Canadian MineralogistVol. 25, pp. 695-705 (1987)

CRVSTAL CHEMISTRY AND GENESIS OF Nb., V.,AND AI-RICH METAMORPHIC TITANITE FROM EGYPT AND GREECE

REGINA BERNAU AND GERHARD FRANZInstitut filr Angewondte Geophysik, Petologie und Lagerstiittenforschung, EB 310, Technische tlniversitdt Berlin,

1000 Berlin 12, Ilest Germany

ABSTRACT

Titanite CaTiOSiOa from calc-silicate rocks of GebelUmm Shagir, Egypt, associated with garnet, p,'roxene,scapolite, calcite and quartz, contains up to 11 wt.9oNb2O5, 6 wt.9o Al2O3 and2 *t.Vo V2Oj. Other elementspresent in minor amounts are Fe, Ta, Sn, and F. The mainsubstiturions are2Ti : Nb + AI, Ti + O = Al + (OH,F),and V for Ti. Up to 50 mol.9o of Ti is substituted. Thecrystals show complex zoning, with Nb contents decreas-ing from core to rim. The grorth of the crystals is correlatedwith different stages of metamorphism. Titanite from low-grade Mn-rich metasediments of Andros, Greece, containsup to 2 wt.q0 V2O, in homogeneous crystals. The valencestate of vanadium is uncertain; possible mechanisms of sub-st i tut ion are Ti+O=V3* + (OH,F), Ti=V4+, and2 Ti = V5+ + (Al,Fe)3+.

Keywords: niobian titanite, vanadian titanite, calc-silicaterock, Mn-rich metasediment, chemical composition,Egypt, Greece.

Soutrernr

La titanite qui se trouve dans des calc-silicates i GebelUmm Shagir, en Egypte, est associ6e ir gxenat, pyroxbne,scapolite, calcite et quartz; elle contient jusqu'd 1l% enpoids de Nb2O5, 6Yo de A12O3 et.2i/0 de V2Or. Sont aussipr€sents en quantit6s moindres Fe, Ta, Sn et F. Les substi-tutions importantes sont Nb + Al pour 2Ti, Al + (OH,F)pour Ti + O, et V pour Ti. Ces couples mdnent au rempla-cement de 50q0 du Ti. Les cristaux montrent une zonationcomplexe otr le Nb diminue du coeur i la bordure. On 6ta-blit le lien entre la croissance des cristaux et les diffdrenrsstades de m6tamorphisme. La titanite dans les m6tas6di-ments riches en manganbse de Andros, en Grlce, faible-ment m€tamorphisds, forme des cristaux homogbnes quipeuvent contenir jusqu'd 2c/o de Y2O5. La valence duvanadium n'est pas 6tablie, et les couples de substitutionpossibles sont VJ-. + (OH,F) pour Ti + O, V4' pour Ti,et V)* + (Al,Fe)r* pour 2Ti.

(Traduit par la Rddaction)

Mots-cl6s: titanite niobifere, titanite vanadifdre, calc-silicates, m6tas€diments manganifdres, composition chi-mique, Egypte, Grdce.

INTRoDUCTIoN

The crystal chemistry of titanite CaTiOSiOa

695

shows considerable variation in the octahedral sitesnormally occupied by Tia* (Ribbe 1980). Al3* andFe3* may be incorporated by means of the substi-tution Tia+ + 02- = M3* + (OH,F)-, which hasbeen found to replace up to 5090 ofthe Ti in natu-ral titanite (Franz & Spear 1985, and referencestherein). Charge balance for the exchangeTia+ +M3+ can also be maintained by a coupledsubstitution with a pentavalent cation, e.9., Nb orTa. Clark (1974) and Paulet ql. (1981) described thistype of substitution, 2 Ti4+ : M5+ + M3+ , irt tita-nite of pegmatitic origin. In the present note, we des-cribe an occunence of an extremely (Nb,Al)-rich tita-nite from high-grade metamorphic calc-silicate rocksand an occrurence of V-enriched titanite in low-gadeMn-rich metasediments.

GsolocrceL Srrrnc AND PETRoGRApHy

Gebel Umm Shogir, Egtpt

The (Nb,Al)-rich titanite comes from the GebelUmm Shagir basement of southern Eg5pt. It is a Pre-cambrian metamorphic complex consisting mainlyof granitic gneiss and migmatite, minor calc-silicaterocks, marble and amphibolite, and small graniticintrusive bodies. Conditions of metamorphism weredetermined to be on the order of 8 kbar, 800oC forrelics of the granulite facies. An amphibolite-faciesoverprint at approximately 6 kbar, 600 - 700oC,produced the main mineral assemblage of the rocks;a greenschist-facies overprint is also discernible (Ber-nau et al. 1987).

The (Nb,Al)-rich titanite occurs as an accessorymineral in a rock composed of garnet, clinopyroxeneand scapolite, with small amounts of calcite andaccessory quartz and opaque minerals. Other rocktypes from this area include different calc-silicaterocks, extremely coarse-grained marble with scapo-lite, clinopyroxene, zoisite and quartz, and gneissesof granitic composition. Lack of proper er

696 THE CANADIAN MINERALOGIST

9aset

s t o 2 3 7 . 9 6 3 8 . 6 6

T i o 2 0 . 4 3 0 . 3 6

A t 2 O 3 t 5 . 6 0 1 8 . 1 5

I ' s 2 o 3 9 . 7 1 6 . 0 1

F a o 1 . 9 5 2 . 6 4

[ g O 0 . 1 8 O . 2 0

l i n o 0 . 8 9 I . | 3

c a o 3 3 . 1 6 3 3 . 1 0

Na2O

K2o

r 9 9 . 7 8 1 0 0 . 2 5

s l 2 . 9 6 8 2 . 9 8 0

arw ' l .44'r 1.6,!9

! ' o 3 r 0 . 5 ? 3 0 . 3 4 9

F . 2 t 0 . 1 2 8 o . 1 z o

u 9 0 . 0 2 1 0 . 0 2 3

i l n 0 . 0 5 9 0 . 0 7 4

c a 2 . 7 8 5 2 . 7 3 4

T i 0 . 0 2 5 0 . 0 2 1

aoalp11to

4 1 . 6 9 4 4 . 1 9

2 9 . O 2 2 7 . 1 2

ol' ol'0 . 0 5 0 . 1 4

2 0 . 1 1 1 9 . 1 3

1 . 4 7 ' l . 8 0

0 . 1 8 0 . 3 1

9 3 . 4 4 9 3 . 9 0

rBLE 1. REFNESENTATIW CO!{FOSITION OF ACCOHPANYING UINERLSI M,.srTao.e1Fd +o.r,Mns.13O4, whereas calculation onthe basis of 12 negative charges, with all Fe assumedto be present as Fd+, gave unsatisfactory results.Therefore, we conclude that the inclusions are notminerals of the columbite-tantalite or tapiolitegroups, (Fd*,Mn) (Nb,Ta)2o6.

Titanite occurs as oval-shaped inclusions (<I mm) in garnet, clinopyroxene and scapolite, as wellas larger anhedral grains between the main silicateminerals (Fig. l). It was also observed in contact vrithcalcite and qvartz. The crystals are pleochroic yellow-brown to brown. Optical extinction is not homogene-ous over the grains, but reveals a zonation thatmatches the chemical zonation; three distinct zonescan be distinguished (see below, in "Chemistry").Furthermore, most of these crystals have manyminute inclusions in the core. Somewhat larger inclu-sions, situated in the inner rim, are composed ofclinopyroxene, scapolite and garnet. Their compo-sitions do not differ from that of the matrix minerals(Table l). Small round grains of zircon occur asinclusions in the titanite. In contrast to the core andthe inner rim, the outer rim is free of inclusions.

Besides the zoned grains of titanite describedabove, smaller (= O.OS mm) subhedral grains wereobserved on grain boundaries between the matrixminerals. Their composition is similar to that of theouter margin of the larger anhedral grains of titanite(see number 13, Table 2).

Vitali, Andros, Greece

Vanadium-bearing titanite was found in apiemontite-albite gneiss from Vitali, Andros Island,Greece. The gneiss belongs to the highly oxidizedMn-rich metasediments intercalated in a series ofmetapelitic quartzose schist, marble and basicmetavolcanic rocks (Reinecke 1980. The meta-morphic rocks on Andros form a part of the Eoceneblueschist belt of the Hellenides and were affectedby a high-P, low-T type of regional metamorphism(P > 9 kbar, T 400 - 500'C) and a later Barrovian-type metamorphism (P 5 - 6 kbar, T 400 - 500oC),which generated widespread greenschist-facies assem-blages (Matthews & Schliestedt 1984).

The accessory V-rich titanite coexists withpiemontite, albite, muscovite or phengite, calcite,hematite and Mg-chlorite; small amounts of quartz,cymrite and apatite are also present (for more details,see Reinecke 1983). The titanite forms almost color-less, slightly brownish gtey, euhedral crystals up toI mm across in contact with all other minerals @ig.2).

CHEMISTRY

The titanite grains were analyzed on a Camebaxmicroprobe (WDS) using the following standards:

Iryrox6ne

5 0 . 7 6

0 . 0 6

1 . 4 6

9 . 0 6

0 . 7 3

2 3 . 9 9

, r . r ,

slAIF€-

'

NA

A n e q " 1 0 0 ( A 1 - 3 ) / 3

M-{ - '100( (b* !Nq+Fe)ola+R€+li!+el

6 . 5 9 1 7 . 0 2 7 6 t r . 9 5 05.408 4 .gg i a l rv o .oso0.042 o .o8o a tv r o .o te0 . 5 0 9 3 . 2 0 4 p . 3 * O . O a Z0.451 0 .544 Fe2* 0 .39?0 . 0 3 6 0 . 0 6 t u 9 0 . 5 1 9

! {n 0 .024

80.3 66 .6 ca 0 '988

12.0 r5 .4 Na o 'o12

8 8 . 0 8 4 . 6Er! 25.9P E 1 9 . 9r f o 4 5 . 8T S 4 . 8t u 1 , 2J o l 2 . 4

Aneq

llar

llei

lualtGlo aaPytALBSIEae

2 4 . 7 ' , 1 7 . 9

6 4 . 3 ? 3 . 80 . 7 0 . 84 . 3 5 , 52 . 0 2 . 5

r Saupte no. Ro 05A, O@ Shagllr I Jo - Johannseaite

The main mineral assemblage of the titanite-bearing rock is garnet, clinopyroxene and scapolite.Representative compositions are compiled in Table1. The garnet is a grossular-rich variety (62 - 740/ogross), with subordinate (18 - 30 mol.9o) andradite.Minor abnandine, spessartine and pyrope compo-nents are rather constant in all grains. The garnetis anhedral, and no distinct zoning could be deter-mined. The clinopyroxene is essentially a diopside--hedenbergite solid solution, with Fd*/(Fd* +Mg)values between 0.34 and 0.44, and 44 - 46 mol.9oCaSiO3 component. Scapolite is an extremely Ca-rich variety, with 85 to 88Vo meionite component and< I Vo of the K-bearing end member. Chlorine andsulfur could not be detected by microprobe analyses,and the mineral is assumed to be predominantlyCOr-scapolite. Opaque minerals are extremely rare;in the thin section analyzed, only three grains werefound as inclusions in titanite. They are too smallfor a quantitative analysis; the data obtained (ana-lyses recalculated to 100q0 after subtraction of Ca,Si and Ti of the surrounding titanite) indicate thatit is a (Nb-Fe)-oxide mineral, probably of thewodginite-ixiolite type. The M/Ta ratio is very high,similar to that in the surrounding titanite. Calcula-tion of the formulae on the basis of 8 negativecharges, with the assumption that all the Fe is presentas Fe3*, yielded the average chemical formula

METAMORPHIC TITANITE FROM EGYPT AND GREECL 697

rb:D / , ' ( J , . , ' .

M

Frc. l. Photomicrographs of zoned niobian titanite from Cebel Umm Shagir, Egypt. (A, D) plane-polarized light; second-ary electron image (B, E) and chemical zoning of niobian titanite (C F). The continuous line in Figure lC, F delimitsthe core, and the dashed line, tle inner rim of zoned grains. Numbers refer to Nb2O5 contents in wt.9o. FigurelA shows niobian titanite as an inclusion in clinopyroxene. The large inclusion (black in Fig. lB) is also clinopyrox-ene. Titanite in Figure lE is surrounded by garnet and scapolite and has inclusions of garnet, Note the concentrationof small inclusions in the inner rim and core of the titanite.

698 THE CANADIAN MINERALOGIST

TABLE 2. COMPOSITION OF TITANITE AND STRUCTURAL FORMULAE

rdtSo:(1de

&zosTa205v205Si02Tio2SnO2A12o3 -Fe203 t 'CaO!{noNa2OF

[ - r . 1 6 /38

Ca

T1SnA1Fe3+MlTav5+

f, oct

c t

oH*2O r l

E O, OI t , F

0 . 9 8 0 . 9 9 I . 0 0

0 . 5 5 0 . 5 8 0 . 7 6- 0 . 0 1 0 . 0 1

0 . 2 4 0 . 1 7 0 . 1 30 . 0 4 0 . 0 3 0 . 0 30 . 1 6 0 . 1 0 0 . 0 50 . 0 1 0 . 0 10 . 0 3 0 . 0 4 0 . 0 4

1 . 0 3 1 . 0 4 1 . 0 2

0 . 9 9 0 . 9 8 0 . 9 80 . 0 8 0 . 0 7 0 . 0 6

U . U I

4 . 9 4 4 . 9 4 4 . 9 4

5 . 0 2 5 . 0 1 5 . 0 1

0 . 9 9 1 . 0 0

0 . 8 0 0 . 5 50 . 0 10 . 1 2 0 . 2 20 . 0 2 0 . 0 40 . 0 3 0 . 1 7

- 0 . 0 10 . 0 4 0 . 0 2

1 . O 2 1 . 0 1

0 . 9 8 0 . 9 8- o . 0 2

0 . 0 7 0 . 0 44 . 9 4 4 . 9 4

5 . 0 1 s . 0 0

0 . 9 9 1 . 0 0

0 . 6 7 0 . 7 00 . 0 1 0 . 0 10 . 1 8 0 . ' t 70 . 0 3 0 . 0 30 . 0 9 0 . 0 90 . 0 1 0 . 0 10 . 0 4 0 . 0 3

1 . 0 3 1 . 0 4

0 . 9 8 0 . 9 70 . 0 8 0 . 0 1

- 0 . 0 64 . 9 4 4 . 9 3

5 . 0 2 5 . 0 0

ldleaL 50 It lt. Nb-Al la

' t 6 . 4 4 1 0 . 9 4 1 0 . 4 0 6 . 4 1 3 . 3 9 1 . 9 6 1 1 . 1 8 6 , 1 7 5 , 7 90 . 5 7 . 0 . 9 7 0 . 8 1 0 . 3 9 0 . 1 7 0 . 8 3 0 . 7 7 0 . 6 9' t . 2 9 1 . 3 1 1 . 6 4 1 . 7 8 2 . 0 2 1 . 1 1 1 . 6 8 1 . 5 2

3 0 . 6 5 2 9 , 7 3 2 9 . 3 2 2 9 . 7 2 2 9 . 6 2 2 9 . 8 4 2 9 . 9 0 2 9 . 0 2 2 9 . 5 4 2 9 , 6 94 0 . 1 5 1 9 . 7 7 2 1 . 4 0 2 1 . 8 4 2 7 . 4 2 3 1 . 0 8 3 2 . 3 6 2 1 . 7 1 2 6 . 9 7 2 8 . 4 9

0 . 3 4 0 . 3 3 0 . 6 3 0 . 8 7 0 . 7 6 0 . 3 4 0 . 7 0 0 . 5 66 . 1 3 5 . 8 9 6 . 0 4 4 . 4 1 3 . 4 7 3 . 1 5 5 , 6 2 4 . 5 4 4 . 3 6

' 1 . 5 5 1 . 4 2 1 . 2 4 1 . 1 2 0 . 9 1 1 . 5 2 1 . 2 2 1 . 1 22 8 . 5 0 2 7 . 7 5 2 7 . 9 0 2 7 , 5 0 2 8 . 1 9 2 8 . 4 2 2 7 . 9 8 2 7 , 5 4 2 7 . 9 9 2 8 . 3 6

0 . 0 2 - 0 . 0 5 0 . 0 30 . 0 8 0 . 0 1 0 . 0 2 0 . 0 10 . 6 3 0 . 7 2 0 . 6 8 0 . 5 6 - 0 . 2 2 0 . 7 7 0 . 1 3

' 1 0 0 . 0 0 1 0 0 . 0 0 9 9 . 6 6 9 9 . 9 6 . t 0 1 . 8 3 1 0 0 . 7 1 9 9 . 2 1 9 9 . 0 1 1 0 0 . 1 3 1 0 0 . 6 6

7b6 a^ C3b2a

1 . 0 0

0 . 5 4

v . z 5

0 . 0 40 . 1 70 . 0 10 . 0 3

1 . 0 2

0 . 9 80 . 0 70 . 0 14 . 9 3

5 . 0 1

NbZOSTa2Ogvzo5S1O2T1O2SnO2A12O3Fe2O3* rcaoMnONa2OF

E-F . 1 6 / 38

T1SnA1Fe3 +NbTav5+

! oct

s1FOllt 2

O r 3

I O , O H , F

3 . 3 0 1 0 . 1 50 . 4 s 0 . 9 01 . 9 1 1 . 4 7

2 9 . 9 2 2 9 . 0 33 0 . 7 6 2 4 . 1 9

0 . 8 5 0 . 4 43 . 6 9 4 . 2 10 . 9 9 1 . 7 8

2 9 . 0 3 2 8 . 2 50 . 0 1 0 . 0 30 . 0 2 0 . 0 1v . a t

1 0 1 . 1 0 1 0 0 . 4 6

1 . 0 1 1 . 0 1

0 . 7 5 0 . 6 10 . 0 1 0 . 0 10 . 1 4 0 . 1 70 . 0 2 0 . 0 40 . 0 5 0 . 1 5

- 0 . 0 10 . 0 4 0 . 0 3

1 . 0 1 ' t , 0 2

0 . 9 7 0 . 9 70 . 0 30 . 0 4 0 . 0 24 . 9 2 4 . 9 7

4 . 9 9 4 . 9 9

11" 12"

4 . 3 1 1 . 5 50 . 2 8 0 . 1 71 . 7 9 2 . O 2

3 0 . 0 4 3 0 . 3 43 0 . 5 3 3 4 . 2 0

0 . 7 3 0 . 8 03 . 7 9 3 . 0 51 . 1 2 0 . 8 6

28 .52 29 .030 . 0 1 0 . 0 30 . 0 1 0 . 0 1

1 0 1 . 1 3 1 0 2 . 0 7

0 . 9 9 0 . 9 9

o . 7 5 0 . 8 20 . 0 1 0 . 0 10 . 1 5 0 . 1 20 . 0 3 0 . 0 20 . 0 6 0 . o 2

0 . 0 4 0 . 0 4' t . 0 4 I . 0 3

0 . 9 8 0 . 9 7

0 . 0 7 0 . 0 74 . . 9 4 . , 4 . 9 4

5 . d 1 5 . o l

14

0 . 2 10 . 0 91 . s 0

2 9 . 9 03 4 . 6 8

2 . 2 60 , 6 7

2 8 . 1 40 . 0 30 . 0 70 . 6 3

9 9 . 3 2 9 7 . 9 2

' I . 0 0 1 . 0 0

0 . 8 0 0 . 8 70 . 0 10 . 1 4 0 . 0 90 . 0 2 0 . 0 20 . 0 1

:0 . 0 3 0 . 0 3

1 . 0 1 1 . 0 1

0 . 9 8 0 . 9 90 . 0 4 0 . 0 70 . 0 74 . 8 9 4 . 9 3

5 . 0 0 5 . 0 0

0 . 1 0 0 . 2 0- 0 . 0 9

t . o J z . u )

2 9 . 7 4 3 0 . 3 43 5 . 2 1 3 5 . ? 8

1 . 5 6 2 . 0 40 . 9 4 0 . 6 7

2 7 . 9 3 2 8 . 7 s

0 . 0 4 0 . 0 50 . 8 4 0 . 7 2

9 7 . 9 7 9 9 . 8 9

1 . 0 0 1 . 0 0

0 . 8 8 0 . 8 6

0 . 0 5 0 . 0 80 . 0 2 0 . 0 2

0 . 0 4 0 . 0 4' I . 0 0 1 . 0 0

0 . 9 9 0 . 9 90 . 0 9 0 . 0 7

4 . 9 2 4 . 9 5

5 . 0 1 5 . 0 2

r ^ a9c 13

u . o I

| . o u5 V . Z Z

32 .7 40 . 8 3I C (

0 . 7 3z 5 . t z

0 . 0 7

0 . 4 3

Coe f f l c l en t s on t he bas i s o f E ca t . 3 . 00 (1 -13 :14 -16 : samp le 80 /179 , V l t a l i And ros ) . a co re i br : ca l cu l a ted : (A l i Fe3+ ) -F - (Nb iTa+vs+ ) = 611 ; * r

sample RU 05A, Unun Shagir ;l nne r r lm ; c ou te r r im , * t Fe to t as Fe2O3 tca l cu la ted : { ( t ca t i on cha rges ) - oE -F } / 2 .0 .

METAMORPHIC TITANITE FROM EGYPT AND GREECE

FIc.2. Photomicrograph of euhedral vanadian titanite from Vitali, Andros Island,Greece, coexisting with albite (alb) and strongly zoned piemontite (pie); plane-polarized light.

699

wollastonite (Ca, Si), orthoclase (K) and albite (Na),synthetic F-phlogopite, TiO2, MgAl2O4 and Fe2Or,and metallic Nb, Ta, Sn and Mn. The acceleratingvoltage was 15 kV; counting time was 20 seconds forF, Nb, Ta and Sn, and l0 seconds for the other ele-ments. We used Zo lines for Nb, Ta and Sn, andKa lines for all other elements. Raw data werereduced using the ZAF algorithm. All titanite grainswere routinely checked for the presence of other ele-ments such as REEwith EDS (detection limit 0.5 -190). Representative compositions are presented inTable 2. Formulae were calculated on the basis ofD cations = 3.00, since H2O was not determinedand there is no fixed number of oxygen atoms performula unit. The chemistry of the (M,Al)-richtitanite from the calc-silicate rocks is described first.

The major oxide CaO shows only a slight varia-tion, between 27.95 and28.69 wt.t/0, which is closeto the ideal value for pure titanite and for 50Vo ofthe hypothetical2Ti = Nb + Al substitution (Table2). SiO2 shows a slightly larger variation than CaO,especially toward lower values than the stoichiomet-ric composition. Similar results were obtained byFrarz & Spear (1985) for [Al + (F,OH)]-titanite;these are discussed below. TiO2 is always lower thanthe ideal value of 40.75 wt.t/0, with a minimumrecorded value of 20.9 wt.Vo. Al2O3 varies from 2.7to 6.1 wt.9o, and correlates negatively with TiO,(Fig. 3), positively with Nb2O5, and also positivelywith FerOr, which is always between 0.6 and 2.0

Ti1.0

0.9

0.8

0.7

0.6

0.50 0.05 0.10 015 0.20 0.25

AtFrc. 3. Al versus Ti in titanite. Dots: niobian titanite from

Gebel Umm Shagir, crosses: vanadian titanite fromGreece. The straight line shows the ideal substitution2Ti = Ms+ + M3+ . Cation coefficients calculated onthe basis of 3 cations per formula.

1 t . .

700 THE CANADIAN MINERALOGIST

- o2cr\+J3c/)

oN

t I '1

wt.ryo (Fig. 4). Nb2Oj attains ll.3 wt.9o, and Ta2O,invariably is below I wt.Vo. Fluorine varies, but notsystematically with AlrOr, as determined previously

A1203 wt.o6Frc. 4. Proportion of

titanite. Dots: GebelN2O3 versus Fe2O3 (wt.Vo) inUmm Shagir, crosses: Greece.

t1 l

a .a o

00 .5 1

SnOz wt.oFtc.5. Proportion of SnO2 versusY2O5 (wt.9o) in titanite

from Gebel Umm Shagir.

p o fa a a

. t

t t t '

"/

by Franz & Spear (1985); it invariably is below 1wt.Vo. SnO2 (0.3 to 0.9 wt.Vo), is positively cor-relared with V2O, (1.0 to 2.1 wt.t/o) only (Fig. 5),and both oxides are negatively correlated withM2O5 @ig. Q and Ta2O5. Other elements, like Mg,Mn, K, Na and the REE, are mostly at concentra-tions below the detection limit of the miooprobe.

The chemistry of the vanadian titanite from Greeceis rather simple. Y2O, attains 2.3 wt.0/0. Fe2O3 islow (< l.l fi.90) and is not correlated wilh Al2O.(maximum 2.5 wt.c/o, see Fig. 4). All samples showdecreasing TiO, contents with increasing VrO, andAl2O, contents (see Figs. 3, 7). Representative com-positions are given in Table 2, numbers 14 - 16.

Figure 7 (Nb + Ta +\5* versus Ti) shows thewide compositional range of the titanite, with TiO,values for the (Nb,Al)-rich grains approaching about5090 of 2 Ti : M5* + M3+ substitution. However,the sum of M5" is always lower than that for ideal(Nb,Ta,V,Al)-rich titanite, indicating that other sub-stitutions occur as well. This is also evident from aplor of (Al+Fe3+) versas (Nb+Ta+V5+) (Fig. 8),which shows M3* always higher than expected foran ideal (M5* + M3*) substitution. Additional ele-ments that are present in minor amounts are respon-sible for these phenomena: Sn probably enters theTi site by the simple substitution Ti4+ : Sn4*, andF and OH replace O by the coupled substitutionTi4+ + 02-: Al3+ + (F, OH)-.

In contrast to the previously described (M,Ta)-rich titanite from granitic pegmatites (Clark 1974,Paul et ul. 1981, Groat et al. 1985), the Nb,/Ta valueof the (Nb,Al)-rich titanite is extremely high (Fies.9b, c), Both elements are positively correlated. Thevanadian titanite shows a poor correlation betweenNb and Ta (Fig. 9a) and a large scatter in the M/Tavalues.

The calculation of formulae yielded cation coeffi-cients for both Si and Ca near 1.00, and we assumethat substitution for these elements is not important'On the other hand, substitution for Ti can be sub-stantial, as has been shown above and also in Figure10. The deviation from the 1:l slope could reflecta systematic error in the microprobe data, but itmight also be due to a slight substitution of Al forSi in tetrahedral positions. There is also the pos-sibility that the titanite is not exactly stoichiometric.

Each crystal of (Nb,Al)-rich titanite can be sub-divided into three separate regions, a core, an innerrim and an outer margin, with decreasing substitu-tion for Ti from core to margin (Fig. 1). Neverthe-less, tlere is an overlap of composition from all zonesof different crystals, with no indications of a misci-bility gap. The sharp, discontinuous zoning inindividual crystals must therefore be due to chang-ing P - T conditions or to changing activities ofchemical species. The grains ofvanadian titanite arerather homogeneous, with only a limited variation.

METAMORPHIC TITANITE FROM EGYPT AND GREECL,

DrscussroN

Crystal chemistry

The chemical data of the titanite samples analyzedallow some conclusions to be derived about possi-ble substitutions. It is important to note that in thetitanite from Gebel Umm Shagir, almost 50 mol.9o,of the Ti is replaced by other elements @ig. l0). Themost important substitution is 2 Tia+ = Nb5++ Al3*; only small amounts of Ta are incorporated

by 2Tf = Ta5+ + Al3+. Instead of Al, these cou-pled substitutions may also operate with Fe3*.There is a large surplus of (Al + Fe3*) over the sumof pentavalent cations, and another substitutionmechanism that is extensively involved isTi4* + 02-: (Al,Fe)3+ + (OH,F)-.

There is uncertainty concerning the valence stateof V. It is common usage to list V in titanite com-positions as V2O, (e.g., Sahama 1946, Deer et al.1982, p. 449),but it might be present ̂ y3+, y4+or V5* @vans 1969). That V is trivalent in kyaniteis established experimentally (Langer 1976), and thisseems also to be the case in the V-mica roscoelite(Heinrich & Levinson 1955) and in V-bearing gar-net (Schmetzer & Otteman 1979). Staples et al, (1973)described two vanadian silicates (cavansite and pen-tagonite) with V4". Nevertheless, ionic radii in VIco-ordination (Shannon^1976) ofV3+, Va+ and V5*are0.64,0.5! and 0.54 A, respectlvely, and because56s+ (0.64 A) and Ap+ (0.535 A) Uottr subsriturefor Tia+ (0.605 A), all these valence states may bepossible from a crystqlochemical point of view.

There is also little information about dependenceof oxidation state of V on/(O). The data of Waki-hara & Katsura (1970) show that complex oxidesbetween VrO, and VO2 (V2-"O3, V3Or, V.OT) arestable over a limited range of T-f(O) below thehematite - magnetite buffer, and the V2O3 - V2_,O3stability limit is below the wiistite - magnetite buffer.Kosuge (1967) presented a phase diagram showingthat up to 700oC, ten different non-stoichiometricoxides (magneli phases) are stable between V2O3and VrOr. Therefore, it must be considered that Vmay be incorporated according to

D tto* + 02-: V3* + (OH,F)-,ii) 1ia+ = !4+, or

i i i) 2Tia* = V5+ +(Al,Fe)3+.The oxidation state of the whole rock in which the

vanadian titanite occurs is above the hematite -magnetite buffer (Reinecke 1986), and probablywithin the range of VrOr stability. Moench & Mey-rowitz (1964) and Wood (1982) have shown thatV3+ is preferentially incorporated in garnet (gold-manite) and clinopyroxene. The calc-silicate rocksfrom Gebel Umm Shagir are composed essentiallyof these two minerals, and, if trivalent vanadiumwere present, it would have been a trace element inthese minerals. Its concentration in the accessory

*i

Ti1.0

0.9

0.8

0.7

0.6

0.5

-o 10

+i3ro

oN

-oz5

\ .

\ '\ . .\ "

\

2

VzOs wt.o

Flc.6. Proportion of V2O5 rersas Nb2O5 m.q0 in titanitefrom Gebel Umm Shagir.

0 0.05 0J0 0J5 0.20 025

Nb +To +Vs*

Frc.7. Sum of pentavalent elements versusTi (symbols asin Fig, 3). Note the deviation from the trend of the idealsubstitution ZTi __ Ms+ + M3*, which seems some-what more pronounced at low contents of IUF+.

701

702

+io

+o

F+

oz

THE CANADIAN MINERALOGIST

0 0.05 0.10 015 0.20 0.25

Al+ Fe3*

Frc.8. Sum of trivalent elements versrs sum of pentava-lent elements. The straight line shows the ideal substi-tution 2 Ti: Ms+ + M3+. Symbols as in Figure 3;other symbols: square: Craveggia (Clark 194), triangle:Huron claim @aul er al. 1981), diamond: Irgon claim(Groat et al. 1985), filled square: Kola (Sahama 1946).

titanite indicates a different valence-state, unsuita-ble for substitution into garnet and clinopyroxene.Therefore, the compositions were calculated on thebasis of V5". Another argument for pentavalentvanadium is its positive correlation with the pentava-lent niobium (Fig. 6). However, it must be kept inmind that the other valence states are possible.

The end members (mol.9o) of titanite were calcu-lated in the following sequence: l) Sn end member(malayaite), 2) Fst+ + Nb, 3) Fe3* + (OH,F), 4)Al + Nb, 5) Al + Ta, 6) Al * V5*, 7) Al + (OH,F),and 8) Ti end member (titanite). For a graphicpresentation of the non-Ti end members in a trian-gular plot (Fig. 1l), the (Fe3*,M), (Al,Nb) and(Al,Ta) end members were grouped, and the[Fe3+ +(OH,F)] and [Al+(OH,F)] as wel l ;malayaite is neglected since it represents only 0.5(core) to 1.0 mol.9o (rim).

The diagram thus shows the variation in these endmembers, and also the type of zoning. If V is calcu-lated as V4* or V3* and plotted as vanadian titaniteor [V3+ + (OH,F)]-titanite, the relative distributionwould be different, of course, with a higher percen-tage of [(Al,Fe3+)+(OH,F)]-titanite, but thegeneral trend would be the same.

t '

n t t r

0 1 0

A . 'a

o l

' 9 "o

ar:i;*tW

0.3

f302to

ON

-oz.0.1

a- t - . a

o - t

aO

ox

x x xx xx

x tx

r;r1.20.10

o)

X 1x

x

oo

.. !.1o o

O 1

0I

c)

o+o0.2 0.3 0

b)

Ftc.9. Proportion of Ta2O5 versas M2O, (wt.Vo) in titanite. Note the different scales. a. Greece, b. Gebel Umm Shagir,c. Gebel Umm Shagir (shaded area) compared with titanite from Craveggia (square), Huron claim (triangle), andIrgon claim (diamond).

To 205 wt.oh

METAMORPHIC TITANITE FROM EGYPT AND GREECE 703

The vanadian titanite from Greece shows a widerrange of (Al+Fe3*) and V contents, no regulartype of zoning, and low contents of (M + Ta). The(Nb,Al)-rich titanite shows a large variation in(Nb + Ta), but only a very limited variation in V. Thecore of grains of (Nb,Al)-rich titanite is highlyenriched in the [(Al,Fe3*; + (Nb,Ta)] component.The inner margin is characterized by a large varia-tion in [(Al,Fe3+)+(Nb,Ta)], with a constant ratioof (Al+v)/t(Al,Fe3*)+(OH,F)1. In the ourer mar-gin, the content of the (Al + V) end member israther constant, and only the ratio[ (A l ,Fe3

* ) + (Nb ,Ta ) ] / [ (A l ,Fe3 * ) + (OH,F ) ]decreases. This type of zoning is interpreted asgrowth zoning, with relatively sharp discontinuitiesbetween the different zones. The shape of the zonesis irregular, as observed earlier in natural[Al+ (OH,F)]-titanite @ranz & Spear 1985). The Nb-rich core typically is embayed (Fig. l) and thereforeprobably corroded as a result of a reaction with anew Al-rich titanite component. The outer rim ispartly an overgrowth on euhedral crystal shapes,partly an overgrowth on embayed crystals. The for-mation of the zones can be correlated with differentmetamorphic stdges (see below).

Paragenetic and geochemicql considerotions

The calc-silicate marble association at Gebel UmmShagir probably represents the high-grade equivalentof a marly limestone. The possibility of a metamor-phosed carbonatite cannot be ruled out, but thereis no other supporting evidence for this hypothesisexcept the high Nb-contents in titanite. However, agranitic origin of the Nb is improbable because ofthe geological setting.

The mineral assemblages possibly developed in thefollowing way. A carbonate-marl sediment wasdeposited, together with heavy minerals such as zir-con, apatite, rutile and those that provided the Nb.These could have been, for example, complex oxidesof the pyrochlore-microlite group, the euxenite-aeschynite group, the tantalite or the ixiolite-wodginite group. The only other M-mineral besidestitanite present now in the sample is an oxide, prob-ably of the ixiolite-wodginite type, in the core of thetitanite, where Nb contents are extremely high.Therefore the core of the titanite can be interpretedas a product of the first metamorphism, and lheixiolite-wodginite inclusions are either relics or haveformed during this metamorphism from other com-plex oxides. The Al content of the rock is rather high,as documented by the presence of garnet and scapo-lite, and the Al necessary for the coupled substitu-tion was supplied by this assemblage. The high Ca-content of the scapolite indicates peak temperaturesof metamorphism on the order of 800oC (cl, Oter-doom & Gunter 1983), but the (Nb,Al)-rich titanitecould have been formed also in a prograde stage of

0 0l o.2 0.3 0.4 0.5

Nb+V+To+ Sn+ Fe&+Al

Frc. 10. Substitution for Ti by other cations in titanite.Dots, triangles and squares represent different crystalsfrom Gebel Umm Shagir (filled: core, half-filled: innermargin, open: outer margin). Crosses represent titanitecrystals from Greece.

this metamorphism. However, we suppose that thehigh Nb-content of the titanite is rather due tounusual chemical conditions than to unusual P - Tconditions.

The development of the titanite continued with theformation of an inner rim, with increasing propol-tion of [Al + (OH,F)] and decreasing proportion of[(Nb,Ta) + (Al,Fe3")] end members. The inner rimis considered to be the product of mineral reactionsinvolving the previously formed (Nb,Al)-rich cores(in part) and the matrix minerals, e.g., breakdownof scapolite, leading to titanite compositions moreenriched in the CaAI(OH,F)SiO4 component. Thissecond slage of titanite $owth is tentatively cor-related with the amphibolite-facies metamorphismthat produced the main mineral assemblages in thegneisses, and also led to the emplacement of smallgranitic stocks. The generally sharp and straightboundaries between the core and the inner rim ofthe crystals (Fig. 1) may suggest that migmatite for'mation did not follow immediately after granulite-facies metamorphism. The small syntectonic graniticstocks could not have provided the Nb, because thecore zone of the titanite was already formed at thegranulite-facies stage. The outer rim is mainlyenriched in the [AI+(OH,F)] componenu the(Al+V) component does not increase further, andthe boundary between inner and outer rim is lesssharp. This stage of mineral formation is correlated

iTl f

(At,Fe3.)*(Nb,To)

ex"'" \\9

/a\/o o\

/r%i innerI l i l m

{ ' /t o /l o lE/

704 THE CANADIAN MINERALOGIST

AI+V (At,Fe3.)+(0H,F)

Frc. l l . Relat ive distr ibution of [(Al,Fe:+) + (Nb,Ta)], (Al,V) andKAl,Fe3+) + (OH,F)I end members in the Gebel Umm Shagir (dots) and vitali(crosses) titanite crystals.

50

with the retrograde greenschist-facies overprint. Atrather low temperatures, incorporation of theCaAl(OH,F)SiOa component continues, but there isonly minor Nb available. The small grains of titanitein the matrix (number 13, Table 2)have the samecomposition as the outer rim and were probablyformed at this stage as well.

The rocks from Greece come from a completelydifferent tectonic environment. An early high-pressure, low-temperature event with a later over-print at greenschist-facies conditions is welldocumented (Reinecke 1980. The vanadian titanitein the piemontite-albite gneiss could have beenformed on the prograde path of metamorphism.Alternatively, the vanadian titanite could have beenformed by the creenschist-facies overprint of a high-P assemblage containing, e.9., V-bearing jadeiticclinopyroxene. V-bearing clinopyroxene fom high-P metamorphic rocks was described by Wood (1982).A late titanite-forming reaction resulting from thebreakdown of piemontite and rutile was describedby Reinecke (1986) for spbssartine-bearing rocksfrom the same locality.

The variable M and Ta contents, shown in Figures9a, b and c, clearly demonstrate that titanite has nopreference for either one of these elements. Theextent to which it incorporates M and Ta may onlybe a question of the geochemical environment.

CONCLUSIONS

The importance of titanite as a mineral that incor-porates many other elements is documented by theoccunence of a complex (Nb,Al)-rich variety and an(Al,V)-rich variety. Geochemical studies of meta-morphic rocks must consider titanite as a sink forNb, Ta and V, in addition to the well-known incor-poration of Sn and the REE.

The substitutions of Nb, Ta and V for Ti are com-plex. Therefore, they cannot be completely inter-preted without knowledge of the hydrogen contentand the valence state of V and Fe. Nevertheless, thisstudy shows that the most important substitutionsare:

1) 2Ti4+ : M5+ + (Al,Fe)3*,2\ 2lTl* : V5+ +(Al,Fe)3*, and3) 2Ti4+ + 02- : Al3" + (OH,F)-.

Zoning is very strong and irregular, and reflectsgrowth conditions at different stages of metamor-phism.

Vanadium is probably pentavalent in titanitebecause of the high oxidation state ofthe rocks. Ifvanadium were trivalent, it would have been incor-porated also in garnet and clinopyroxene.

METAMORPHIC TITANITE FROM EGYPT AND GREECE 705

AcKNowLEDGEMENTS

We express our sincere thanks to T. Reinecke, whomade available the V-bearing specimen, for helpfuldiscussions and for critical reading of the manuscript,and also to F. Lucassen for his help during the prepa-ration of the manuscript. Many improvements in themanuscript resulted from critical reviews by P. Hen-derson, C. Tennyson and P. Cernf. All microprobeanalyses were carried out at ZELMI, TU Berlin, withthe invaluable help of F. Galbert. This work was sup-ported by the DFG as a part of the research projectSFB 69, AI.

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Received December 12, 1986, revised manusciptaccepted March 13, 1987.