have become partially substituted for the major elements in a crystalline lattice.
Some of these elements could either replace the major elements or even form other
individual minerals. For example, the most common metallic trace elements
forming minerals and exploited by industry are gold, iron, silver, nickel, platinum,
copper, lead, cobalt, zinc, antimony and palladium.
In the 1960’s, before the invention of Microprobe Analyses, which could
determine the chemical composition of individual crystals, we performed chemical
analyses on rock samples by using a mass-spectrometer. Our samples needed to be
ground into a fine dust before inserting them into a spectrometer, which could
indicate their chemical composition. By analyzing glass, one could have a better
idea about the bulk-melt composition than when studying a crystalline sample
where the individual minerals forming a particular rock might significantly vary in
their chemical contents. Analytical uncertainty could also result if the whole rock
powdered-sample had not been properly homogenized. Mineralogy was done
using onionskin thin slices of rocks (called ‘‘thin sections’’) that were placed under
a microscope so we could see the mineral crystals and recognize them on the basis
of their particular shapes and colors. Therefore, crystallized samples were best for
mineralogy studies, however glassy samples were the best for mass spectrometry
since they did not have any crystals that might unduly influence the analyses’ final
results.
Basaltic Rocks
The top layer of the sea floor is primarily composed of about 50 % basalt, which is
the most common type of volcanic rock to be erupted. Basaltic rocks have
solidified after the eruption of lava composed of several minerals such as plagioclase (silicates of aluminum, calcium and sodium), clinopyroxene (silicates of
aluminum, magnesium, calcium and iron), and olivine (silicates of iron and
magnesium), which could be enriched in forsterite (Fe-rich olivine = Fo 70-88 ).
Less than 5 % of basalt’s mineral content is composed of opaque metallic-oxides
such as spinel, titanomagnetite or ilmenite.
The mineral composition of basalt reflects the composition of the upper portion
of the Earth’s oceanic lithosphere. Partial melting starts at the borders of mineral
grains where interstitial liquids are formed and then escape towards shallower
depths. If a basaltic melt circulating in the lithosphere solidifies prior to its
extrusion at the surface, it will be called a dolerite. The mineral composition of
dolerite is close to that of basalt, but since dolerite rocks have solidified more
slowly within the crust, they are therefore more crystalline than are the erupted
basaltic rocks.
The name ‘‘basalt’’ has an uncertain origin according to Albert Johannsen
(1931). It could have come from the Greek word meaning ‘‘touchstone’’, due to its
shiny appearance and its use in engraving coins. A historical naturalist, Pliny the
Younger, has written that most statues from Egypt were carved from basalt, and
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4 Sea Floor Rocks
Some of these elements could either replace the major elements or even form other
individual minerals. For example, the most common metallic trace elements
forming minerals and exploited by industry are gold, iron, silver, nickel, platinum,
copper, lead, cobalt, zinc, antimony and palladium.
In the 1960’s, before the invention of Microprobe Analyses, which could
determine the chemical composition of individual crystals, we performed chemical
analyses on rock samples by using a mass-spectrometer. Our samples needed to be
ground into a fine dust before inserting them into a spectrometer, which could
indicate their chemical composition. By analyzing glass, one could have a better
idea about the bulk-melt composition than when studying a crystalline sample
where the individual minerals forming a particular rock might significantly vary in
their chemical contents. Analytical uncertainty could also result if the whole rock
powdered-sample had not been properly homogenized. Mineralogy was done
using onionskin thin slices of rocks (called ‘‘thin sections’’) that were placed under
a microscope so we could see the mineral crystals and recognize them on the basis
of their particular shapes and colors. Therefore, crystallized samples were best for
mineralogy studies, however glassy samples were the best for mass spectrometry
since they did not have any crystals that might unduly influence the analyses’ final
results.
Basaltic Rocks
The top layer of the sea floor is primarily composed of about 50 % basalt, which is
the most common type of volcanic rock to be erupted. Basaltic rocks have
solidified after the eruption of lava composed of several minerals such as plagioclase (silicates of aluminum, calcium and sodium), clinopyroxene (silicates of
aluminum, magnesium, calcium and iron), and olivine (silicates of iron and
magnesium), which could be enriched in forsterite (Fe-rich olivine = Fo 70-88 ).
Less than 5 % of basalt’s mineral content is composed of opaque metallic-oxides
such as spinel, titanomagnetite or ilmenite.
The mineral composition of basalt reflects the composition of the upper portion
of the Earth’s oceanic lithosphere. Partial melting starts at the borders of mineral
grains where interstitial liquids are formed and then escape towards shallower
depths. If a basaltic melt circulating in the lithosphere solidifies prior to its
extrusion at the surface, it will be called a dolerite. The mineral composition of
dolerite is close to that of basalt, but since dolerite rocks have solidified more
slowly within the crust, they are therefore more crystalline than are the erupted
basaltic rocks.
The name ‘‘basalt’’ has an uncertain origin according to Albert Johannsen
(1931). It could have come from the Greek word meaning ‘‘touchstone’’, due to its
shiny appearance and its use in engraving coins. A historical naturalist, Pliny the
Younger, has written that most statues from Egypt were carved from basalt, and
78
4 Sea Floor Rocks