They are mainly composed of magnesioferrite
, spinelloid, and lepidocrocite.
Magnesioferrite from the Cretaceous-Tertiary boundary, Caravaca, Spain.
(1990.) The significance of magnetic spheroids and magnesioferrite occurring in K/T boundary sediments.
Magnesioferrite spinel in Cretaceous/Tertiary boundary sediments of the Pacific basin: Remnants of hot, early ejecta from the Chicxulub impact?.
1992; Kyte 2002); and presence of magnesium and nickel-rich spinels with varying compositions and high oxidation states, also reported in the literature as magnesioferrites (Smit & Kyte 1984; Kyte & Smit 1986; Robin et al.
Although there are some differences between these works, they agree pointing to the impact-generated magnesioferrites or Mg, Ni-rich spinels as the main magnetic phase within the K-T boundary microspherules.
All these features (and the absence of Verwey transition reported by Villasante-Marcos et al., 2007) can be explained by the presence of highly oxidized (maghemitized) magnesioferrites (Mg, Ni-rich spinels) accompanied by a high proportion of goethite and haematite, which seem to be concentrated within the Fe-O spherules.
The magnetic residues of sample 75-503, consist primarily of chromite and magnesioferrite, which are highly enriched in Ir, Ni and Cr (Table 1, cf.
At these localities the spherules were associated with dark, clay-rich spherules full of quench crystals of magnesioferrite spinel, and enriched in iridium opposite the Ir concentration of the K/T boundary clay itself (33 and 2 ppb, respectively).
The composition of larger lobate grains shows their pertinence to titaniferous magnetite with a higher content of ulvospinel molecule ([Fe.sub.2]Ti[O.sub.4]) and low contents of Mn, Mg, Al, thus forming very low proportions of jacobsite, magnesioferrite
, and hercynite, respectively, and sometimes V.
, like ludwigite, has been found as microscopic inclusions in native arsenic from the Bor pit.
The iron oxide will go readily into solution in the magnesia lining forming magnesioferrite
or magnesiowustite, depending upon the partial pressure of the available oxygen.