Monticellite is a mineral in the olivine group. Its chemical composition is intermediate between forsterite and calcio-olivine. It is considered a valid mineral because the Ca and Mg atoms are ordered in different positions in the molecule, and there is very limited replacement of calcium. It would appear, however, that monticellite forms a series with kirchsteinite CaFe(SiO₄).

Monticellite is not an uncommon mineral, and is listed from 199 localities in Mindat (2022), including slag dumps and meteorites. It occurs in two distinct parageneses:
1) In metamorphic siliceous and magnesian limestones, and in skarns at the contacts between metasomatized limestones and instrusive rocks.
2) In intrusive mafic rocks such as kimberlite and alnöite, as well as in many carbonatites.

Pseudomorphs of pyroxenes and serpentines after monticellite are known, and some wonderful “fassaite” pyroxene pseudomorphs after monticellite crystals up to 5 cm are known from the Monzoni Mountains, Fassa Valley, Italy.

Well developed crystals are rare, but can occur in both of the typical parageneses. The best specimens and crystals are those from the Crestmore Quarries, California, USA, where prismatic crystals up to 8 cm in length have been reported in the contact aureole between a quartz monzonite and a limestone.

Canada

St Lawrence Columbium Mine, Oka, Deux-Montagnes RCM, Laurentides, Québec, Canada

Pyrochlore was discovered in the Oka carbonatite complex in 1953, and numerous mining companies conducted exploration work following the discovery. The St. Lawrence Columbium Mine, which opened in 1961, was one of largest niobium mines in the world before it ceased operations around 1977. Significant reserves of pyrochlore remain at the deposit.

Lentz et al. (2006) give a summary of the geology and petrology of the mine area, where coarse-grained calcite-carbonatite (søvite) is the dominant rock. They describe nine intrusive events, and two of these events contain monticellite-bearing søvite with up to 10 vol% monticellite.

Unfortunately, the mineralogy of the St. Lawrence Columbium Mine is poorly described, but it appears that monticellite mainly occurs as anhedral to subhedral grains, although relatively well formed crystals can be found.

References:

Lentz D., Eby N., Lavoie S. and Park, A. (2006) Field trip B4, Diatremes, dykes, and diapirs: Revisiting the ultra-alkaline to carbonatitic magmatism of the Monteregian Hills. GAC/MAC Joint Annual Meeting, Montréal, Québec, Post-conference field trip, 48 pages.

Sabina, Ann P. (1983): Rocks and minerals for the collector: Kingston, Ontario to Lac St. Jean, Quebec. Geological Survey of Canada, Miscellaneous Report 32, 142 pages.

Treiman, Allan H. and Essene, Eric J. (1985): The Oka carbonatite complex, Quebec: geology and evidence for silicate-carbonate liquid immiscibility. American Mineralogist, Vol. 70, pp 1101-1113.

Germany

Caspar quarry, Ettringen, Vordereifel, Mayen-Koblenz, Rhineland-Palatinate, Germany

Volcanism in Eifel is a part of a series of intra-plate volcanic fields in Central Europe that have been active throughout the Tertiary and Quaternary and is largely related to rifting of the Rhine Graben and broad uplift of pre-Tertiary basement in the Rhenish Shield. Xenoliths and ejecta of various rocks are common, and host a wide range of different minerals.

Monticellite is a relatively rare mineral in the Eifel volcanic field. It has nevertheless been found in several parageneses in Bellerberg.

It was first found in calcium-rich xenoliths as multiple mm-long, colorless to light gray, short prismatic crystals with ellestadite, mayenite and pyrrhotite. It was later found as well formed brownish crystals up to 2 mm in a matrix of anhydrite, gypsum and wollastonite, as well as brown-red to orange-red, tabular, complex crystals up to 1 mm with melilite.

Colorless clear crystals have been identified as forsterite - monticellite intergrowths. The core of these crystals is forsterite while the outer parts are monticellite. Similar zoning has been found in carbonatites where monticellite has formed from forsterite.

References:

Blaß, G., Emmerich, F.J., and Graf, H.W. (2006): Die Minerale der Vulkaneifel, DVD.

Italy

Somma-Vesuvius Complex, Naples, Campania, Italy

A large number of mineral species have been found in the Somma-Vesuvius Complex. Multiple cycles of magma with variable composition, originating from magma chambers at different depths, have created a range of mineral-forming environments. Many of the eruptions have brought to the surface xenoliths or loose ejecta of rocks formed in the contact between different magmas and the rocks surrounding the magma chambers.

Montecillite is found in crystalline calcareous ejected blocks of Monte Somma, along with mica, vesuvianite, cuspidine, spinel, and green pyroxene. The small (mm sized) prismatic crystals are colorless or yellowish. H.J. Brooke, who first described and named monticellite, found that the crystals resembled quartz. Due to the small crystal size, he could not accurately measure crystal angles, but it was later confirmed that monticellite is closely related to olivine.

References:

Brooke, H.J. (1831): On Monticellite, a new Species of Mineral. The Philosophical Magazine, Or Annals of Chemistry, Mathematics, Astronomy, Natural History and General Science, Vol. 10, pp 265-266.

Pelloux, Albert (1927): The minerals of Vesuvius. American Mineralogist, Vol. 12, pp 14-21.

USA

Magnet Cove, Hot Spring County, Arkansas, USA

The Magnet Cove complex is an alkaline ring-dike complex that crops out over an area of approximately 12km². The complex has a core of ijolite and carbonatite, an intermediate ring of trachyte and phonolite, an outer ring of nepheline syenites, and two masses of jacupirangite, one on the west edge of the complex and the other on the northeast edge. Smaller dikes of tinguaite, trachyte porphyry, nepheline syenite, miscellaneous trachytes, pegmatite, aplite, gabbro, fourchite, and carbonatite, and a variety of veins are widespread.

Erickson and Blade (1963) concluded that phonolites and trachytes were intruded first (101.4 +/- 1.0 Ma), followed by jacupirangite and the syenites of the outer ring, then the inner-ring ijolites, and finally carbonatite (95.9 +/- 0.4 Ma), which is found in the core of the complex. Metasomatism has played an important role in the petrogenetic history of these rocks, contributing to the presence of unusual minerals known from the intrusive complex. Mindat lists 143 approved species from this locality.

Montecillite occurs in the carbonatite bodies, which consist largely of medium- to coarse-grained calcite with several accessory minerals. These are, in approximate order of decreasing age, carbonate-fluorapatite (light yellow-green), monticellite (brown), biotite, magnetite, pyrite and perovskite. Also present in the assemblage is kimzeyite, a dark brown zirconium-rich garnet.

Monticellite occurs as large crystals and grains ranging from 1 to 20 mm, and as nodular masses and bands. The color is light brown. Larger prismatic crystals of 3 and 5 cm respectively were used by Pirrson (published in 1891) and referred by Williams (1890) to to make crystallographic measurements and to perform chemical analyses.

Good crystals are rare, but their rarity is not sufficiently appreciated, probably because of the modest color and some of the unusual associated minerals. There has been no proper description of the monticellite crystals since the extensive account on the petrology and mineralogy given by Williams in 1890.

References:

Erickson, Ralph Leroy and Blade, Lawrence Vernon (1963): Geochemistry and petrology of the alkalic igneous complex at Magnet Cove, Arkansas. USGS Professional Paper 425.

Flohr, Martha J.K. and Ross, Malcolm (1989): Alkaline igneous rocks of Magnet Cove, Arkansas: Metasomatized ijolite xenoliths from Diamond Jo Quarry. American Mineralogist, Vol. 74, pp 113-131.

Howard, J. Michael and Chandler, Angela (2007): Magnet Cove, A synopsis of its geology, lithology and mineralogy. Arkansas Geological Survey. Brochure Series 004.

Landes, K.K. (1931): A paragenetic classification of the Magnet Cove minerals. American Mineralogist, Vol. 16, pp 313-326.

Williams, J.F. (1890): The igneous rocks of Arkansas: Arkansas Geological Survey, Annual Report, Band 2, 457 pages.

Crestmore quarries, Crestmore, Jurupa Valley, Riverside County, California, USA

The minerals at Crestmore are formed in two irregular, lenticular bodies of magnesium-rich limestones, the Chino Limestone and Sky Blue Limestone. They were penetrated by a quartz diorite and a quartz monzonite porphyry, forming a high-temperature, low-pressure contact aureole with marbles, skarn and pegmatites.

Montecellite is found in the contact aureole of the quartz monzonite intrusion, which can be as thick as 50 feet. Within this aureole, most of the
complex mineral assemblages are fund in a well-defined zonal distribution of mineral groups, listed outward from the intrusion:
1) mainly garnet, with lesser wollastonite and diopside
2) mainly vesuvianite
3) primarily monticellite, but characterized by the presence of a considerable variety of minerals.

Monticellite is commonly associated with blue calcite, and is most commonly massive or granular. The color is usually pale to dark brown and, more rarely, practically colorless. It is also known to have occurred in nodular masses from 5 to 45 cm in diameter, containing brown to gray and colorless monticellite. Other minerals, such as green vesuvianite, blue calcite and crestmoreite, can also be found in these nodules. Gray, vitreous grains and crystals of monticellite up to 10-20 mm occur in bands in spurrite-calcite.

Prismatic brown crystals extending into the blue calcite have provided the best specimens. Individual crystals up to 8 cm have been reported by Murdoch and Web b (1966), but it appears that crystals exceeding 2 cm are very rare.

References:

Devito, Fred, Parcel, Robert. W., and Jefferson, G.T. (1971): Contact metamorphic minerals at Crestmore quarry, Riverside, California. Field Trip No.5: 94-124.

Murdoch, Joseph and Web b, Robert W. (1966): Minerals of California, Centennial Volume (1866-1966). California Division Mines & Geology, Bulletin 189.

Woodford, A.O., Crippen, R.A. and Garner, K.B. (1941): Section across Commercial Quarry, Crestmore, California. American Mineralogist, Vol. 26, pp 351-381.

Mindat locality page: http://www.mindat.org/loc-3449.html

Commercial Quarry, Sky Blue Hill, Crestmore quarries, Crestmore, Jurupa Valley, Riverside County, California, USA

See Crestmore quarries above.


Olav Revheim Aug. 2014
Reviewed by Becky Coulson Aug. 2014