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

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

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

Albitised pegmatite dyke located at the base of the north face of Pizzo Marcio, at an elevation of about 1750-1790 m, hosted in metaperidotite and serpentine schists of the Antrona-Zermatt Sass ophiolitic nappe.
Vittorio Mattioli on June 30, 1974, discovered the occurrence that would soon become a classic Alpine locality, due to the presence of remarkable specimens of beryl var. emerald.

The dyke, also known as the Pizzo Marcio emerald dyke among the mineral collectors, outcrops for about 60 metres. It shows a pinch-and-swell structure and reaches up to 2.5 m in width. Along the talus, below the dyke, the presence of large boulders of pegmatite indicates that the pegmatite body was significantly eroded. The pegmatite forms along the exocontacts a green-grey, decimetre-sized, sheared micaceous unit composed of phlogopite + talc + actinolite-tremolite, which in some cases embeds gem-quality emerald crystals. The sheared micaceous unit also embeds large boudins of albitised pegmatite. Several sets of fractures, up to a few millimetres wide, crosscut the albitised pegmatite mass. These fractures are locally lined by green radiating aggregates of actinolite-tremolite, greenish sheets of phlogopite + talc and prismatic crystals of a brownish-black tourmaline (dravite80–schorl20). Most of the pegmatite body is composed of the albitised unit, characterised by massive albite (Ab90–95) with a fine- to medium-grained texture, containing pale green to milky beryl crystals up to several centimetres in length. This unit locally contains abundant millimetre-sized secondary cavities lined with albite and minor acicular crystals of actinolite-tremolite with a Mg/(Mg+Fetot) ratio between 0.85 and 0.90. Oxides of Nb–Ta, mainly columbite-tantalite and microlite, occur in the secondary cavities of the albitised unit or embedded in the massive albite, associated with beryllium silicates, such as bavenite, bertrandite, meliphanite, milarite, roggianite, and phenakite, and abundant zeolites, including chabazite-Ca, gismondine, phillipsite-Ca, stilbite-Ca and thomsonite-Ca. The core of the pegmatite was left unaffected by the albitisation processes. Coarse-grained, blocky, white perthitic K-feldspar, large blades of muscovite and K-feldspar with quartz in graphic textures are preserved here, in addition to accessory phases such as prismatic crystals of pale blue beryl, occasionally covered by crystals of bavenite, prisms of schorl, corroded spessartine, and tiny platy columbite-(Mn).

Notes on the mineral list:

— Green Cr- and V-bearing beryl crystals (emerald) occur along the exocontacts, characterised by a green to grey, sheared micaceous unit composed of phlogopite + talc + actinolite-tremolite. Beryl crystals embedded in massive albite commonly show a green vitreous rim and a whitish, milky core. According to analytical data, obtained by means of atomic absorption spectroscopy, the content of Fe is 0.5 wt.% in the crystal core and 0.45 wt.% in the crystal rim; that of Cr, Ni, and Li is ≤0.05 wt.% in both cases; OH is absent in the core and reaches 2.8 wt.% in the rim (Boscardin et al., 1976). Recent electron microprobe analyses on emerald crystals show the presence of significant amounts of Na2O and MgO, while the content of Cr2O3 reaches 0.03 wt.% (Guastoni et al., 2005; Guastoni et al., 2008). The content of trace elements measured in emerald is 196 ppm of Cs; 196 ppm of Rb; 38 ppm of Sc (Guastoni, 2012).

— 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 re-examination, by means of micro-Raman spectrometry, of specimens from 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).

— Oxides of Nb–Ta typically occur in the albitised unit. In particular, minerals of columbite–tantalite series form brown-grey granular rounded crystals or millimeter-sized, brown to reddish, platy, prismatic crystals, hosted by massive albite or within secondary albite cavities. These crystals prevalently consist of Mn-dominant members, i.e. columbite-(Mn) and tantalite-(Mn).

— Microlite group minerals vary in colour, according to their composition, from yellow (almost pure calcium microlites) to dark brown (U-bearing microlites). All samples analysed by Guastoni et al. (2008) are now ascribable to hydroxycalciomicrolite (Vignola et al., 2018) or oxycalciomicrolite (Menezes da Silva et al., 2020).

— Tapiolite-(Fe) and ferrowodginite form grey-brown pseudo-octahedral crystals associated with microlite and embedded in massive albite. Chemical analysis of tapiolite-(Fe) shows Ta2O5 contents up to 86.53 wt% and FeO up to 8.91 wt%. Ferrowodginite contains Ta2O5 up to 76.58 wt% and variable Fe/Mn values, with Fe slightly predominant over Mn.

— Fersmite forms yellowish or greenish, millimeter-sized, striated, prismatic crystals, in some cases with a dark reddish to brownish core composed of columbite-tantalite + microlite. Idiomorphic crystals of fersmite have been observed in the secondary cavities associated with albite and actinolite-tremolite or embedded in massive albite with other oxides of Nb–Ta. Usually, these crystals are rather homogeneous with Ta/(Ta+Nb) in the range of 0.15 to 0.19 p.f.u., but cases of Ta-rich fersmite are known; in Ta-rich fersmite the Nb2O5 content dramatically decreases down to 41.79 wt%, and consequently the Ta2O5 increases up to 43.04 wt% with Ta/(Ta+Nb) up to 0.38 p.f.u. (The latter value probably represents the highest Ta2O5 content for fersmite measured in the literature). Fersmite from Pizzo Marcio does not incorporate other elements except for Ce2O3 (up to 0.32 wt%).

— Vigezzite forms brown to reddish, millimeter-sized, platy, prismatic crystals, in some cases with a dark core composed of thorian vigezzite (Th content up to 0.13 p.f.u.) embedded in massive albite and associated with albite and actinolite-tremolite (Guastoni et al., 2005; Guastoni et al., 2008; Guastoni, 2012).

— The tourmalines of this locality, which seem to be prevalently oxy-dravites with variable Fe/Mg ratio, vary in colour from brownish-black for the crystals embedded in massive albite, to bluish-dark green for the prisms lining sets of fractures crosscutting the albitised pegmatite associated with actinolite-tremolite, and bluish-green for the idiomorphic crystals lining the secondary cavities of the wall-intermediate albitised pegmatite zone.

— Garnet can be referred to as spessartine (Guastoni, 2012), but the presence of small yellow-brown crystals of grossular-andradite in secondary cavities, as observed at Alpe Rosso, is possible.

Other Regions, Features and Areas containing this locality

References