Mineral composition of  silicocarbonatites from Fuertaventura Island.

Groznova M.V., Kogarko L.N.. Senin V.G.

Vernadsky Institute of Geochemistry and Analytical Chemistry RAS,Moscow  alkaline@geokhi.ru

 

Carbonatites of  Fuertaventura island was researched. Carbonatites are submitted as sövites and silicocarbonatites. Carbonatites presented everywhere by dikes up to meter by thickness, or small local weights - shapeless formations up to 2-3 meters thickness, or the most thin veins of 1-5 sm  thickness, chaotically located in «Basal complex» islands.

Prominent feature of these silicocarbonatites rocks in difference of sövites is raised contents > 20 % on weight SiO₂. Part of silicates of these rocks contains albite, stronalsite, a granate - andradite, pyroxene - aegirite-augite, mica ferruterous phlogopite, sphene, zircon. Part of carbonate presented by calcite and strontic calcite. Also at these rocks there is a graphite, phosphates of the rare grounds (La- Ce- Nd- Sm- Pr), alumosilicate La- Ce-Nd- Sm- Pr, barite, magnetite, ilmenite and wüstite.

Identification of structures of minerals was made by a method of roentgenospectral microanalysis on a spectrometer “Camebax-SX 100”. Results of analyses of minerals are counted on mineral formulas (table 1). Graphite was defined qualitatively in raster radiation of carbon (photo 1). For the first time in carbonatites of Fuerteventura it was founded stronalsite -a mineral from group of firm solutions stronalsite-banalsite. These minerals are isostructural, also they form some continuous mutual transitions due to full isomorphism Ba and Sr.

Wüstite is the indicator of  regenerative mode of environment minerogenesis of silicocarbonatites .Wüstite is stable at temperature above 600˚С. Rather high activity of the strongest reducers are necessary for formation of wüstite in natural processes minerogenesis (H₂, CO, CH₄ and other hydrocarbons). This fact speaks about low fugacity of oxygen.

Wüstite in rock forms fine (up to 1 mm) allocation of  spheroidal forms. Frequently meets in joints with ilmenite. In the wustite there are impurity Ti (0,34 %-TiO₂),

Mn (0,28 %-MnO), Mg (0,1-0,2 %-MgO), Si (0,71-1,2 % SiO₂), P (0,13-0,25 %-P₂O₅).

Photo 1.Left- Back-scattered electron images showing relationships between minerals of silicocarbonatite : graphite, calcite, barite,albite, stronalsite. Right- raster radiation of carbon.

 

Table 1. Microbrobe analyses of albite, garnet, ilmenite, mica, titanite, wüstite, pyroxene.

 

	albite	garnet	ilmenite	stronalsite	mica	titanite	wustite	piroxene
SiO2	68.96	38.03	0.05	38.71	37.73	30.33	0.71	51.64
TiO2		0.04	51.19	0.06	3.21	35.75	0.34	0.35
Al2O3	18.73	21.11	0.1	32.07	12.02	2.32	0.21	1.22
FeO	0.11	1.01	35.9	0.14	15.29	1.20	92.65	20.02
MnO		0.15	11.79		0.35	0.07	0.26	0.74
MgO	0.02	0.07	0.12		15.77	0.01	0.14	5.55
CaO	0.09	37.02	0.11	0.02	0.04	28.83	0.1	14.34
Na2O	11.48			9.84	0.26	0.06		5.89
K2O	0.21	0.01	0.04	0.03	9.8		0.02
SrO	0.02			14.51				0.01
BaO			0.03	3.21				0.03
P2O5	0.15	0.17	0.21	0.22	0.14		0.25
Cl	0.04		0.04
Totals	99.8	97.62	99.57	98.81	94.59	98.57	94.81	99.78
	Cations	Cations	Cations	Cations	Cations	Cations	Cations	Cations
	p.f.u.	p.f.u.	p.f.u.	p.f.u.	p.f.u.	p.f.u.	p.f.u.	p.f.u.
Si	3.023	2.960	0.001	4.032	2.869	1.007	0.009	2.029
Ti		0.002	0.982	0.005	0.184	0.893	0.003	0.010
Al	0.968	1.937	0.003	3.939	1.077	0.091	0.003	0.057
Fe	0.004	0.066	0.766	0.012	0.972	0.033	0.963	0.658
Mn		0.010	0.255		0.023	0.002	0.003	0.025
Mg	0.001	0.008	0.005		1.787	0.000	0.003	0.325
Ca	0.004	3.087	0.003	0.002	0.003	1.026	0.002	0.604
Na	0.976			1.988	0.038	0.004	0.002	0.403
K	0.012	0.001	0.001	0.004	0.951		0.000
Sr				0.877				0.000
Ba				0.131				0.000
Totals	4.987	8.070	2.016	10.989	7.904	3.056	0.988	4.111

 

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