Trrn AMERICaw MINERALoGISTJOURNAL OF THE MINERALOGICA]. SOCIETY OF AMERICA

Vo]. 54 JANUARY_FEBRUARY Nos. 1 and 2

TUNISITE, A NEW CARBONATE FROM TUNISIA

Zoorqor JonaN,r Pir.vBr, Povonnna, aNo Envrn Srl,wsrv,2Geological Institute oJ the Czechosl,oaak Acad.emy oJ

S c,iences, Progue, C zechosloaakia.

Assrnlcr

Tunisite, NaHCazAlr[(COr)r(OH)t,l is a new carbonate found at the Pb-Zn deposit,Sakiet Sidi Youssef, Tunisia. It occurs in fine-grained white aggregates or in small tabularcrystals associated with pyrite, marcasite, sphalerite and calcite in hydrothermal veins.

The new mineral is tetragonal; D7a1,-P4f nmm; a:11.22+0.01, c:6.582+0.001 A,c/a:0.5866; I/ :828.6 43, cell contents: 2{NaHCa:Alr[(COr)n(OH),0.]. The strongestlines in the X-ray powder photograph are: 5.615 (10), 2.592 (9),3.551 (8), 3.288 (7),2.s26 (7) A.

Tunisite is colorless. H:45; p2.51+0.02 (meas.),2.48 (calc.). Basal and prismaticcleavage very good. Optically uniaxial (+), @:1.573 +0.001, e :1.599+0.001.

The chemical analysis: NazO 4.77, KzO 0.35, CaO 18.08, AIzOt 32.56, COz 28.66, HzO+15.04, HrO- 0.51,> 99.97 wt./6. DTA shows a strong endothermic peak at 440" (escape ofSCOz*7HzO) and an exothermic peak at 850' (formation of new crystalline phases andescape of 2HzO). IR spectrum shows absorption bands at 680 s, 850 s, 1155 s, 1530 vs,1725 iv, 1860 vw, 1920 w,2330 vw, 2650 w, 3500 s cm-l.

INrnonucrroN

A mineral, later established as a new species, was found by one of theco-authors (Z.J) on the dumps of the Pb-Zn ore deposit of Sakiet SidiYousseff, Tunisia, in 1965. This new Na, Ca, Al carbonate was calledtunisite after the country of its first occurrence (Tunisia).

The ore deposit of Sakiet Sidi Yousseff is situated in the vicinity of theAlgerian frontier at a road connecting the town of Le Kef with that ofSouk Ahras. The now abandoned mine l ies at about 2 km to the south-east of the vil lage on the slope of Djebel-er-Ressas (elevation 828 m).

Similar to the majority of TunisianPb-Zn ore deposits, the deposit ofSakiet Sidi Yousseff is developed on the tectonic boundary between theso-called extrusive Triassic and other geological units represented in thiscase by the Upper Cretaceous (Cenomanian-Maestrichtian) sediments(Sainfeld, 1952).

1 Present address: B. R. G. M , B. P. 818, 45-Orl6ans-La Source, France.2 Present Aclclress: Department of Geology, Kuwait University, Kuwait.

JOIIAN, POVONDRA AND SLANSKY

The mineralizatron is developed in a series of small veins parallel to theanomalous Triassic boundary occuring in a zone of approximately 10 m

width and 160 m length (Sainfeld, 1952), as well as in the transversal

dislocations. The Iatter type of veins reaches a thickness of up to l'5-2m and a length of about 300 m.

The gangue material is almost exclusively calcite. Quartz occurs only

very sporadically. On the basis of the study of material found on the

dumps it appears that some parts of the veins were enriched in barite.

The following ore minerals were identif ied by Sainfeld (1952): galena,

sphalerite (colloform, "schalenblende"-type), marcasite, and pyrite.

So far only two specimens of tunisite have been found. It fills the cavi-

ties in the white calcite part of the veins, or it is developed as fine covers

on the calcite crystals. It apparently belongs to the latest hydrothermal

stage.

Quer-rrarrve SpnctnocunltcAl ANArYsrs

The analysis was performed (analyst Z.Houdkova) by means of a Zeiss (Jena) Q 24 UV-

spectrometer (intermittent arc, slit width 0.0045 mm, electrode distance 5 mm, time of

exposure 90 sec.). The elements determined were visually divided into five concentration

ranges (the elements underlined are near to the boundary of the higher concentration

range, those in brackets are close to the upper limit of the lower concentration range). The

specimen used for the spectrochemical analysis gave the following results: 10r: AI,Ca,Na;

100: K,Mg; 10 1: Mn,(Pb),(Si); 10-': Ag,Cr,Zn,V,Sr; lj-37a: Cu,Fe,Zr(?).

Cnnurcer, ANlr-vsrs

Tunisite is easily soiuble in concentrated inorganic acids, except nitric acid. If dilute

inorganic acids are used, the decomposition is accompanied by a strong hydrolysis of the

Al-salt thus produced; especially so during prolonged boiling. The hydrate formed is then

practically insoluble in acids.A sample weighing about 0.5 g was used for the analysis. It was dissolved in concen-

trated hydrochloric acid at room temperature and then carefully at higher temperatures.

When the decomposition was completed, the solution was oxidized by a few ml of nitric

acid, then diluted to the volume of 100 ml and boiled shortly in order to expel the nitrogen

oxides. After cooling, the solution was transferred to a 250-ml volumetric flask and brought

up to the 250-ml volume. Amounts suitable for separate determinations were then taken

from this solution. Calcium was determined by volumetric titration in a strongly basic

medium using the solution of Complexon III and fluorexon as a metafluorescent indicator.

Aluminum was analyzed indirectly by determining the amount of ethylendiamintetracetic

acid released after the reaction oI complexonates with fluoride ions. The determination was

made with the aid of aZn-salt solution in a weakly acid medium (pH 5) during a biampero-

metric indication using fenoryanide/ferricyanide oxy-redox system added in a very small

amount. Alkalies were determined from the reserve solution by means of flame photometry

in the medium of hydrochloric and phosphoric acids.

For the analysis of water and COz separate weights of 0.2g were used. Water was de-

termined gravimetrically according to Penfield using lead sesquioxide as a flux. Carbon

dioxide was determined by means of volumetric-absorption gas analysis method.

TLTNISITIa

T,q.r:r,r 1. Csrmclr- ANalvsts oF TuNrsrrE

NazOKrO

fiaOAlsOsCOzHzO*H:O-

0 . 3 518 0832 5628.6615 040 . 5 1

ee.e7%

4 8 00 . 3 5

1 8 . 1 83 2 . 7 428 -81t J . t z

r00.00%

0. 1548\0.0074Jo.32420.64220. 6548t .6790

1

2 .003 . 9 64-O4

10 .35

4 . 9 8

18 .033 2 . 7 728.3015.92

100.00%

Themula:

P. Povondra, Analyst.

1 Weight 70.2-Weight 0/p recalculated.

3-Atom proportions.4-Mole ratios.5 Theoretical composition for NaHCazAla[(COr4(OH)s].

results of the chemical analysis (Table 1) yield the empirical for-

(Nao.gsKo os)r .ooCar ooAla.soHo.zs[(COa)e on(OH)n.ur ] ,

f ormula weight : 616.96.The ideal chemical formula of tunisite can therefore be expressed as

NaHCarAl+ [ (cor) n(oH) ro1.

X-Rav Srupv

Despite the small size of tunisite crystals, oscillation and Weissenberg

photographs (Cu/Ni) about the [001] axis were obtained and the t param-

eter:6.58*0.01 A was derived from them. The zero layer-l ine was in-

dexed with the use of the linear relation of dr,r.o and o. Tunisite is tetrag-

onal with the following unit cell dimensions:

a : 1 1 . 2 2 + 0 . 0 1 Ac :6 .582 - t0 .001 A

c/a:0.586; V:828.6 ]rt ' Z:2; the calculated specific gravity is 2.48.

The specific gravity of tunisite determined by the pycnometer method is

2.51 + 0.O2. The unit cell contains 2 { NaHCazAl4[(COJ4(OH)10] ].Laue class Dqn-4fmmm. The results of thd indexing of the Weissen-

berg photographs show only systematic absence of hk| reflexions with

h*hl2n. Space group of the mineral is D*'7 - P4lnmm. The indexed

X-ray powder pattern of tunisite is given in Table 2. The intensities of

TOIIAN, POVONDRA AND SLANSKY

T.q.lr,n 2. X-ney Powopn Dar.q lon TuNrsrrl,(Cu/Ni ; I :1.5418 A; Camera Diameter 114.59 mm)

I

7o

7I25I

I

361I

.)

I

5

7 . 9 26. 595 . 6 1 55 0704 . 2 6 7s .9933 . 5 5 13.4033 . 2 8 83.r283 035

2.8r1a , < A

2.5922 . 5 2 62.4542 412L , J + I

2.269

7 .9346. 5825 . 6 1 05 .0664 . 2 7 03.9923 .5493.3993 . 2 9 23. r243.0402 8142.8052 .7522 .5812 533z +54

2.4132.3452.2612.2002 . 1 9 4

2 1242.O972.0432 0r01 .986r .9831 .9031 . 8 9 91 . 8 7 01 . 8 6 6

11000r2001 1 12012113102210023llr123214002 t 2401222J.) I

3 1 2421322510003103501; 431412203213521440303441600J13

d(meas)

1 . 8 3 11 8001 . 7 6 0r . 7 2 91 698

1 . 6 5 0

| . 6 2 8

1 5871 576

1 558

1 . 5 4 9

1 . 5 1 4

1.492

l . + / . )

d(ca lc )

r .8291 7991 . 7 6 01 . 7 2 81 6991 6521 . 6 4 51.628| 6261 . 5 8 7| 579I .).) /

r 556.517. J _ t 4

5 1 1.493491

. 4 4 1

hht

J t z

601522403442423004104f f i 25 5 0 ; 7 1 020470164054264152331463273070273r650603414642334722424

821660

d(meas)A d(calc)A hht

34-5

106358176

3-4

36346

6

3

zt-2

1

1

I

J

l-2

3

2

2

3-4

t-22-3r-2r-2

J\

I

2.200

2 1532 . 1 2 52.0962.O402.010

1 .988

1.900

I .866

I 4381 . 4 3 7

1 . 3 7 71 .345I .334r.322

| . 4 2 31 .408t .407| 397L396r . J / o1 .345| 332| 322

001 diffraction lines on the X-ray powder pattern are enhanced due topreferential orientation resulting from the very good basal cleavage.

Pnvsrcar, PnopBnrros

fn the specimens studied tunisite most frequently forms fine-grainedaggregates of pure white color which are composed of microscopic sub-hedral crystals. occasionally, the aggregates consist of euhedral or skere-ton crystals of the same size. In the latter case they are developed as thin

TANISITE

Fro. 1. Tabular tunisite crystals in a cavity of calcite. 4X.

covers on calcite crystals. Tunisite occurs very rarely in macroscopictabular crystals which form parallel groups (Fig. 1). The form of themicroscopic crystals can be seen on Figure 2. The tetragonal crystals oftunisite are tabular with strongly developed basal pinacoid. This facepredominates even on skeleton crystals. The size of tunisite crystals maybe expressed mostly in hundredths of mml quite exceptionally microscop-ic crystals of 0.2 mm in size were observed. The dimensions of macro-scopic crystals range between 2 to 8 mm. Beside the (001) and the (110)there is also observed the (100) and (101). Two crystal types were ob-served, the schematic drawings of which are given in Figure 3. Unfortu-nately, owing to the size of the crystals and their parallel grouping, it wasnot possible to perform any goniometric measurements.

The hardness of the crystals measured on the (001) face is 4.5 (in termsof Mohs scale); the hardness of the aggregates is 3.5. Tunisite has a per-

JOITAN, POVONDRA AND SLANSKY

Frc. 2. Microscopic tetragonal and skeleton crystals of tunisite. 120X.

fect basal and prismatic cleavage. The mineral is optically uniaxial (f).The indices of refraction (for sodium light) are: a:1.573 *0.001 e :

1 .599 + 0 .001 , e - a : 0 .026 .

TnpBlrar, Sruov

The DTA curve of tunisite (Fig. a) is characterizedby the presence ofan endothermic peak at 4400C and an exothermic one at 850oC. The formof the curve on heating the sample in an inert atmosphere (argon) is notchanged, except that the peak temperatures are slightly shifted towardsIower values (420 and 830oC respectively).

BFrc. 3. Schematic drawings of two tunisite crystal types.

A

TUNISITE

100 200 300 400 500 600 700 800 900 /000"e

Fro. 4. The DTA (B) and thermogravimetric (C) curves of tunisite.

Curve A is the first derivative of C.

100

90

80

60

50

40

30

t0

100

90

EO

70

60

50

10

30

20

/o

0

t00

90

80

70

60

40

/000 " c

ctti'

3000 2000 tE00 /600 hao 200 1000 600 650

Infrared absorption curves of tunisite heated to 650,950, and1000"C respectively. N- Nujol peaks,

- 5000 +000

Frc. 5

TUNISIT'E

According to the thermogravimetric curve (Fig 4) the endothermic re-action corresponds to the loss of weight oI 38.2 percent. The entire lossof weight up to 1000oC calculated from the thermogravimetric curveequals 40.8 percent. The exothermic reaction at 850'C is accompanied bya loss of weight of 2.6 percent. Taking into account the amount of COzand HzO determined by the chemical analysis it may be concluded thatthe endothermic reaction is caused by the simultaneous escape of COzand H2O. By calculation it can be established that the reaction at 4200Cis due to the decomposition of the mineral accompanied by the simulta-neous escape of 8 COz*7 HzO. Theoretically, the loss of weight of 38.4percent corresponds to this escape which is in good agreement with theloss of weight recorded by the thermogravimetric curve. At 850oC, 2H2Oare expelled (the theoretical loss of weight is 2.97d and at the same timethe crystallization of several phases takes place. Comparing the resultso{ the thermogravimetric analysis with the COr and HzO contents deter-mined by the chemical analysis it is apparent that at the temperature of1000'C two water molecules are still present in the specimen. In fact,their presence can be detected from the infrared absorption curves oftunisite heated to 1000'C (Fig. 5). On the same figure the curves of infra-red absorption spectra of tunisite specimens heated to 650 and 950oC re-spectively are given for the purposes of comparison. The O-H absorptionband at 3500 cm-l is for obvious reasons also distinctly recorded on them.

The products resulting from heating the tunisite specimens to the mentioned tempera-tures were subjected to X-ray powder difiraction study with the Iollowing results:

1. After the escape of 8 COz*7 HzO, the specimens are composed for the most part ofan amorphous phase. Besides, but in a smaller amount, a crystalline phase is present givinga simple X-ray powder pattern that ca.n be indexed on the basis of a cubic unit cell witha:2.818 A. The chemical composition of this phase which we denote as phase a could notbe established.

2. The amorphous product occurring at 440"C crystallizes at the temperature of 850'C(exothermic reaction). As can be concluded from Table 4, in addition to phase a, thepresence of 12CaO. TAbO: and CaO.ALOg was detected. There are still some other phasespresent in this material. However, they could not be identified.

TeslE, 3. X-nev Pomnn Dera ron TuNrsrrr Hnerro ro 650' C(Cu/Ni; I:1.5418 A; cu-era diameter 114.59 mm)

d(meas)A

2.8231.993t .625| .4091.26 t1 . 1 5 1

d(cab)A

2.818r .993t . 6 2 7t .4091.2 f f i1 . 1 5 1

hktr

109J

2

1001101 1 1200210ztt

.IOHAN, POYONDRA AND SLANSKY

T,q.em 4. X-nev Pomnn Dere ror TuNrsne Hrarrn ro 950o C

Tunisite heated to9500Cn

2 v b5132 b1o

Diffuseband

I108I739562 b72I292 v b63 bz

I32 b5452I9 v b38 b122312312

s .s4 A4 9 04 6 64.3964.0r43 7463 4803 . 2 7 3 -3 1863 0582 9662 8302 . 7 3 52 6642 5852 5122 . 4 3 62.3922 3292 1812 1322 0922.0492.002| 9491 . 9 1 81 .833| . 7 9 7| . 7 6 0| . 7 2 8| 6921 6581 .6301 . 6 0 11 5791 . 5 5 21.4901 . 4 7 8I . 4 J J

1 . 4 3 21.4091 . 3 9 21 . 3 7 11 .3561 .3401 .3081.2981 . 2 7 91 263

Phase a

1001 a

50

45

40

2.823

| . 2 6 1

9

30

1 17

309

30

.)7

l a

377.)55

12 CaO.7 APzO(ASTM

Card 9-413)

4 8 9

3 . 7 9

3.204

2.998

2.6802 . 5 5 6

2 . M 7

2.189

2.054

| 7301 . 6 9 5r . 6 6 2I .6301 . 6 0 1

I .498t . 4 / J

1 3561 .340t .307| . 2 9 2t . 2 7 7r zo,7

l 32.')71

I

10071

4227

7105.J

5

20

205

35

CaO.Al:Or(ASTM

Card 1-0888)

5 . 6

4 . 6 94 . 4 14 . 0 53 7r3 5 0

3 . 2 0

2 9 72 8 52 7 5

2 5 22 . 4 1

2 3 32 2 02 1 32 . 0 8

1 . 9 21 8 4

t . / J

1 . 6 9l . o J

1 . 5 8

1 . 5 3

' (CulNi; Camera diam. 114.59 mm)

T| ;NISITE L l

Tenr,B 5. X-n.tv PowoBn Dar,q lon Tunrsrm Hslrno ro 1000"C

Tunisite heated to

1000'c'

12CaO.7AhO:iG. Hentschel,

1964)

CaO'AhOa(ASTM

Card 1-0888)

3CaO'SAl:Oa(ASTM

Card 1-0572)

L

o

1L

I21I

11

103I

7

94

71II328211L

1A

I

17

s .s2 A4 .9 r4 .684.444.2204.0503 .8103 .6053.5213 .2033 .0792.9792 .8712.7622 .6952.ffi32.5252.4482.41r2 . 3 M2 . t962.1452.0972 .0582.0231.948t .9251 .8341 .799r .767r . 73 r1 . 6 9 41.662r .6371 .605t .579r . 5 M1 .532

InS

4 .23

3 . 8 2

2 . 6 9

a A <

2 . 3 62 . 1 9

2 . 0 6

t . 7 3t . 7 0| . 6 6t .641 6 0

3 .00

m

mwm

lV

m

13z

277

10

40273340

100

1007

5 6

4 . 6 94 4 1

4 . 0 5

3 . 2 0

2 . 9 72 . 8 5

2 . 5 2

2 . 4 12 . 3 32 . 2 0

2 . 0 2

t . 6 9

1 . 6 5

4.46

3 . 5 0

? o o2 . 9 62 . 8 7z - l J

2 .6 )

2 .+4

2 .05

1 80r . 7 6

t . 6 2

1 . 5 3

" (Cu/Ni; Camera diam. 114.59 mm)

t2 JOEAN, POITONDRA AND SLANSKY

Taera 6. Trenltar, RlecrroNs or TuNrsnr

Reaction Temperaturec/6 Loss of

Weight Interpretation of the reactions

Endothermic 3 8 2 Escape of THzO*SCOz, occurrenceof the cubic a phase and an X-rayamorphous phase

Exothermic Escape of 2HsO, crystallization ofCaO'Al:Or, 12 CaO' TAlzOe and ofother unidentified ohases

3' The diffraction lines of the phase a were not found on the X-ray powder pattern ofthe specimen heated up to 1000'C (Table 5). Of the other phases, CaO.AlzO:,3 CaO..5AlrOs, and 12 CaO' 7 AhO: were identified.

Taking into account the chemical composition of tunisite, it is noteworthy that in theheated specimens no phase containing sodium in essential quantity has been identifiecl.The results of the thermal study of tunisite are summarized in Table 6.

fNrn,qnBn AnsonprroN Arer,vsrs

The infrared absorption curve of tunisite (Fig. 6) was obtained with aUNICAM 200 infrared absorption recording spectrometer using Nujolmethod. The infrared spectrum of tunisite is characterized. by the pres-ence of absorption bands with peaks at 680 s, 850 s, 985 s, 1155 s, 1530 vs,1725 w,1860 vw, 1920 w,2330 vw, 2650 w, and 3500 s cm-1.

Because of the similarity in the chemical composition of dawsonite and

,00 3000 2000 t800 ,600 /400 ,200 1000 a00

Frc. 6. Infrared absorption curve of tunisite. N-Nujol peaks.

440'C

TLINISITE

tunisite it was possible to use the results of the paper by Frueh andGolightly (1967) for the interpretation of the infrared absorption spectraof tunisite. To the COa2- vibrations correspond the absorption bands at850 s, 1155 s, and 1530 vs cm-1. Compared with the results obtained byStubi6an and Roy (1961), it is possible that the absorption at 985 s cm-lis caused by O-H-AI vibrations. The latter authors expressed the opin-ion that these vibrations were essentially due to OH- vibrations the oc-curence of which could take place in the presence of another element, inthis case Al. The absorption at 680 s cm-l may be due to Al-O vibrations(Frueh, Golightly, 1967). The remaining weak absorption peaks could'not be explained. It is noteworthy that within this region some absorp-tion bands were established by Frueh and Golightly (op. cit.) for daw-sonite as well.

The specimens of tunisite are kept in the mineralogical collection ofthe National Museum in Prague (specimen No. 53823). The new mineralwas approved by the Commission of New Minerals and Mineral Namesof I.M.A. in November 1967.

AcxNowlBnclmNts

The authors wish to thank the members of the Service Gdologique de Tunisie in Tunis,especially M. A.. Azzouz and MIle. L. Memmi, for generous help in providing transport andother facilities for the field work at Sakiet Sidi Youssefi. They also extend their thanks tofng. Z. Houdkova for spectrochemical analysis of tunisite and to Dr. Janglova for infraredabsorption analysis of the specimens of this mineral.

The journey of two coauthors of this paper.(2. J., E.S.) to Tunis in 1967 was madepossible by a grant from the Czechoslovak Academy of Sciences, which is gratefully ac-nowledged.

Rnlrnnucrs

Fnuur, A. J., eNo J. P. Gor.rcnrr,v (1967) The crystal structure of dawsonite NaAl(CO)(OH)". Con. Minud.,9, 51-56.

HrNtscrnr,, G. (1964) Mayenit, l2CaO.7AlzOs, und Brownmillerit, 2CaO.(Al,Fe)zOr,zwei neue Minerale in den Kalksteineinschliissen der Lava des Ettringer Belleberges.Neues f ahrb. Mineral,., Monotsh. 1,22-29.

Sarulrr-n, P. (1952) Les gites plombo-zincifEres de Tunisie. Ann. Mines GCol. Tunis 9.StuuiaN, V., nr.rl R. Rov (1961) A new approach to assignment of inftared absorption

bands in layer-structure si licates. Z. Rr'i s t oll o gr., ll1, 200-27 4.

Monuscript recehted, June 11, 1968; aecepted Jor publicati,on, October 10, 1968.

I J