Desert Varnish: An Electron Probe Study
1969; University of Chicago Press; Volume: 77; Issue: 3 Linguagem: Inglês
10.1086/627435
ISSN1537-5269
AutoresRoger LeB. Hooke, Houng‐Yi Yang, P. W. Weiblen,
Tópico(s)Heavy metals in environment
ResumoDesert varnish samples from Deep Springs Valley and Death Valley in California were studied with the use of the electron microprobe. Varnish can be interpreted as having two layers, an inner subordinate layer rich in $$SiO_{2}$$ and usually $$Al_{2}O_{3}$$ and an outer main layer rich in FeO and MnO. The subordinate layer may be altered rock which has lost all evidence of crystal structure and to which Fe and Mn have been added. These layers are not distinguishable optically. In varnish on quartzites, FeO, MnO, $$Al_{2}O_{3}$$, and $$K_{2}O$$ all increase in concentration outward through the varnish from the rock-varnish contact. However, in varnish on argillites, $$Al_{2}O_{3}, K_{2}O$$, and sometimes FeO decrease in concentration outward. Comparison of these variations with data on the bulk composition of the fresh rock suggests that elements which increase outward may be supplied largely from external sources and that elements which decrease outward may be supplied predominantly from the underlying rock. The FeO : MnO ratio generally decreases outward through the varnish. This suggests that some of the Mn in varnish is dissolved whenever Fe-Mn-bearing solutions arrive on the rock and that fractionation occurs during subsequent precipitation such that Mn is precipitated last. This fractionation may be responsible for the existence of the orange bottom coating, the groundline band, and the darker varnish in hollows on the rock surface.
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