Possible implications of glass phases present in volcanic rocks from Cape Verde are analysed along two lines of influence: incorporated in the bedrock of engineering works and connected by an hydraulic binder for further use as construction materials. Palagonization of basaltic hyaloclastites and formation of iron-rich gels and clay-like nanophases resulting from olivine oxidation are considered. Representative examples from Santiago Island are reported.
Volcanic rocks display a variety of textures ranging from very compact mineral assemblages in basalts to loosely bound components - e.g. in pyroclasts, hyaloclastites, lahar deposits - not seldom including vitreous (glass) phases. The mechanical strength of these rocks may then be a serious concern in engineering works. Glassy phases are very liable to alteration, either under a physical point of view once the thermodynamic trend determines long term crystallization and subsequent volume decrease, or in a chemical way, mainly due to the oxidative weathering of ferromagnesian silicates and subsequent hydrolysis of iron-rich phases. These processes are non-isochemical with intense lixiviation and precipitation of secondary materials in available interstitial spaces - two features with major implications on the mechanical behaviour of bulk rock. Reaction mechanisms involved in the alteration of mafic components in volcanic rocks from Cape Verde are analysed, focusing on glass palagonitization and, due to its local relevance, also on iddingsitization of olivine followed by intensive formation of iron-rich gels.
Located about 450km of the West African coast, the Cape Verde Archipelago - ten islands and various islets - has originate from a within-plate volcanism by a hot-spot mechanism (Duncan 1984) in a plateau rising over 2 km above the oceanic floor (Dash et al. 1976). The islands are underlain by "a thickened crust composed of uplifted, deformed Jurassic Ocean crust, plutonic rocks and sediments" (in Gerlach et al. 1988). The magmatism has been essentially alkaline sub-saturated at least since the Oligocene (Torres et al. 2002). Similarly to Canary Islands, Cape Verde archipelago has a semi-arid climate (Daveau 1969) with successive periods of drought (Fonseca 1956). The rainy season is restricted to three months (August to October) and most of the annual precipitation concentrates in a few days (Correia 1996), with short, intense rainfalls combined with an easy surface run off due to steep slopes and sparse vegetation (Figures 1 & 2), as a whole accounting for generalized erosion processes (Costa 2002).
The word "palagonite" was introduced by von Walterhaiisen in the nineteenth century to designate altered basaltic glasses from Palagonia in Sicily (Honorez 1972). It is herein opportune to clarify the distinction between glass - a non-crystalline, compact solid displaying no connected porosity, only isolated spherical vacuoles - and vitreous phase in general - still an amorphous solid, conformable with granular texture and presenting low intergranular cohesion. Accordingly, glass is used for a primary rock forming material, while vitreous phase refers to a secondary product, even a gel - a condensed phase displaying peculiar rheology.