Anatomy of a porphyroblastic schist: Alkali-silica reactivity: Fid-Bau Portal

Anatomy of a porphyroblastic schist: Alkali-silica reactivity

Thomson, Margaret L. / Grattan-Bellew, P.E.

Abstract Concrete made with quartz-bearing, metamorphically deformed rock as aggregate may expand due to alkali-silica reaction. A deformed metamorphic rock is commonly composed of coarser-grained porphyroblasts and finer-grained matrix. Current petrographic practise is to measure the deviation of the angle of undulatory extinction in quartz to predict the degree of potential reactivity, presuming that the quartz grains measured are in fact those which participate in the alkali-silica reaction. The porphyroblast and matrix components of a quartz-rich porphyroblast schist are separated using electromagnetic and heavy liquid techniques. The separate obtained consist of porphyroblast quartz with and without significant subgrained mantles (two samples), quartz-mica-chlorite matrix (two samples) and feldspar-quartz matrix (one sample). Miniature mortar bars are molded using 300–600-μm-sized grains and placed in an autoclave filled with 1N NaOH to induce alkali-silica reaction. Changes in length indicate that the feldspar-dominated fraction expanded the least and the matrix quartz and the porphyroblast quartz fraction with significant subgrained mantles expanded the most, strongly suggesting that the fine-grained subgrain matrix and porphyroblastic mantle are the reactive component. This calls into question the usefullness of determining the deviation of the undulatory extinction angle for determinining the potential alkali-silica reactivity of a rock. It is suggested that determining the proportion of subgrained quartz within a deformed rock to be used as aggregate is more valuable for predicting the potential for alkali-silica reactivity.