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Mont'Orfano (Montorfano), Mergozzo, Ossola Valley, Verbano-Cusio-Ossola Province, Piedmont, Italy

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Location is approximate, estimate based on other nearby localities.
 
Latitude & Longitude (WGS84): 45° 59' 2'' North , 8° 25' 0'' East (est.)
Margin of Error:~0km
Name(s) in local language(s):Mont'Orfano (Montorfano), Mergozzo, Val d'Ossola, Provincia del Verbano-Cusio-Ossola, Piemonte, Italia
Other regions containing this locality:The Alps, Europe


Mont’Orfano is a strikingly prominent landmark 794 meters high, rising steeply from the surrounding alluvial plain of River Toce. Because of its isolated position at the confluence of Strona and Ossola valleys, this mount earned the name of “Mont'Orfano” (i.e. the orphan mount).

It is formed by one of the five late Variscan plutons that intruded the Serie dei Laghi crystalline basement and mainly consists of a white medium-grained granite. The country rocks (medium grade metapelites of the Scisti dei Laghi formation) are exposed only in small outcrops on the northwestern flank of the mount. Here, in the vicinity of Brusco village, the so-called "green granite" of Mergozzo occurs as a narrow band near the contact with the metapelites. This is clearly the result of episyenitisation, i.e. subsolidus, low-temperature transformation of a preexisting intrusive rock, because, despite its igneous appearance, it is composed of albite, chlorite, quartz, sericite, titanite, and carbonates.
The mineral assemblage of white granite is: albite, quartz, K-feldspar, biotite, apatite, zircon, and rare allanite. Green hornblende is only present in a restricted zone in the northwestern part of the granite stock (Gallitelli,1938; Sassi & Sesana,1986). In the amphibolic granite, allanite is abundant in large, automorphic crystals.
At Cava Donna (also known as Cava Montorfano), in the easternmost part of the Montorfano stock, the granite is full of enclaves of a greyish heterogranular facies, in turn containing mafic fine-grained enclaves. The mineral assemblage of the grey granite is the same as that of the white granite: the minerals are present in two distinct generations. The margin towards the white granite is sharp, without any particular contact textures or compositional variations.
Microgranular, compact, dark enclaves are widespread in the Montorfano stock, whilst they are rare in the Mottarone-Baveno pluton. The maximum concentration is within the heterogranular facies of Cava Donna; their size normally ranges from a few millimetres to some decimeters (rarely up to 1-2 m). The enclaves show an intersertal texture of biotite, hornblende and plagioclase; new biotite is formed from hornblende. K feldspar and quartz, introduced by granitisation, are present as integranular material and patches.
A small distinct granite body, crowded with schistose xenoliths, occurs in the northern part of the stock. It is composed of white medium- to fine-grained rocks, sometimes slightly porphyritic in texture, with pale pink K-feldspar phenocrysts; it does not contain plagioclase clots of early crystallisation. The size of the xenoliths ranges from microscopic to more than 1 m across. They preserve the original schistosity and are oriented at random in the enclosing granite; their mineral assemblage includes sillimanite, andalusite, spinel, corundum, and cordierite (Boriani, Burlini et al., 1988).
A dyke of granite porphyry cuts across the northern contact of the granite and penetrates into the country rocks.
Acidic differentiates are represented by many aplitic dykes and very few pegmatitic pockets.


White granite quarries are located on the southern foothills of the mount. Nowadays, this white granite is utilized in buildings and urban décor, while in the past it was mostly used as building stone although in a lesser extent than the famous pink granite of Baveno (which is quarried just a couple of kilometres to the south-east). Its use started in the 16th century and continued up to now: in Milan numerous columns of this granite can be observed in the cloister of San Pietro in Gessate monastery (16th century), Sant’Angelo church, cloisters of San Vittore monastery, courtyard of Ospedale Maggiore (17th century), etc. In the 19th century, 146 monolithic columns (up to 14.5 m in length and 4.5 m in diametre) were used for the recostruction of the patriarchal basilica of San Paolo fuori le Mura in Rome and transported to Rome by lake, rivers and sea.

The green granite, occurring on the northwestern slope of Mont'Orfano, was surely exploited since 1700 as evidenced by buildings in Milan (e.g. Palazzo Mellerio), but its quarrying ceased at the end of the 20th century. The major problem for the exploitation of this rock is the presence of iron carbonates, that are easily weathered to give yellow spots and decrease the resistance to polishing.

Yellowish hexagonal crystals occurring in a crust, consisting of chlorite, stilbite, chabazite, fluorite, and pyrite, or perched on quartz crystals were classified as parisite, on the basis of chemical and physical investigations (Tacconi, 1905). The material of the original description is no more available for modern investigations. According to Gramaccioli (1975), it should be attributable to synchysite-(Ce). In Palache et al. (1951) the presence of parisite at Mont'Orfano is erroneously reported from veinlets in riebeckite-granite. Mont'Orfano granite does not contain riebeckite and, obviously, the name riebeckite does not appear in the original literature.



Mineral List


37 valid minerals. 1 erroneous literature entry.

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References

- Barelli V. (1835): Cenni di statistica mineralogica degli Stati di S.M. il Re di Sardegna, ovvero Catalogo ragionato della Raccolta formatasi presso l'Azienda Generale dell'Interno. Tipogr. G. Fodratti, Torino, 686 pp.
- Strüver G. (1866): Minerali dei graniti di Baveno e di Montorfano. Atti R. Acc. Sci. Torino, 1, 395-397.
- Strüver G. (1867): Sopra alcuni minerali italiani. 5. Pirrotina di Montorfano. Atti R. Acc. Sci. Torino, 3 (1867-68), 132-133.
- Strüver G. (1871): Note mineralogiche. 4. Apatite e arsenopirite del granito di Baveno e di Montorfano. Atti R. Acc. Sci. Torino, 6 (1870-71), 367-368.
- Jervis G. (1873): I tesori sotterranei dell'Italia. Vol. 1: Regioni delle Alpi. Ed. Loescher, Torino, 410 pp.
- Leuze A. (1892): Mineralogische Notizen. In: Bericht über die 25. Versammlung des oberrheinischen geologischen Verains zu Basel am 21. April 1892. Druck von Alfred Müller & Co., Stuttgart, pages 20-33.
- Viola C. (1899): Sopra alcuni minerali italiani. II. Moscovite della cava di Monte Orfano presso il Lago Maggiore nel comune di Mergozzo (Novara). Rend. R. Acc. Naz. Lincei, Classe sci. fis. mat. nat., ser. 5, 8, 1, 540-542.
- Viola C. (1899): Optische Studien über italianische Mineralien. II. Muskovit von Monte Orfano (Lago Maggiore). Zeitschr. Kryst. Miner., 32 (1900), 2, 118-120.
- Tacconi E. (1903): Sopra alcuni minerali del granito di Montorfano. Rend. R. Acc. Naz. Lincei, Classe sci. fis. mat. nat., ser. 5, 12, 1, 355-359.
- Tacconi E. (1905): Ulteriori osservazioni sopra i minerali del granito di Montorfano (parisite, ottaedrite, zircone, ecc.). Rend. R. Acc. Naz. Lincei, Classe sci. fis. mat. nat., ser. 5, 14, 2, 88-93.
- Gonnard F. (1910): Notes cristallographiques sur le feldspath potassique du mont Orfano, près de Baveno (province de Novare, Piémont). Bull. Soc. franç. minéral. crist., 33, 251-254.
- Gallitelli P. (1937): Analisi chimica del granito di Montorfano e del granito e della tormalina di Alzo. Rend. R. Acc. Naz. Lincei, Classe sci. fis. mat. nat., ser. 6, 26, 2, 103-106.
- Gallitelli P. (1938): Ricerche petrografiche sul Montorfano (Lago Maggiore). Mem. R. Acc. Sci. Lett. Arti Modena, 3, 1-92.
- Scaini G. (1939): Itinerari mineralogici. Comune di Mergozzo. Località: cave di granito bianco di Mont’Orfano. Natura, 30, 3, 155-159.
- Palache C., Berman H., Frondel C. (1951): The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Seventh edition, Volume II, John Wiley & Sons, New York, page 284.
- Pigorini B., Soggetti F., Veniale F. (1964): I minerali pesanti accessori dei graniti sub-alpini (dal Lago Maggiore al Canavese) (Nota prelimiare). Rend. Soc. Mineral. Ital., 20, 229-234.
- Pigorini B., Soggetti F., Veniale F. (1964): Studio morfologico-statistico dello zircone accessorio nei graniti di Alzo, Baveno e Montorfano. Atti Soc. Ital. Sci. Nat., 103, 1, 18-48.
- Pigorini B., Veniale F. (1966): Studio mediante microsonda elettronica dei diversi tipi di zircone accessorio nei graniti di Baveno, Monte Orfano e Alzo. Atti Soc. Ital. Sci. Nat., 105, 2, 207-264.
- Veniale F., Pigorini B., Soggetti F. (1968): Petrological significance of accessory zircon in the granites from Baveno, Monte Orfano and Alzo (North Italy). In: Slávik J. (ed.) (1969), Proceedings of Section 13: Other Subjects, International Geological Congress, Report of the Twenty-Third Session Czechoslovakia 1968. Academia, Prague, 243-268.
- Roggiani A.G. (1969): Opale jalite e scheelite nel granito bianco del Montorfano. Rend. Soc. Ital. Mineral. Petr., 25, 2, 549-552.
- Morten L., Rossi P.L. (1971): La clorite del «granito verde» di Mergozzo. Miner. Petrogr. Acta, 17, 85-89.
- Boscardin M. (1972): Informazioni di mineralogia italiana. 21) Opale jalite e scheelite del Montorfano. Notizie Gruppo Mineral. Lombardo, 3, 1 (marzo 1972), 8.
- Del Caldo A., Moro C., Gramaccioli C.M., Boscardin M. (1973): Guida ai minerali. Fratelli Fabbri Editori, Milano, 208 pp.
- De Michele V. (1974): Guida mineralogica d'Italia. Istituto Geografico De Agostini, Novara, 2 vol., 408 pp.
- Ravagnani D. (1974): I giacimenti uraniferi italiani e i loro minerali. Gruppo Mineralogico Lombardo - Museo Civico di Storia Naturale Ed., Milano, 188 pp.
- Gramaccioli C.M. (1975): Minerali alpini e prealpini. Istituto Italiano Edizioni Atlas, Bergamo, 2 vol., 472 pp.
- Sassi A., Sesana E. (1986): Montorfano and Mottarone-Baveno granites. In: Proceedings of the Meeting «Advanced Methods and Techniques for the Study of Stone Decay, Cleaning and Conservation», Group Petrography of the Stone Committee ICOMOS (International Council for Monuments and Sites), Pavia, pp. 65-74.
- Boriani A., Burlini L., Caironi V., Giobbi Origoni E., Sassi A., Sesana E. (1988): Geological and petrological studies on the hercynian plutonism of Serie dei Laghi - geological map of its occurrence between Valsesia and Lago Maggiore (N-Italy). Rend. Soc. Ital. Miner. Petrol., 43, 2, 367-384.
- Boriani A., Caironi V., Oddone M., Vannucci R. (1988): Some petrological and geochemical constraints on the genesis of the Baveno-Mottarone and Montorfano plutonic bodies. Rend. Soc. Ital. Miner. Petrol., 43, 2, 385-394.
- Cavallo A., Bigioggero B., Colombo A., Tunesi A. (2004): The Verbano Cusio Ossola province: a land of quarries in northern Italy (Piedmont). Per. Mineral., 73, (special issue 3), 197-210.
- Piccoli G.C., Maletto G., Bosio P., Lombardo B. (2007): Minerali del Piemonte e della Valle d'Aosta. Associazione Amici del Museo "F. Eusebio" Alba, Ed., Alba (Cuneo), 607 pp.

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