2012

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Magnesian kirschsteinite in melilitolites of the Pian di Celle volcano, Umbria, Italy

Sharygin V.V.

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

sharygin@igm.nsc.ru

Kirschsteinite (mainly CaFeSiO₄) is quite rare mineral in natural and technogenic systems. Its compositions with high Fe were found in meteorites, kimberlites. technogenic parabasalts and slags. Magnesian kirschsteinite and ferroan monticellite are most common of magmatic alkaline mafic rocks and high-temperature skarns (Andersen et al., 2012; Konev, Samoilov, 1974; Melluso et al., 2010; Platz et al., 2004; Sahama, Hytönen, 1957; 1958; Stoppa et al., 1997). This report is devoted to the finding of magnesian kirschsteinite in melilitolites of the Pian di Celle volcano, Italy. Only monticellite was previously described in melilitic rocks of this volcano (Stoppa et al., 1997).

Melilitolites represent the final event in the activity of the Pian di Celle volcano. They form peculiar pegmatoid vienlets with numerous vesicles in one of lava flows of melilitites - venanzites (Stoppa, 1995). The rocks contain coarse-grained melilite (up 5 cm), olivine, leucite, fluorophlogopite and Ti-magnetite as essential phenocrystal minerals that resembles in moda to fine-grained groundmass of the hosted venanzites. The interstices between essential phases are filled with fine-grained groundmass consisting Ti-rich magnetite, fluorapatite, fluorophlogopite, nepheline, kalsilite, clinopyroxene, Zr-Ti-disilicates, umbrianite, kirschsteinite, westerveldite, sulfides, aenigmatite, carbonate globules, and brown or green glass (Bellezza et al., 2004; Sharygin et al., 1996; 2011; Stoppa et al., 1997). Majority of the above minerals occur as well-shaped crystals in numerous rock vugs (up to 3 cm), in which the post-magmatic phases (zeolite, apophyllite, calcite, etc.) and vanadinite are also common.

Magnesian kirschsteinite was found in the groundmass, in which it is closely associated with coarse-grained olivines and sometimes forms the overgrowths around them (Fig. 1). The central part of olivine has composition Fo_85-80, whereas narrow rim (up to 20 μm) is richer in FeO (Fo_70-60). The concentration of CaO is approximately constant (1.5-1.7 wt.%, Table 1). The composition of kirschsteinite is also variable. The earliest zones, overgrowing olivine, have Mg# = 51-45, and most magnesian compositions nominally belong to ferroan monticellite. The latest zones of kirschsteinite are related to more ferroan compositions (Mg# = 40-30). In general, evolution of the olivine group minerals during melilitolite crystallization is directed towards compositions with high Fe and Ca: from forsterite to magnesian kirschsteinite. The similar evolution character for olivines is also common of other melilitic volcanic rocks around the world (Andersen et al., 2012; Melluso et al., 2010; Platz et al., 2004; Sahama, Hytönen, 1957; 1958).

Fig. 1. Kirschsteinite in melilitolite of the Pian di Celle volcano, Italy (scanning microscope).

Symbols: Ol - forsteritic olivine; Kir - kirschsteinite; Lc -leucite; Ks - kalsilite; Ne - nepheline; Phl - fluorphlogopite; Ap - fluorapatite; Mgt - Ti-magnetite; Cpx - clinopyroxene; Gl - glass. Numerals of analyses see Table 1.

The relationships of olivine and kirschsteinite in melilitolites (Fig. 1) have shown that during rock crystallization olivine was getting unstable and reacted with high-Ca silicate melt to form kirschsteinite and other Mg-Fe-Ca-silicates. The study of silicate-melt inclusions in minerals (Sharygin, 1999; 2001; Stoppa et al., 1997) and the presence of carbonate globules in the groundmass glass of melilitolites are the evidences of the existence of immiscible carbonate liquid during evolution of peralkaline silica-undersaturated melt with high fluorine content. Namely the occurrence of carbonate melt is seems to be responsible to the appearance of kirschsteinite.

This work was financially supported by Russian Foundation for Basic Research (grant 11-05-00875-a).

Table 1. Chemical composition (wt.%) of kirschsteinite and olivine from melilitolites of the Pian di Celle volcano.

	Минерал		SiO₂	TiO₂	FeO	MnO	MgO	CaO	Сумма	Fo	Fa	Tph	La	Mg#
18	Kir		34.10	0.03	26.04	0.22	9.53	29.90	99.83	20.82	31.93	0.28	46.97	39.47
19	Kir		34.49	0.02	24.00	0.28	11.27	29.79	99.86	24.33	29.08	0.35	46.24	45.55
32	Ol	c	39.21	0.04	17.59	0.11	41.66	1.50	100.10	79.10	18.74	0.12	2.04	80.84
33	Ol	m	39.16	0.02	17.22	0.13	41.78	1.47	99.78	79.47	18.38	0.14	2.01	81.22
34	Ol	r	37.34	0.01	26.90	0.20	33.67	1.66	99.79	67.24	30.15	0.23	2.38	69.04
39	Ol	r	36.77	0.00	30.43	0.26	30.95	1.71	100.12	62.65	34.57	0.29	2.49	64.44
35	Kir		34.53	0.02	23.92	0.21	11.39	29.82	99.89	24.56	28.95	0.25	46.24	45.90
36	Kir		34.72	0.00	22.65	0.16	12.41	29.81	99.75	26.60	27.25	0.20	45.95	49.40
37	Kir		34.85	0.00	22.09	0.16	12.59	30.17	99.86	26.92	26.51	0.19	46.38	50.39
38	Kir		33.47	0.04	28.50	0.23	6.82	30.64	99.70	15.16	35.56	0.29	48.98	29.89
40	Kir		34.56	0.01	24.31	0.26	10.93	30.13	100.19	23.57	29.41	0.31	46.71	44.48

c, m, r - core, middle and rim of grain. Fo - forsterite, Fa - fayalite; Tph - tephroite; La - larnite, Mg# - Mg/(Mg+Fe).

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