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emainder being detrital minerals such as quartz, plagioclase, potassium-feldspar,
pyroxene, phillipsite, and authigenic carbonate fluorapatite (CFA; Table 5). The older
parts of thick crusts are phosphatised and may contain up to 30% CFA in that part of the
crust, but CFA is generally less than 10% of thick bulk crusts. Another major phase in
crusts is X-ray amorphous iron oxyhydroxide (?-FeOOH, feroxyhyte; 47), which is
commonly epitaxially intergrown with ?-MnO2 48. In about 6% of 640 samples analysed,
the feroxyhyte crystallizes as goethite in the older parts of thick crusts. In Pacific crusts,
the quartz and part of the plagioclase are eolian, whereas the remainder of the
plagioclase and the other volcanogenic minerals derive from local outcrops.
Table 5
Mineralogy of Pacific Fe-Mn crusts
Dominant Common Less Common Uncommon or Uncertain
?-MnO2
CFA, quartz,
(iron-vernadite), plagioclase,
Iron oxyhydroxide
(feroxyhyte)
smectite
CFA = carbonate fluorapatite
Phillipsite, goethite,
todorokite, calcite, Kfeldspar,
pyroxene,
opal-A, barite,
amphibole, magnetite,
amorphous
aluminosilicates
Halite, illite, clinoptilolite,
lepidocrocite, manjiroite,
manganite, palygorskite, chlorite,
dolomite, stevensite, kutnahorite,
mordenite, natrojarosite,
hematite, manganosite,
maghemite, lithiophorite, analcite
Todorokite, which is common in diagenetic Fe-Mn nodules and hydrothermal
manganese deposits, is rare in hydrogenetic crusts. Of 640 X-ray diffraction analyses
done by the USGS on Pacific crusts, only 5% (2% if offshore California samples are
excluded) contain todorokite; 30% of crust samples from offshore California contain
todorokite, which, because of very high biological productivity, may reflect the lower
oxidation potential of seawater there compared to Pacific sites farther to the west. CFA
occurs in 28% of the crust samples analysed, but not in any crusts from the east (offshore
California) or far north Pacific; if those crusts are excluded, 34% of central and west
Pacific samples contain CFA. Phosphatisation of Fe-Mn crusts generally occurred during
two periods of time, about 34 Ma and 24 Ma ago 49. Smaller percentages of Atlantic and
Indian Ocean crusts contain CFA because those crusts are thinner and younger compared
to Pacific crusts.
2.3. Ages and Growth Rates of Fe-Mn Crusts
Hydrogenetic Fe-Mn crusts grow at incredibly slow rates of