KISH GEOCHEMICAL STUDIES OF PUMICE 217

 

70

65

60

SiO2 [wt%]

99

50

45

40
1.480

1.540

 

1.600 1.660

Refractive index [n]

Figure 4. Plot of the measured refractive index of glass in pumice samples versus the calculated anhydrous SiO, concentration
for the bulk rock in the same sample. The curve shown in the figure represents the observed refractive index values for a wide
range of volcanic glass samples (Fisher and Schmincke 1984). The samples that fall outside the boundaries of the curve have
an andesitic composition and are characterized by having moderately high (>10 %) concentrations of mafic and plagioclase
phenocrysts. Samples for the Whitebelt and Wynnhaven sites are not shown due to possible alteration of the sample material.

have predicted SiO, contents that are much higher than their
observed bulk-rock silica composition (Figure 6). Petrographic
examination of one of these samples (MDC.1.222.1 1a) reveals
the presence of >15 percent mafic mineral phenocrysts, which
include both biotite and amphibole. These mineral phases have
low SiO, contents. A mixture of high silica glass and low silica
mafic minerals will produce the observed bulk-rock SiO,
composition. The index of refraction for the glass from the
mafic sample (EGLIN 056) taken from the Wynnhaven Beach
site is far lower than would be indicated by it bulk chemical
composition. The glass in this sample appears very cloudy and
contains a very high volume of microlites (microcrystals). The
presence of the microlites may explain the discrepancy between
observed and predicted SiO, content for the sample.

Geochemical Characterization
Major Element Chemistry

Since most volcanic rocks are either very fine grained or are
composed of a large percentage of glass, a complete, conven-
tional lithologic classification and characterization of volcanic
material based upon mineralogy is not possible. Most studies
of volcanic rocks make use of the chemical composition of
individual samples or a suite of cogenetic samples to provide a
more exact characterization of the volcanic material. Petrologic
classification methods that utilize the major element chemistry
can relate the major element chemical composition of the
volcanic rock to a hypothetical mineral assemblage know as
“normative minerals” (Irvine and Baragar 1971). Volcanic