Minerals Report - International Seabed Authority

3.2. Mineralogy
The mineralogy of bulk crusts is relatively simple compared to
hydrothermal and diagenetic iron and manganese deposits. The dominant
crystalline phase is iron-rich δ-MnO2 (ferruginous vernadite (46)); with
generally two X-ray reflections at about 1.4 Å and 2.4 Å that vary widely in
sharpness as the result of crystallite size and manganese content. δ-MnO2
generally makes up more than 95% of the X-ray crystalline phases, the
remainder being detrital minerals such as quartz, plagioclase, potassiumfeldspar,
pyroxene, phillipsite, and authigenic carbonate fluorapatite (CFA;
Table 5). The older parts of thick crusts are phosphatized 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 (δ-FeOOH, feroxyhyte; (47)), which is commonly epitaxially
intergrown with δ-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
δ-MnO2
(Iron-vernadite),
Iron oxyhydroxide
(Feroxyhyte)
CFA = carbonate fluorapatite
CFA, quartz,
plagioclase,
smectite
Phillipsite, goethite,
todorokite, calcite,
K-feldspar,
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 Xray
diffraction analyses done by the USGS on Pacific crusts, only 5% (2% if
offshore California samples are excluded) contain todorokite; 30% of crust
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