3 Microanalyses, Macro Implications: Using Microscale Analyses to Decipher System-to-Regional Scale Processes

Monday, 25 March 2019
8:00 AM - 12:00 PM
Kings Room

MICRO-MINERALOGY OF A SINGLE KEOKUK GEODE; HOW COMPLEX CAN IT GET?

Paper number: 3-10
Program: Theme Sessions
Time: 10:55 AM – 11:15 AM

Authors

Abstract

From a suite of Keokuk geodes collected in Hamilton, Illinois, we randomly selected a single geode for a detailed micro-mineralogical study. These geodes are specific to the dolomitic beds of the lower Warsaw unit in the Keokuk Formation. This sedimentary marine deposit is a consequence of a regressing epicontinential sea during the Mississippian period around 340 million years ago. Optical microscopy using a scanning electron microscope with an electron dispersive x-ray spectrometer reveals a surprisingly complex mineralogy. A comprehensive micro-analysis exposed an elaborate network of intergrowths linking multiple generations of quartz, calcite, and siderite which exhibit a wide array of crystal habits. Platy hexagonal kaolinite crystals are littered throughout the geode as inclusions in quartz and calcite and are also associated with siderite. Micro-crystals of acicular siderite grew from the edges of the hexagonal crystals creating spike-rimmed kaolinite grains. Kaolinite included calcite exhibits several habits including: flow structures with ripple marks, a stair-stepped box texture, and euhedral crystals. Manganese carbonate micro-spheres grew from the points of micro-druzy quartz and exhibit a variable iron composition corresponding to the rhodocrosite-siderite series including ponite. Siderite appears as hallow spheres with growth rims indicating multiple phases of varying iron concentrations. Siderite also coats calcite euhedra with a multi-layered crust a few microns thick and incorporates spike-rimmed kaolinite grains in some areas. Most unusually, siderite appears as a box-like rectangular-to-rhombic lattice of sub-botryoidal textured walls which incorporate the siderite spheres. This siderite structure creates a ‘framework’ where some of the resultant ‘boxes’ are filled with calcite. Several additional minerals have been tentatively identified including: chalcedony, goethite, pyrite, sylvite, hollandite, and an unidentified REE carbonate. Organics are also present in the form of micro-sized amorphous hydrocarbon particles. The mineralogy of this geode offers a microcosm illustrating the complex geologic process of secondary mineralization that has occurred in the Midwest region.

Geological Society of America Abstracts with Programs. Vol. 51, No. 2

doi: 10.1130/abs/2019SC-326459

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.