Mineralogical changes associated with deep weathering of gabbro in Aberdeenshire

Abstract

The distribution, profile morphology and mineralogy of deeply weathered gabbroic rocks in the basic igneous masses of lnsch and Bogancloch, Aberdeenshire, have been studied. The saprolite is characterized by granular disintegration and spheroidal weathering with a relatively low degree of secondary mineral development. The upper part of the profile often shows evidence of mass movement and incorporation into overlying drift but below this zone the degree of alteration is generally uniform with depth. Feldspar and hornblende are largely unaffected, except by disintegration, but pyroxenes show varying degrees of weathering from the incipient development of amorphous iron oxides to replacement by reticulate networks or complete pseudomorphs of trioctahedral vermiculite. Biotite weathers to hydrobiotite and vermiculite, giving rise also, under certain conditions, to kaolinite-gibbsite vermiforms. The correspondence between occurrences of deep weathering and coarser textured biotite-bearing rocks can be related to the instability of this mineral. Consideration of the mineralogical changes found and the morphology of the profile of weathering suggest their formation under a warm, humid climatic regime, probably during the mid Tertiary times.

Weathering of Enstatite to Talc Through a Sequence of Transitional Phases

“…Because the gabbro has no phyllosilicate minerals from which vermiculite could have formed, they concluded that the mineral crystallized from "elements put into solution." Our observations suggest that vermiculite could have formed by the topotactic transformation of pyroxene or amphibole from the gabbro, as Basham (1974) found.…”

“…In the upper part, where the rock texture was modified, smectite and vermiculite dominate. From optical microscopy and X-ray diffraction analyses Basham (1974) described the progressive replacement of hypersthene by trioctahedral vermiculite during the weathering of a gabbro. The layer silicate occurred as yellow-brown pseudomorphs, biaxial (-), 2V = 10 ~ oriented with its 'c'-axis normal to the 'c'-axis of the pyroxene.…”

Weathering of Hornblende to Ferruginous Products by a Dissolution-Reprecipitation Mechanism: Petrography and Stoichiometry

“…Previous petrographic studies of naturally weathered pyriboles have noted what appeared to be early-stage weathering products in fractures and cleavages (see, e.g., Basham, 1974;Cleaves, 1974;Colman, 1982;Proust, 1982Proust, , 1985. Alteration apparently originated at grain boundaries and intragranular fractures and grew inward to the present contact.…”

“…The initial weathering products of ferromagnesian silicate minerals may be iron oxides, iron oxyhydroxides, or clay minerals, depending on the intensity of leaching in the weathering profile (Cleaves, 1974). Continued weathering produces either pseudomorphs of clay minerals after pyribole (e.g., Eggleton, 1975;Cole and Lancucki, 1976;Delvigne, 1983;Eggleton et al, 1987) or a boxwork structure in which fracture fillings are preserved, but the pyribole itself is dissolved (Basham, 1974;Cleaves, 1974;Delvigne, 1983).…”

“…Most pyribole weathering products exhibit preferred crystallographic orientation (e.g., Basham, 1974;Wilson, 1975;Delvigne, 1983), but not all (Wilson and Farmer, 1970;Berner and Schott, 1982). Eggleton (1975) and Cole and Lancucki (1976) suggested that the crystallographic orientation of the products with respect to parent structures resulted from "solid-solid" transformation involving minimal rearrangement of tetrahedra and octahedral cations.…”