6.12 Phosphorite (or “Phosphatic
Deposits”)
Phosphorites are sedimentary layers in which phosphate minerals are the main components, mainly apatite Ca 5 PO 4
ð
Þ 3 F; Cl; OH
ð
Þ . These crystals may contain
fluor (F
À ), chlorine (Cl
À ) or hydroxyl (OH
À ).
Fluorapatite Ca 5 PO 4
ð
Þ 3 F is the most important in
marine sediments. We also have francolite
Ca; Na
ð
Þ 5 F; OH
ð
Þ PO 4 ; CO 3
ð
Þ 3 , where some carbonate
has substituted for phosphate and where sodium
substitutes for calcium. Iron apatites such as strengite
FePO 4 Á 2H 2 O
ð
Þare also relatively common minerals
in secondary (weathered) phosphatic deposits.
Phosphorite rocks are an important source of fertilizer. Large deposits such as those found in Morocco,
Spanish Sahara and Senegal in West Africa, and in
Florida, are of great commercial value. Understanding
how phosphate deposits form is therefore a matter of
considerable economic interest. In most sedimentary
rocks phosphorus is a trace element, and very special
conditions are necessary for phosphate enrichment to
take place. These deposits therefore tell us something
important about the environment of deposition.
Guano deposits are formed on land from bird or bat
excrement in which the phosphate gradually becomes
concentrated as the other organic components are
leached out. For the phosphates to be preserved, the
rainfall must not be too high, because it will dissolve
phosphate sediments on land. We also have freshwater
phosphatic deposits, but it is the marine ones that are
the most important.
The first prerequisite for marine phosphate deposits
is that sedimentation must proceed very slowly, i.e.
there must be virtually no clastic sedimentation. In
consequence we find phosphate beds associated with
major or minor breaks in sedimentation, or with
periods of very slow sedimentation. It has long been
known that phosphate is formed in areas with strong
upwelling and high organic productivity. We have
good examples of this along the edge of the continental slope off Chile and Peru. Water welling up from
great depths brings with it nutrients which are
liberated when marine organisms disintegrate in
deeper water. When the water flows up to the surface
the nutrients are consumed by organisms, mostly plant
plankton, which provide a high primary production.
These organisms contain about 1% P (dry weight),
which is an enrichment of 140,000 compared to
dissolved phosphorus in ordinary seawater.
Organisms with an amorphous silica skeleton (e.g.
diatoms and radiolaria) will readily dissolve in seawater, and in some cases carbonate (calcite and particularly aragonite) will also dissolve, resulting in
further phosphate enrichment. Phosphate minerals
then crystallise out of the phosphate-rich sediments
and often replace other minerals, e.g. carbonate. Apatite is a heavy mineral (specific weight 3.18) and may
also be enriched mechanically by weak traction
currents.
Apatite will often crystallise out as concretions in
bottom ooze, and erosion by traction currents may
concentrate these nodules into a conglomerate. On
the continental shelf, phosphate forms at depths of
between 100 and 400 m, i.e. below the photic zone.
However, phosphate nodules and massive beds of finegrained phosphate mud (phosphate micrite) may also
form in lagoons where the water is less clear. These
phosphate deposits will be easily eroded even as a
result of minor regressions, and form conglomerates
of phosphate mudstone (micrite). Today phosphate
deposits are forming only in a few areas with strong
upwelling, but in previous geological periods we find
very extensive phosphate beds, often associated with
transgressions. The transgressions will hold clastic
sediment back for a while, enabling biogenic matter
to be concentrated. Phosphate beds are often
associated with other authigenic (formed in situ)
minerals which take a long time to form, particularly
glauconite and also manganese deposits.
In Florida there are large phosphate deposits of
mid-Tertiary age, and the same beds are exposed on
the floor of the continental shelf off South Carolina
(Blake Plateau), where the manganese-rich phosphatic
deposits total 10
9 tonnes.
Marine phosphate deposits may also be formed by
fossils with a phosphatic skeleton, for example fish.
These deposits may also be extensive, and are often
called “bone beds”.
Phosphate minerals such as apatite may contain
considerable amounts of uranium and rare-earth
metals which substitute for calcium. Weathering of
phosphate deposits will lead to oxidation of uranium
to UO
2þ
2 , which is soluble in the form of uranyl ions,
and uranium may be precipitated again in the reduced
state (UO 2 ) when it comes into contact with organic
228
K. Bjørlykke
Deposits”)
Phosphorites are sedimentary layers in which phosphate minerals are the main components, mainly apatite Ca 5 PO 4
ð
Þ 3 F; Cl; OH
ð
Þ . These crystals may contain
fluor (F
À ), chlorine (Cl
À ) or hydroxyl (OH
À ).
Fluorapatite Ca 5 PO 4
ð
Þ 3 F is the most important in
marine sediments. We also have francolite
Ca; Na
ð
Þ 5 F; OH
ð
Þ PO 4 ; CO 3
ð
Þ 3 , where some carbonate
has substituted for phosphate and where sodium
substitutes for calcium. Iron apatites such as strengite
FePO 4 Á 2H 2 O
ð
Þare also relatively common minerals
in secondary (weathered) phosphatic deposits.
Phosphorite rocks are an important source of fertilizer. Large deposits such as those found in Morocco,
Spanish Sahara and Senegal in West Africa, and in
Florida, are of great commercial value. Understanding
how phosphate deposits form is therefore a matter of
considerable economic interest. In most sedimentary
rocks phosphorus is a trace element, and very special
conditions are necessary for phosphate enrichment to
take place. These deposits therefore tell us something
important about the environment of deposition.
Guano deposits are formed on land from bird or bat
excrement in which the phosphate gradually becomes
concentrated as the other organic components are
leached out. For the phosphates to be preserved, the
rainfall must not be too high, because it will dissolve
phosphate sediments on land. We also have freshwater
phosphatic deposits, but it is the marine ones that are
the most important.
The first prerequisite for marine phosphate deposits
is that sedimentation must proceed very slowly, i.e.
there must be virtually no clastic sedimentation. In
consequence we find phosphate beds associated with
major or minor breaks in sedimentation, or with
periods of very slow sedimentation. It has long been
known that phosphate is formed in areas with strong
upwelling and high organic productivity. We have
good examples of this along the edge of the continental slope off Chile and Peru. Water welling up from
great depths brings with it nutrients which are
liberated when marine organisms disintegrate in
deeper water. When the water flows up to the surface
the nutrients are consumed by organisms, mostly plant
plankton, which provide a high primary production.
These organisms contain about 1% P (dry weight),
which is an enrichment of 140,000 compared to
dissolved phosphorus in ordinary seawater.
Organisms with an amorphous silica skeleton (e.g.
diatoms and radiolaria) will readily dissolve in seawater, and in some cases carbonate (calcite and particularly aragonite) will also dissolve, resulting in
further phosphate enrichment. Phosphate minerals
then crystallise out of the phosphate-rich sediments
and often replace other minerals, e.g. carbonate. Apatite is a heavy mineral (specific weight 3.18) and may
also be enriched mechanically by weak traction
currents.
Apatite will often crystallise out as concretions in
bottom ooze, and erosion by traction currents may
concentrate these nodules into a conglomerate. On
the continental shelf, phosphate forms at depths of
between 100 and 400 m, i.e. below the photic zone.
However, phosphate nodules and massive beds of finegrained phosphate mud (phosphate micrite) may also
form in lagoons where the water is less clear. These
phosphate deposits will be easily eroded even as a
result of minor regressions, and form conglomerates
of phosphate mudstone (micrite). Today phosphate
deposits are forming only in a few areas with strong
upwelling, but in previous geological periods we find
very extensive phosphate beds, often associated with
transgressions. The transgressions will hold clastic
sediment back for a while, enabling biogenic matter
to be concentrated. Phosphate beds are often
associated with other authigenic (formed in situ)
minerals which take a long time to form, particularly
glauconite and also manganese deposits.
In Florida there are large phosphate deposits of
mid-Tertiary age, and the same beds are exposed on
the floor of the continental shelf off South Carolina
(Blake Plateau), where the manganese-rich phosphatic
deposits total 10
9 tonnes.
Marine phosphate deposits may also be formed by
fossils with a phosphatic skeleton, for example fish.
These deposits may also be extensive, and are often
called “bone beds”.
Phosphate minerals such as apatite may contain
considerable amounts of uranium and rare-earth
metals which substitute for calcium. Weathering of
phosphate deposits will lead to oxidation of uranium
to UO
2þ
2 , which is soluble in the form of uranyl ions,
and uranium may be precipitated again in the reduced
state (UO 2 ) when it comes into contact with organic
228
K. Bjørlykke