Pizzo Marcio, Trontano, Verbano-Cusio-Ossola Province, Piedmont, Italyi

Regional Level Types
Pizzo Marcio	Mountain
Trontano	Commune
Verbano-Cusio-Ossola Province	Province
Piedmont	Region
Italy	Country

Pizzo Marcio, Ossola Valley, Verbano-Cusio-Ossola Province, Piedmont, Italy

Pizzo Marcio (1924 m) is a mountain located north of Monte Togano (2301 m), connected with the latter by Costa Ragozzo, which separates the valley of Rio Margologio (to the west) from Antoliva Valley (to the east).

Albitised pegmatites occur on the north, south and southeast slopes of Pizzo Marcio Mt. These pegmatites, hosted in metaperidotite and serpentine schists of the Antrona-Zermatt Sass ophiolitic nappe, are characterised by replacement processes producing total albitisation and metasomatic reactions along the exocontacts with the formation of phlogopite + talc + actinolite-tremolite.

The dykes can be reached from the Coimo railroad station, following the path that leads to the pastures of Dalovio (948 m a.s.l.) and Alpe Miucca (1498 m a.s.l.). A westwarding horizontal path leads then to the northern cirque below Pizzo Marcio. The Pizzo Marcio North dyke, located at the base of the north face of Pizzo Marcio, can be reached ascending the talus slope which mainly consists of metaperidotite and serpentine schists boulders, coated with a typical reddish-brown weathering crust, with some white boulders of albitised pegmatite on the left hand. Ascending the talus slope more to the right, it is possible to reach the crest line connecting the summit of Pizzo Marcio (1924 m a.s.l.) to Dente di Sassoledo (1830 m a.s.l.) and then descend to the Pizzo Marcio South dyke. The Pizzo Marcio South-East dyke is located on the southeastern slope of Pizzo Marcio in Antoliva Valley, along the bed of a streamlet, just above the path connecting Alpe Miucca to Passo Biordo.

The most known of these albitised pegmatites, i.e. the Pizzo Marcio North dyke, which is located at the base of the north face of Pizzo Marcio, is renowned since the middle of the 1970s for the occurrence of collection specimens, including green Cr- and V-bearing beryl crystals (emerald) and rare accessory minerals. Some emerald crystals have been cut into gemstones up to one carat.
The crystal structure of roggianite was revised and refined by Giuseppetti et al. (1991) on needle-shaped crystals from this occurrence, already described by Mattioli (1979 a, b) and Vezzalini & Mattioli (1979).

A recent reexamination, by means of micro-Raman spectrometry, of specimens from the Pizzo Marcio North dyke classified as "thomsonite" has shown that most of them consist of bavenite. In particular: individual stout tabular crystals are ascribable to bavenite; most of the radiating aggregates are phillipsite-Ca, especially when gismondine is perched on their terminations or forms their core; in the remaining cases, the radiating aggregates are ascribable to thomsonite-Ca and only rarely to bavenite. The stout tabular crystals of bavenite, similar in appearance to those of thomsonite figured by Goldschmidt in Atlas der Kristallformen, may be considered, because of their perfection, the best in the world (Boscardin et al., 2016).

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References