"Sr-bussenite" - potentially new mineral specie from alkaline-ultra-basic rocks (melteigite-urtites) of Khibiny massif (Kola peninsula, Russia)

 

Azarova Yu.V. *, Krinov D.I. **

* - Institute of geology of ore deposits, petrography, mineralogy and geochemistry RAS, Moscow, Russia; ** - Unclosed joint-stock company "Conducting cresearch institute of chemical technologies", Moscow, Russia

azarova_yu@mail.ru

 

The melteigite-urtites - rather widespread rocks in alkaline and alkaline-alkaline-ultra-basic complexes of the world. It is considered, that it is pleased "poor"-mineralogycal, in most cases, the rocks - rock-forming minerals in them are represent nepheline and aegirine-diopside, accessory minerals apatite, titanite, lamprophyllite, sometimes - ilmenite, pyrrhotine, is rare by others.

In the ultra-basic rocks s of a series melteigite-urtites of Khibiny alkaline massif, as well as in others alkaline and alkaline-alkaline-ultra-basic massifs, rock-forming minerals represent nepheline and aegirine-diopside, aegirine-hedenbergite, typical accessory minerals - titanite, eudialyte, lamprophyllite, apatite.

By the authors of representative article in massive urtites, select from a core of well, go past at foot of  Mt. Poachvumchorr some minerals extremely rare meeting in natural geologic objects set. It - arctite, khanneshite, bussenite Na₂Ba₂FeTi[Si₂O₇](CO₃)(OH)₃F, which until recently was considered endemic of a Kukysvumchorr deposit, of Khibiny massif and was described only once (Khomaykov, 2001). Except for naturally bussenite, the phase sets, in which at recalculation of its composition on the formula, the content of strontium (in formulation unities) exceeds the content of barium - Sr/Ba=1.2-1.5. Its idealized formula - Na₂(Sr,Ba)₂FeTi [Si₂O₇](CO₃)(OH)₃F. Formally such phase can be name "Sr-bussenite" and be a potentially new mineral specie.

Presence and forms of excretions "Sr-bussenite"

"Sr-bussenite" set as very shallow (40-50 microns) plate crystals of isometric form among accumulations of aegirine and aenigmatite (Fig. 1). The relations with minerals of urtites and localized near to cracks in rocks indicated on more later formation "Sr-bussenite" in relation to pyroxenes and nepheline, and in relation to titian-silicates. Probably, this mineral, crystallized at late, low-temperature (hydrothermal) stages minerogenesis. Contacts between "Sr-bussenite" and naturally bussenite by us the time relation between them is not revealed also remains yet clear.

 

Naturally bussenite forms larger prismatic and plate crystals and their intergrown pieces from 50 up to 200 microns and more in length, sometimes with original "apophises", among accumulations of aegirine-gedenbergite and aegirine and on contacts of grains of titanite and aenigmatite with aegirine-hedenbergite (Fig. 2). Except for numbered minerals, it associated also with baritolamprophyllite, pirrhotine and djerfisherite, developing as borders on pyrrhotine.

Chemical composition of "Sr-bussenite" and bussenite.

On the average, the composition of a phase (see tab.), for which has been established the content of strontium, exceeding content of barium (at calculated of composition on the formula) – of "Sr-bussenite", corresponds to empirical formula: (Na_2.29K_0.01)_2.30(Sr_0.83Ba_0.69Ca_0.25)_1.77(Fe_0.96Mn_0.17Mg_0.02)_1.15Ti_0.91 [Si_1.87O₇](CO₃)(OH)₃F_1.23 (quantity of fluorine is calculated, proceeding  from requirement of balance of charges in the formula, accepting quantities of groups OH and CO₃ relevant to the idealized formula of bussenite). A relation Sr/Ba=1.2-1.5, on the average 1.3.

Naturally bussenite, set in massive urtites, on the composition (table) is close to composition of a sample of bussenite Kukisvumchorr’s from hydrothermal veinlets characterized A.P. Khomaykov (2001). Feature of bussenite, set in urtites, is more considerable content Na₂O (11-13 wt. %, instead of 8-9, as in bussenite of A.P. Khomaykov (2001)) and SrO (10-13 wt. % in our sample and approximately 6-7 wt. % in sample Kukisvumchorr’s) and lower - BaO (18-22 and 28 wt. % accordingly). The medial composition of bussenite in urtites corresponds the empirical formula: (Na_2.34K_0.02)_2.36(Ba_0.75Sr_0.68Ca_0.26)_1.69 (Fe_0.95Mn_0.19Mg_0.03)_1.17Ti_0.86 [(Si_1.91Al_0.01)_1.92O₇](CO₃)(OH)₃F_1.15. The oscillations in the contents of barium and stontium allow to suppose existence of an isomorphous series of minerals: naturally bussenite - "Sr-bussenite" and, probably, characteristic for bussenite isomorphism Ba→Sr. The medial composition of bussenite from urtites, calculated on the empirical formula, disposed, probably, practically in the middle of guessed isomorphous series. Composition of bussenite, described A.P. Khomaykov (2001) answers the dominant hits baric members, and set by the authors "Sr-bussenite" - accordingly strontium-dominant members.

 

The table. Elemental compositions bussenite and "Sr-bussenite" in massive urtites of Khibiny massif.

 

	Bussenite		«Sr-bussenite»
	Average1	Range	Average2	Range
Na2O wt. %	12.43	11.29-13.27	11.05	10.21-12.16
K2O	0.14	0.00-0.29	0.10	0.00-0.22
CaO	2.50	2.08-2.64	2.18	2.03-2.26
SrO	12.07	10.10-12.93	14.53	13.58-15.66
BaO	19.72	18.32-21.78	16.63	16.01-17.63
MnO	2.32	1.91-3.00	2.27	1.95-2.64
MgO	0.18	0.00-0.83	0.04	0.00-0.12
FeO*	11.56	10.16-12.51	10.68	10.26-10.72
Al2O3	0.13	0.00-0.29	-	-
SiO2	19.64	18.27-22.57	18.64	17-79-18.99
TiO2	11.80	11.44-12.26	12.22	11.52-13.00
Total**	92.49		88.36

1 - twelve points by a method of an energy-dispersive analysis

2 - five points by a method of an energy-dispersive analysis

 * - the total FeO+Fe₂O₃ is given

** - the contents of H₂O and CO₃ was not detected, in total it were not taken into account.

 The crossed out section – element is absented in limits of a threshold of definition by a sectional method.

 

The conclusions.

1. In melteigite-urtites of Khibiny alkaline massif the phase sets which on features of the composition can be named "Sr-bussenite" and is, probably, potentially new mineral species. Besides the oscillations in the contents of barium and strontium allow to suppose existence in rocks of Khibiny massif of an isomorphous series of minerals: naturally bussenite - "Sr-bussenite".

2. The mineralogy of melteigite-urtites is much richman, than it was considered till now. Probably, lots of minerals are present at them as very shallow and microns of excretions and missed during previous examinations. Radiating from this, plenty of rare and unique minerals, including of endemics, can appear, actually, rather widespread. At detailed analysis in alkaline and alkaline-alkaline-ultra-basic massifs of the world, in particular, in Khibiny massif, rocks considered poor mineralogical, with use of precision methods in necessary volume, the rare phases characterized in sectional clause and other rare phases, can become their ordinary low-temperature minerals.

Authors are very grateful to Z.V. Shlukova  for the collection of samples given for studying

 References:

Khomaykov A.P, Men’shikov Yu.P., Yu Huyun. Bussenite Na₂Ba₂Fe²⁺TiSi₂O₇(CO₃)(OH)₃F - new similar to mica titanosilicate  from of Khibiny alkaline massif (Kola peninsula) // Zap. Vser. Miner. Obshchest. 2001. 130. 3. P. 50-55 (in Russ.)

Yakovenchuk V.N., Ivanyuk G.Yu., Pakhomovsky Ya. A. and Men‘shikov Yu. P. Khibiny. Apatity: Laplandia Minerals, 2005. 468 pp. Russia.