New method of semi-automatic modal analyses for phenocrysts of porphyritic rocks based on quantitative digital colour

Abstract This paper proposes a new method for modalanalyses of centimetric phenocrysts and porphyroblastsusing a digital rock image. The dot matrix image of therock is imported into vectorial draw software asbackground. The outline of all of the phenocrysts ismarked on the overlay by closed vectorial curves andtheir internal area is painted in black. After thisoperation, the rock image on the background is deletedand the background is painted in white. The vectorialobjects composed of the background and the overlayare exported as a matrix image of BMP format. Thepercentage of the areas painted in black correspondsto modal abundance of the phenocrysts, and it iscalculated by the Wilber colour specification software.This method has high precision in graphic processing.However the translucency of the groundmass causesuncertainness in vectorial phenocryst marking.Therefore, it is recommended for rocks with phenocrystslarger than 5 mm. Keywords : Modal analyses, digital image, phenocryst,porphyroblast, Wilbur, colour specification.