MICROPROBE ANALYSIS OF ERIONITE, KILLDEER MOUNTAINS, NORTH DAKOTA
The presence of erionite in North Dakota was reported by Forsman (1986 NDGS RI 87) as occurring in the Killdeer Mountains (KDM), one of a number of prominent buttes in western North Dakota. It was discovered in volcanic tuffs of the late Oligocene to early Miocene Arikaree Formation. Other formations throughout western North Dakota, South Dakota, and Montana have been recognized as erionite-bearing (Goodman and Pierson, 2010 GSA Abstr. 42(3):5).
For the study reported here, 23 rock and/or soil samples were taken from the KDM. Samples were prepared by flotation in a water column followed by vacuum filtration to separate any zeolite fibers from larger size fractions. Powder X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) were carried out at NDSU. Eight samples were selected for electron microprobe analysis (EMPA) at University of Minnesota. Of those, 28 individual fibers (77 points) were measured.
EMP analyses were evaluated using the guidelines established for erionite identification by Dogan and Dogan (2008 Environ Geochem Health 30:355) and Passaglia et al (1998 Am Min 83:577). Most analyses show Mg (ions/72 oxygen) > 0.8; E > +/- 10%, and Mg/(Ca+Na) > 0.3. These values point toward the majority of KDM fibers not meeting the criteria for identification as erionite. In fact, only 3 analyses met the criteria.
The results of this study are consistent with those of Lowers and Meeker (2007 Denver microbeam lab rep 14012007), who analyzed fibers separated from ND soil and roadbed samples. SEM/EDS data plot in both regions of erionite and offretite.
Acknowledgment: NIH grant P20 RR016471 from INBRE/NCRR.