magn esian cordierite and ph logopi te. Am . J . Sci., 273 : 84-9 1 CHAT T ERJE E, N. D. and SCHREYER, W.- 1972 - The reaction enstatite + sillimanite =

= sapphirine + quartz in the system MgO-Alz0 3-SiOz . Cc nt r. Mineral. and Petrol., 36 : 49-62 DEER, W. A., HOWIE , R. A., and ZUSSM AN , J . - 1962 - Rockforming Min era ls. Longma ns, London, 5 vol. FAW CETT, J . J. and YODER, H . S., Jr.- 1966 - Phase relati onship s of chloritcs in the system M g O .A I 20 3- SiO z ~H 20 . Am . Mi neral, 51: 353 ~38 0 H ERD, R . K " WI NDLEY, B. F. and G H ISLER, M.- 1969 - The mode of occ urencc and . petrogene sis of the sapphirine-bea ring and associated rocks of west Greenlan d . Gronl. Geo l. Und ers. Rapport. 24: 44 pp . M ENG, L. K . and MOORE, J. M., Jr. - 1972 - Sapphirine bearing roc ks from Wilson Lake La brador . Can. Mineral, 11 : 777-79 0 MO ORE, P. B. - 1969 - The crysta l structure of sapphirine. Am. Mineral, 54: 31-49 MORSE, S. A. an d T ALLEY, S. H . - 1971 - Sapphi rin e reactions in deep-seat ed granulites near Wilson Lake, Ce ntral Labrador, Ca nada. Ear th Planet. Sci. Latters, 10 : 325-328 NE WT ON, R. C. - 1972 - An experime nta l determination of th e high -pressur e stability limits of magn esian cordieri te und er wet and dr y conditions. J. Geol., 80 : 398-420 SCH REYER, W . and SEL FER T , F. - 1969 - High pressur e ph ases in the system M g O ~A I 2 0 3 ­ -SiO z-H 20: Am. J . Sci. Scha irer Vo lume: 407-443 SC HREYE R, W . and YODER , H . S. - 1964 - T he system Mg-cordierite-H j O and related roc ks. Neues Jarb : Mineralogic Abh ., 101: 27 1 ~342 WILSON, A. F. and H UDSON, D. R . - 1967 - The discover y of bery llium bearing sapphit ine in th e gr anulites of th e Mu sgrave ran ges (Cent ral Austr a lia). Chern. Geol., 2, 209-215 WaNES, D. R . and EU GST ER, H. P. - 1965 - Sta bility of biot ite : experimen t, theory, and applica tion. Amer. Mineral., SO: 1 228-1 272 YODER , H . S, and EUGSTER, H . P. - 1954 - Phlogopite synthesis and sta bility ran ge. Geochim. Cosmoc him, Acta, 6 : 157