Session 3: Petrology and Mineralogy

Tuesday, 19 May 2020
8:00 AM - 12:05 PM

FURTHER STUDIES OF PSEUDO-CUBIC TERMINATIONS OF QUARTZ CRYSTALS IN A KEOKUK GEODE

Program: Virtual Meeting
Time: 9:05 AM – 9:25 AM

Authors

Abstract

Previously, I reported on the occurrence of pseudo-cubic terminations of quartz crystals within Keokuk geodes from the Hamilton, Illinois area. These occur in strata of the Mississippian-age lower Warsaw Formation (Sinotte 1969). Within this environment, low-temperature solutions emplaced primarily quartz but also calcite and other less-common minerals. A majority of these geodes, however, have their terminations covered by whitish chalcedony, which has obscured their recognition as quartz and seemingly led to their misidentification as pseudo-cubic calcite crystals or pseudomorphs of quartz after calcite (Marché 2018).

Frondel (1962) explained pseudo-cubes of quartz as resulting from a positive rhombohedron in which the “r” {101} faces develop to the exclusion of “z” {011} and “m” {100} faces. This is supported by observation of remnant “z” faces on a few terminations that were removed from a geode whose interior was not entirely covered by chalcedony. Nineteen crystals were extracted; ten face pairs on three crystals were photographed and their interfacial angles measured. A mean of 92° was obtained, which closely matches the (supplementary) value reported by Frondel (1962). This also demonstrates that these terminations are not pseudo-morphs of quartz after calcite.

To assess whether these pseudo-cubic terminations might have resulted from trace elements incorporated during late-stage crystal growth, a sample was submitted for SEM/EDS testing to Excalibur Mineral Corporation. No trace elements were found, however, implying that pure quartz alone is present. This leaves unanswered the question of how (or under what conditions) these crystals changed habits from prismatic to pseudo-cubic. Pseudo-cubes of quartz have been reported from other locations in North America (and elsewhere) but without modern analytical studies being conducted upon them.

Geological Society of America Abstracts with Programs. Vol. 52, No. 5

doi: 10.1130/abs/2020NC-346498

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