Non-eutectic, graphic, micrographie and graphic-like “myrmekitie” structures and textures

Abstract

True graphic quartz structures in pegmatites from Carrara/Giggiga and Harrar (town) districts of Ethiopia, are compared with the micrographic quartz textures in the Rapakiwi granite of Finland. Graphic-like textures of uraninite in microcline are also discussed and compared with these graphic structures.

A quartz vein, about 1–2 meters in thickness, intersects a pegmatite in the Carrara/Giggiga district. This quartz vein extends into the microcline of the pegmatite as fine quartz veins which attain the form and character of graphic quartz. Also the graphic quartz of the Harrar pegmatites is observed to extend into and occupy cracks in the microcline.

Comparable in origin to these graphic textures is the micrographic quartz in the Rapakiwi granite. Observations show micrographic quartz following the cleavage directions in the orthoclase as well as the interzonal spaces and the boundaries of inclusions in the K-feldspar.

On the basis of the observed structures and textures these graphic and micrographic intergrowths are considered to be due to solutions penetrating or “infiltrating” into existing structures and not due to simultaneous crystallisation as conditions of eutectic crystallisation would require.

In addition to the well known graphic structures there occur graphic or “myrmekitic”-like intergrowths of uraninite in microcline which, from a structural and physico-chemical point of view, cannot be considered to be due to eutectic crystallisation.