166 Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones

From ore materials to precious gemstones, minerals play an important role in almost every aspect of our lives. This session will focus on characterization of economic minerals employing spectroscopic techniques, geochemical analysis, and microanalytical techniques.

Tuesday, 21 October 2025
8:00 AM - 12:00 PM
HBGCC, 217A

Mineralogical Characterization of Legacy Graphite Resources in the Central Texas Graphite District

Paper number: 166-3
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

The Central Texas Graphite District, situated within the Llano Uplift, represents a significant but underexplored terrane with potential for critical mineral development. The uplift exposes the largest area of Mesoproterozoic basement in the southern U.S and is composed of deformed and metamorphosed sedimentary and igneous rocks formed during the Grenville orogeny (1.2–1.1 Ga). Graphite is primarily associated with the Packsaddle Schist, a metapelitic to semi-pelitic unit that experienced upper amphibolite-facies metamorphism during Grenville deformation (1.1 Ga). These rocks originated as carbonaceous sediments that were thermally overprinted during intrusion of the ca. 1.08 Ga Town Mountain Granite, driving flake graphite formation.

Disseminated graphite occurs within quartz-feldspar-muscovite ± biotite schists, with mineralization concentrated along schistosity-parallel zones traceable for several kilometers. The historic Southwestern Graphite Mine exploited high-grade lenses containing up to 12% graphitic carbon (GC). Early production records are limited, but total graphite production is estimated to be 75Kt. Remaining tailings are estimated at over 2 million metric tons and an average of 3.5% GC. Flake size is predominantly >50 mesh, suggesting favorable material for secondary recovery. Coarser flakes (+30 mesh) are rare and typically restricted to pegmatitic dikes, quartz stringers, and augen-bearing structures.

SEM-BSE analyses reveal graphite occurs as both fine-grained, foliation-aligned flakes and coarser, interstitial segregations. Tailings mineralogy includes 20–35% quartz, 5–8% K-feldspar, 14–36% plagioclase, 2–18% graphitic clay, and 2–12% biotite, with accessory phases such as rutile, monazite, xenotime, zircon, ilmenite, and apatite—indicating additional potential for secondary recovery of Ti, REEs, and Zr. Ongoing characterization of legacy materials points to the near-term viability of these resources for domestic graphite production.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413346

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