Mortars, pigments and saline efflorescence from Canarian pre-Hispanic constructions (Galdar, Grand Canary Island)

Abstract

Construction elements of the ancient caves of Galdar (Grand Canary Island) give details of the life of isolated, prehistoric island inhabitants (pre-Hispanic Canarians) and their adaptation to a dry volcanic environment. Mineralogical study samples of host rocks, mortars, pigments and saline efflorescence by X-ray diffraction and electron microscopy allow us to explain their local origin, development grade and environmental factors of degradation. Regional tephra pyroclasts are composed of volcanic glass and euhedral phenocrysts of pyroxene and olivine, with traces of biotite and apatite. Cream mortars are calcite, apatite and feldspars; white are calcite-halloysite; grey calcite with iron oxides and the argillaceous plasters are mixtures of illite, montmorillonite, kaolinite and chlorites. Red-ochre pigments are oxo-hydroxides of iron-manganese, whites are halloysite, and blacks are silica mixtures of Mg, Fe, Mn, Ti, K and Ca, collected from the host-tephra (pyroxene, biotite, spinel, pyrolusite and iron oxides). The main saline efflorescences consist of thenardite and trona (sodium salts) from the watering of modern crops fields and gypsum from the restoration works (concrete) of the archaeological park in 1997. The ancient population used numerous combinations of local minerals to prepare beautiful paintings, which were protected underground below pyroclastic deposits until modern times, when they were dug out and flooded with saline waters, which produced serious alterations. Additional care must be taken to protect these prehistoric constructions.