Fe-rich delhayelite from nephelinite lavas, Oldoinyo Lengai, Tanzania

Sharygin V.V.

* V.S.Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

 

Delhayelite from the Oldoinyo Lengai volcano were firstly described by Dawson (1998) and Dawson and Hill (1998) in combeite and wollastonite-combeite nephelinites. During study of the the Oldoinyo rocks we also observed the delhayelite-group mineral in nepheline-hosted melt inclusions and in the groundmass of wollastonite-combeite nephelinite. The rock studied (sample Ol7-2000, 1917 eruption) are porphyritic and contain abundant euhedral phenocrysts (>1-5 mm) of nepheline and clinopyroxene, and rarer euhedral combeite, titanite, Ti-andradite and apatite. The groundmass consists of microphenocrysts (<1 mm) of the above mentioned minerals and green to brown glass. Delhayelite, perovskite, magnetite, wollastonite with combeite corona, pyrrhotite, K-feldspar, Sr-bearing barite and calcite are minor or accessory in the groundmass.

Delhayelite occasionally occurs in large silicate melt inclusions (>40 m) in the core of nepheline phenocrysts. They are partly crystallized and also contain green glass, gas-carbonate globule, unidentified K-rich (20.3 wt.% K2O) alumosilicate and fluorite (Fig. 1). Delhayelite within inclusions forms colorless radial aggregates or individual acicular crystals (up to 10-20 m). In groundmass association this mineral forms acicular crystals (up to 50 m) in green or brown glass (Fig. 1). Some groundmass crystals indicate the presence of alteration rim. It is most common of delhayelite occurring in brown glass, up to complete replacement by water-bearing mineral of the delhayelite group. The same alteration processes were observed for delhayelite from Shahery, DR Congo (Sahama, Hytonen, 1959) and Khibini (Dorfman, Chiragov, 1979) and delhayelite-like mineral from Pian di Celle, Italy (Stoppa et al., 1997).

 

Fig. 1. Delhayelite in nepheline-hosted inclusions and groundmass of wollastonite-combeite nephelinite, Oldoinyo Lengai, sample Ol7-2000, 1917 eruption (ordinary light and scanning microscopy, SEM).


Note: Gl - silicate glass; Dlh - delhayelite; g - gas bubble; Fl - fluorite; Kp - unidentified K-alumosilicate; Cmb - combeite; Ne - nepheline, Alt - altered delhayelite (hydrodelhayelite ?); Cal - calcite; Po - pyrrhotite; SiC - abrasive. Scale bar - 10 m.

 

New EMPA data confirmed very specific composition of the Oldoinyo delhayelite obtained previously by Dawson (1998) and Dawson and Hill (1998). This mineral contains high concentrations of Fe2O3, TiO2, MgO and S and low amounts of Al2O3 (Table 1). These compositional features are remarkable in comparison with the Khibini delhayelite (Dorfman et al., 1961; Stoppa et al., 1997; Ageeva, 2002; Sharygin, 2002; Sokolova et al., 2005). It should be noted that delhayelite from nepheline-hosted inclusions are richer in F, S and poorer in Cl than groundmass mineral (Table 1). Altered rims on the groundmass crystals differ from fresh samples in lower alkalis, F, Cl and higher SiO2 and seem to be hydrodelhayelite or other water-bearing mineral of the delhayelite group (rhodesite, macdonaldite, Chiragov, Dorfman, 1981). The high Fe2O3 and low Al2O3 strongly suggest that iron is incorporated into tetrahedral site. The calculations on the basis of (Si+Al+Fe3+)=8 indicate formula for the Oldoinyo fresh delhayelite close to K4Na2Ca2[(Al,Fe3+)Al7O19]F2Cl, where Fe3+ is up to 0.6 apfu, K is 3.3-3.7 apfu, Ca is 2.1-2.4 apfu and Na is near 2.0 apfu. In general, this approaches formula, proposed for the Khibini delhayelite: HxK8-xNa4Ca4[AlSi7O19]2F4Cl2, where x=0.5-1.5 (Sharygin, 2002). The possible incorporation of iron as Fe3+ in delhayelite is consistent with mineralogy of the Oldoinyo nephelinites, where the groundmass minerals contain high abundance of Fe2O3 and some of them (sodalite, nepheline) may bear Fe3+ in tetrahedral site (Dawson, 1998; Dawson, Hill, 1998)

At present day, the studies for the crystal structure of delhayelite were provided for the Shahery holotype, K7(Na3Ca)Ca4[AlSi7O19]2F4Cl2 (Cannillo et al., 1970), and the Khibini holotype, K3Na2Ca2[AlSi7O19](F,Cl)2 (Chiragov, Mamedov, 1974). However, both structural formulae are so far from real compositions of fresh delhayelite, especially in lower contents of K2O (Stoppa et al., 1997; Ageeva, 2002; Sharygin, 2002; Sokolova et al., 2005). The composition of the Khibini delhayelite with highest content of K2O (20.75 wt.%, Sokolova et al., 2005) shows K up to 3.94 apfu. In addition, structural study of the Khibini delhayelite indicated the presence of domains with composition K4Na2Ca2[Al2Si6O19](Cl,F)2H2O (Chiragov, Mamedov, 1983), which is very close to delhayelite-like mineral from Pian di Celle (Stoppa et al., 1997; Sharygin, 2002). Apparently, the crystal structure of delhayelite requires refinement, especially in respect to positions of K, F, Cl, S, H2O and possible incorporation of Fe3+. Recent chemical data for the Khibini delhayelite show variable contents of K2O, F and Fe2O3 (Stoppa et al., 1997; Ageeva, 2002; Sharygin, 2002; Sokolova et al., 2005). The concentrations of SO3 in different samples of the Khibini fresh delhayelite varies from 0.0 to 1.05 wt. % (authors data), whereas the Oldoinyo samples contain up to 2.3 wt.% SO3 (Table 1). However, the specification of sulfur in the delhayelite structure is not clear. Based on IR spectra of delhayelite from Khibini, Sokolova et al. (2005) suggest the possible incorporation of sulfur as (SO4)-group. Nevertheless, the existence of S2- should not be excluded like in minerals of the hauyne-sodalite group.

 

Table 1. Representative analyses (wt.%) of delhayelite from the Oldoinyo Lengai wollastonite-combeite nephelinite in comparison with fresh minerals from other nephelinites of Oldoinyo Lengai and rocks of Khibini and Pian di Celle.

 

 

n

SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

BaO

SrO

Na2O

K2O

F

Cl

S

H2O

O=F,Cl,S

Total

1

2

44.87

0.56

3.69

4.82

0.05

0.41

14.58

0.00

 

6.79

18.38

5.33

2.64

0.36

 

3.02

99.45

2

1

45.00

0.35

2.99

4.59

0.05

0.34

14.64

0.23

 

7.02

16.92

5.15

1.30

0.93

 

2.92

96.57

3

1

43.21

0.58

4.64

5.51

0.15

0.57

12.75

0.00

 

6.42

18.99

4.45

4.75

0.25

 

3.07

99.20

4

1

43.87

0.18

5.35

3.50

0.08

0.46

13.86

0.00

 

6.76

18.80

4.55

4.46

0.06

 

2.96

98.97

5

1

43.98

0.43

5.11

4.45

0.09

0.58

14.30

0.08

 

6.77

18.59

4.16

4.35

0.10

 

2.78

100.19

6

1

54.08

0.30

4.93

5.28

0.19

0.55

11.06

0.05

 

0.24

4.88

3.05

0.32

0.04

 

1.38

83.59

7

1

51.61

0.30

4.10

5.49

0.16

0.55

15.87

0.22

 

1.63

6.75

3.38

2.25

0.12

 

1.99

90.44

8

1

54.07

0.30

5.93

3.33

0.18

0.68

15.88

0.05

 

0.27

2.80

3.97

0.99

0.02

 

1.90

86.58

9

9

44.60

0.11

6.18

1.83

0.09

0.09

14.70

 

 

6.94

18.00

4.09

3.87

0.05

 

2.62

97.93

10

1

45.60

0.19

4.71

3.17

0.09

0.48

14.90

 

 

7.13

17.80

4.76

3.86

0.04

 

2.89

99.83

11

4

47.31

0.00

5.88

0.59

0.12

0.03

12.78

 

0.30

7.25

20.25

2.45

3.85

 

 

1.90

98.89

12

31

47.58

0.01

6.11

0.14

0.11

0.04

13.40

0.02

0.27

6.77

19.95

4.58

3.74

0.13

0.75

2.84

100.76

13

16

42.74

0.05

11.94

1.36

0.05

0.40

10.99

0.03

0.15

3.57

22.59

3.63

4.98

0.01

1.18

2.65

101.01

Note. 1-10 - Oldoinyo Lengai: 1-8: wollastonite-combeite nephelinite, sample Ol7-2000, 1917 eruption (this work): 1-2 - from nepheline-hosted inclusions; 3-8 - from groundmass: 3-5 - fresh mineral; 6-8 - altered mineral; 9-10 - from groundmass of combeite and wollastonite-combeite nephelinites, respectively (Dawson, Hill, 1998; Dawson, 1998); 11-12 - Khibini massif: 11 - veinlets in ristschorrite, Rasvumchorr (Sokolova et al., 2005); 12 - pegmatite, Yukspor (this work); 13 - delhayelite-like mineral, Pian di Celle volcano, Italy (this work). H2O is determined by SIMS, n means number of analyses.

 

References:

Ageeva O.A. Typomorphism of accessory minerals and evolution of mineral formation in rocks of ristschorrite complex (Khibini massif) // PhD Thesis. Moscow 2002. 187 p. (in Russian).

Canillo E., Rossi G., Ungaretti L. The crystal structure of delhayelite // Rendiconti Societa Italiana di Mineralogia e Petrologia. 1970. Vol. 26. P. 63-75.

Chiragov M.I., Dorfman M.D. Crystal chemistry of minerals of the delhayelite group // Doklady Akademii Nauk SSSR. 1981. Vol. 260. P. 458-461 (in Russian).

Chiragov M.I., Mamedov Kh.S. Crystal structure of delhayelite, Ca2Na2K3[(Si,Al)8O19](F,Cl)2 // Mineralogicheskii Sbornik L'vovskogo Gos. Universiteta. 1974. No. 28. Pt. 1. P. 3-7 (in Russian).

Chiragov M.I., Mamedov Kh.S. Crystal structure of new mineral of the delhayelite group // Abstracts of IX Vsesouznogo soveshchaniya po rentgenografii mineralnogo syrya, Kazan, 1983. P. 24-25 (in Russian).

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Dawson J.B., Hill P.G. Mineral chemistry of a peralkaline combeite lamprophyllite nephelinite from Oldoinyo Lengai, Tanzania // Mineralogical Magazine. 1998. Vol. 62. P. 179196.

Dorfman M.D., Belova Ye.N., Neronova N.N. Delhayelite from Khibini // Trudy Mineralogicheskogo Muzeya Akademii Nauk SSSR. 1961. V. 12. P. 191-195 (in Russian).

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Sokolova M.N., Smolyaninova N.N., Golovanova T.I., Chukanov N.V., Dmitrieva M.T. Delhayelite crystals from ristchorrites of the Rasvumchorr plateau (Khibiny massif) // New Data on Minerals. 2005. Vol. 40. P. 115-118.

Stoppa F., Sharygin V.V., Cundari A. New mineral data from the kamafugite-carbonatite association: the melilitolite of Pian di Celle, Italy // Mineralogy and Petrology. 1997. Vol. 61. P. 27-45.
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