Rock-soil transition in weathering of a porphyritic biotite granite

Author : John Kuna Raj
Publication : Warta Geologi
Page : 1-9
Volume Number : 48
Year : 2022

Warta Geologi, Vol. 48, No. 1, April
2022, pp. 1–9


Rock-soil transition in weathering of a
porphyritic biotite granite


John Kuna Raj

No. 83, Jalan Burhanuddin Helmi 2, Taman
Tun Dr. Ismail, 60000 Kuala Lumpur, Malaysia

Email address:


Abstract: Concentric layers of weathered
materials around core-stones show the porphyritic biotite granite to experience
sequential, but gradational, changes in visible features, textures and
mineralogy as it transforms from ‘rock’ into ‘soil’. The changes start with the
opening-up of grain boundaries and micro-cracks (stage 1) followed by their
dark brown staining (stage 2) and the subsequent alteration (to sericite and
clay minerals) of groundmass plagioclase feldspar grains (stage 3). Biotite
flakes are then bleached and altered (to chlorite and clay minerals) (stage 4)
before there starts alteration (to sericite and clay minerals) of groundmass
alkali feldspar grains (stage 5) and finally alteration (to sericite and clay
minerals) of the alkali feldspar phenocrysts (stage 6). Quartz grains are not
altered during these stages of weathering, but disaggregate and reduce in size
due to continual opening-up of grain boundaries and micro-cracks. Increasing
stages of weathering are marked by decreasing dry unit weights, dry densities
and uniaxial compressive strengths, but increasing apparent porosities. The
transition between ‘rock’ and ‘soil’ occurs during stage 6 when all
plagioclase, and most alkali feldspar, groundmass grains have been altered as
are some alkali feldspar phenocrysts. Stage 6 is marked by large apparent
porosities (>18%) but low values of dry unit weight (<20.81 kN/m3),
dry density (<2,122 kg/m3) and uniaxial compressive strength
(<1.8 MPa).


Keywords: Porphyritic biotite granite,
weathering, rock-soil transition



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Manuscript received 4 September 2021

Received in revised form 11 January 2022

Accepted 13 January 2022

Available online 26 April 2022


0126-5539; 2682-7549 / Published by the Geological Society of


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