The Beola: a dimension stone from the Ossola Valley (NW Italy)

The Ossola Valley (north-western Italy) is well known in Italy both for its geological features and for the exploitation of various rock types. Among these the most valuable gneiss is the Beola used as building or ornamental stone since the Roman Age. The different varieties of Beola are exploited in four main areas according to their geological and structural setting: Vogogna (Fobello-Rimella mylonitic schists - Austroalpine), Beura-Villadossola (Monte Rosa nappe orthogneisses – Upper Penninic), Trontano-Domodossola (Orselina-Moncucco-Isorno orthogneisses – Upper Penninic) and Montecrestese-Crevoladossola (Monte Leone nappe orthogneisses – Lower Penninic). This paper deals with the historical use of the Beola and its quarries and presents new data on the petrographical, geochemical and mechanical features of the different varieties. The mineralogical composition of the Beola varieties is quite homogeneous: Qtz+Kfs+Pl+Bt+WM; the main differences are related to the rock fabric and to the presence of accessory/secondary minerals. The «Quarzite» Verde from Vogogna has the worst value of wear resistance (14.17 mm; Amsler), the Beola Argentea from Montecrestese-Crevoladossola has the best one (4.06 mm); whereas the other rock types have quite homogeneous values (5-8 mm). The flexural strength is remarkably different as function of the load applied parallel or perpendicular to the rock fabric (foliation or lineation) ranging from 19.5 to 11.3 MPa for the Beola Grigia and from 16.1 to 6.0 MPa for the Beola Ghiandonata of the Beura-Villadossola area. The water absorption value is usually low (0.27-0.44%) for all the Beola types. Based on the geo-mechanical features and the aesthetic parameters, the Beola Bianca, Favalle and subordinately the Beola Grigia are the most valuable varieties of the Ossola district.