A large gyromagnetic effect in greigite

Rotational and anhysteretic remanent magnetization (RRM and ARM) acquisitio n curves of two sediment samples containing greigite have been measured up to peak fields of 80 mT at rotation frequencies between 5 and 95 revolution s per second (rps). At 95 rps the ARM (70 muT direct field, applied antipar allel to the RRM) increased almost linearly with peak field but the RRM inc reased approximately exponentially. The effective field (B-g), defined in t his case as 70 x RRM/ARM, was about 1100 muT for the two samples after the application of an alternating field (AF) of 80 mT peak. B-g is approximatel y 10 times higher than previously observed for magnetite of size 1 mum. Alt hough greigite is the dominant ferrimagnetic mineral present in these sampl es, other studies (Snowball 1997a) have shown that low concentrations of de trital multidomain magnetite are also present, so that the high value of B- g must be regarded as a lower limit. Unlike a magnetite sample that was used for comparison, both greigite sampl es had a negative RRM at all rotation frequencies below 50 rps but, like ma gnetite, there was an increase in RRM when it changed from negative to posi tive at 50 rps. In addition, unlike magnetite, the ARM was not constant but approximately halved as the RRM became strong and positive above 50 rps (A RM antiparallel to RRM). Thus there appeared to be an interaction between t he ARM and the RRM. Further investigation of this interaction by applying w eak direct fields parallel and then antiparallel to the RRM (80 mT peak AF. 95 rps rotation rate) showed that the ARM was linear with direct held but was indeed always smaller when the direct held producing it was antiparalle l to the RRM. At present it is not cleat why gyromagnetic remanences are so strong in gre igite. Such high values of B-g have never been observed before except in th e very special case of a self-reversing lithium chromium ferrite near its m oment compensation temperature. These high values of B-g might enable RRM t o be used as an indicator for greigite if they turn out to be unique to thi s mineral.