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The Composition of Sapphirine from Salvador, Bahia and Conditions of Its

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The Composition of Sapphirine from Salvador, Bahia and Conditions of Its 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 .
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