38-1 Occurrence and Geological Implications of the Growth Twin Law in Kyanite from the Piedmont Regolith of Habersham County, Georgia

Session: Petrology and Geochemistry (Posters)

Presenting Author:

Authors:

Abstract:

Kyanite was observed as twinned crystals in regolith formed from the Ordovician-aged Chauga River Formation near Clarksville, Habersham County, GA. The kyanite was observed as tabular crystals that can produce visible twins. The outcrop is approximately 100 m wide and 10 m tall and consists of a highly weathered graphitic schist with dark blue to grey kyanite porphyroblasts. The kyanite was abundant both in the shists and in the regolith. The Chauga River Formation has been metamorphosed to low to medium-grade facies based on the presence of kyanite, andalusite, and sillimanite.

Electron backscatter diffraction (EBSD) data showed the presence of twinning along the (001) plane, matching the lamellar twins, which is descriptive for kyanite, as well as twinning on the (100) and (010) planes. The studied regolith contained 51%-61% kyanite,1.2%-8.5% sillimanite, and trace amounts (>1%) of andalusite, as well as the 2M₁ muscovite polytype (2.2%), and 3T muscovite polytype (0.4%). Twin boundaries as well as an offset extinction of the different limbs by 60°, consistent with the twin law described by Kenngott in 1858.

Two metamorphic events were evident from the petrographic study, one producing medium-grade metamorphic minerals, followed by retrograde metamorphism. The rarity of the growth twins is dependent on the availability of ions in the growth solution, where supersaturated or undersaturated conditions can control the abundance of growth twins. As such, these twins should not be expected in numerous localities in the study area. This growth twinning in kyanite has been reported in two localities worldwide. These twins were compared with previously documented growth twins from Loliondo, Tanzania.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.