Experimental etching of a microcline perthite and implications regarding natural weathering

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Abstract
SUMMARY: Matching 010 and 001 cleavage faces of a microcline perthite from Arendal, Norway, were hydrothermally etched for increasing periods of time. On both faces the interface between the two felspar phases quickly becomes etched, enhancing the contrast between them and indicating dislocations in the plane of the perthitic lamellae. Moreover, on the 010 face, etch lines reveal dislocations which transect the lamellae parallel to the trace of 001 cleavage. In all probability these dislocations result from the strain arising from partial coherence of the two felspars. The microcline matrix develops innumerable rhomboidal‐shaped etch pits on the 001 face and pits with an elongated parallelogram‐shaped outline on the 010 face. The dominant direction of etching is parallel to the pericline twinning, suggesting that dislocations occur within the pericline twin boundaries. At a later stage micro‐crystalline boehmite develops preferentially in the perthitic lamellae and along the dislocations parallel to the (001) plane. Such sites may be particularly favoured for clay mineral formation during natural weathering.