drill holes (usually below 500 m depth). Marine
clinoptilolite forms finer grained (o45 mm), platy
crystals of relatively silicon-rich hydrated potassiumand sodium-rich aluminosilicate (high Si/Al ratio).
Clinoptilolite is more frequently encountered at
depths 4100 m in the sediment column and persists
to great depths and geological age, suggesting that it is
thermodynamically stable in the deep-sea. Analcite is
a rarer, sodium-rich zeolite in deep-sea sediments that
displays a general trend of increasing abundance with
geological age which parallels that of clinoptilolite.
Both clinoptilolite and analcite can form directly from
alteration of more siliceous volcanic glass (andesite,
rhyolite, from active island arcs and explosive
continental volcanism), by diagenetic reaction of
phillipsite and dissolved silica (clinoptilolite) or Na/K
exchange with clinoptilolite (analcite).
Palygorskite and sepiolite Authigenic clay minerals
(o2 mm particle size) in the deep-sea include the
fibrous minerals palygorskite and sepiolite and the
smectite family of expandable phyllosilicates. Both
palygorskite and sepiolite are rare in recent marine
sediments, occuring more often in Eocene and older
sediments. Palygorskite is a hydrous silicate
containing Mg, Al and Fe
3þ , whereas sepiolite is
almost a pure hydrous magnesium silicate. Pore water
solutions of alkaline pH with high concentrations of
dissolved silica and magnesium favor the formation of
both minerals. Fine fibrous textures and overgrowths
of siliceous tests and opal-CT attest to their
authigenesis. Reactions range from the diagenetic
alteration of (mainly silicic and intermediate) volcanic
ash, either directly or indirectly via smectite with
biogenic silica, formation upon magnesium release
after conversion of biogenic opal-A to opal-CT, and
reaction of biogenic silica tests with marine pore
waters, including hypersaline brines enriched in
magnesium (Table 1).
Smectites Most clay minerals in deep-sea
sediments are detrital phases from continental
weathering. These minerals compose the bulk of the
nonbiogenic sediments in the o2 mm fraction and
include clay-sized quartz and the phyllosilicates
kaolinite, illite, chlorite, and smectite. The smectite
Table 1 Summary of observed and suggested diagenetic reactions in deep-sea sediments
Opal-A-opal-CT-quartz
Opal-A-quartz
Basaltic glass-palagonite
Volcanic glass-smectite minerals
Volcanic glass-zeolites
Basalt-smectite minerals
Basalt þ H 4 SiO 4 -smectite minerals
Basaltic glass þ H 4 SiO 4 -phillipsite
Poorly cyrstalline smectite-phillipsite
Andesitic and rhyolitic glass þ H 4 SiO 4 -clinoptilolite
Andesitic and rhyolitic glass-smectite minerals
Phillipsite þ H 4 SiO 4 -clinoptilolite
Phillipsite þ smectite þ H 4 SiO 4 -clinoptilolite þ palygorskite
Opal-A þ Al(OH) 4
À þ K
þ
-clinoptilolite
Opal-A þ Al(OH) 4
À þ K
þ
-opal-CT þ clinoptilolite
Plagioclase þ K
þ þ H 4 SiO 4 -K-feldspar þ Na
þ þ Ca
2þ
-K-feldspar
?-albite
Clinoptilolite þ Na
þ
-analcite þ K
þ þ quartz
Clinoptilolite- K-feldspar þ quartz
Clinoptilolite þ Na
þ
-analcite þ K-feldspar þ quartz
H 4 SiO 4 þ Mg
2þ þ Al(OH) 4
À
- palygorskite
H 4 SiO 4 þ Mg
2þ -sepiolite
Volcanic glass þ Mg
2þ þ H 4 SiO 4 -palygorskite
Smectite þ Mg
2þ þ H 4 SiO 4 - palygorskite
Clinoptilolite þ palygorskite þ calcite- K-feldspar þ dolomite þ quartz
Clinoptilolite þ sepiolite þ calcite- K-feldspar þ dolomite þ quartz
Amorphous hydroxides (mainly Fe) þ H 4 SiO 4 þ Mg
2þ -Fe-montmorillonite
Nontronite þ Mg
2þ þ reduced sulfur-saponite þ FeS 2
Dissolved silica adsorption by clay minerals
Amorphous aluminosilicate reconstitution?
Smectite-mixed-layer illite–smectite?
After Kastner (1981).
AUTHIGENIC DEPOSITS 333
clinoptilolite forms finer grained (o45 mm), platy
crystals of relatively silicon-rich hydrated potassiumand sodium-rich aluminosilicate (high Si/Al ratio).
Clinoptilolite is more frequently encountered at
depths 4100 m in the sediment column and persists
to great depths and geological age, suggesting that it is
thermodynamically stable in the deep-sea. Analcite is
a rarer, sodium-rich zeolite in deep-sea sediments that
displays a general trend of increasing abundance with
geological age which parallels that of clinoptilolite.
Both clinoptilolite and analcite can form directly from
alteration of more siliceous volcanic glass (andesite,
rhyolite, from active island arcs and explosive
continental volcanism), by diagenetic reaction of
phillipsite and dissolved silica (clinoptilolite) or Na/K
exchange with clinoptilolite (analcite).
Palygorskite and sepiolite Authigenic clay minerals
(o2 mm particle size) in the deep-sea include the
fibrous minerals palygorskite and sepiolite and the
smectite family of expandable phyllosilicates. Both
palygorskite and sepiolite are rare in recent marine
sediments, occuring more often in Eocene and older
sediments. Palygorskite is a hydrous silicate
containing Mg, Al and Fe
3þ , whereas sepiolite is
almost a pure hydrous magnesium silicate. Pore water
solutions of alkaline pH with high concentrations of
dissolved silica and magnesium favor the formation of
both minerals. Fine fibrous textures and overgrowths
of siliceous tests and opal-CT attest to their
authigenesis. Reactions range from the diagenetic
alteration of (mainly silicic and intermediate) volcanic
ash, either directly or indirectly via smectite with
biogenic silica, formation upon magnesium release
after conversion of biogenic opal-A to opal-CT, and
reaction of biogenic silica tests with marine pore
waters, including hypersaline brines enriched in
magnesium (Table 1).
Smectites Most clay minerals in deep-sea
sediments are detrital phases from continental
weathering. These minerals compose the bulk of the
nonbiogenic sediments in the o2 mm fraction and
include clay-sized quartz and the phyllosilicates
kaolinite, illite, chlorite, and smectite. The smectite
Table 1 Summary of observed and suggested diagenetic reactions in deep-sea sediments
Opal-A-opal-CT-quartz
Opal-A-quartz
Basaltic glass-palagonite
Volcanic glass-smectite minerals
Volcanic glass-zeolites
Basalt-smectite minerals
Basalt þ H 4 SiO 4 -smectite minerals
Basaltic glass þ H 4 SiO 4 -phillipsite
Poorly cyrstalline smectite-phillipsite
Andesitic and rhyolitic glass þ H 4 SiO 4 -clinoptilolite
Andesitic and rhyolitic glass-smectite minerals
Phillipsite þ H 4 SiO 4 -clinoptilolite
Phillipsite þ smectite þ H 4 SiO 4 -clinoptilolite þ palygorskite
Opal-A þ Al(OH) 4
À þ K
þ
-clinoptilolite
Opal-A þ Al(OH) 4
À þ K
þ
-opal-CT þ clinoptilolite
Plagioclase þ K
þ þ H 4 SiO 4 -K-feldspar þ Na
þ þ Ca
2þ
-K-feldspar
?-albite
Clinoptilolite þ Na
þ
-analcite þ K
þ þ quartz
Clinoptilolite- K-feldspar þ quartz
Clinoptilolite þ Na
þ
-analcite þ K-feldspar þ quartz
H 4 SiO 4 þ Mg
2þ þ Al(OH) 4
À
- palygorskite
H 4 SiO 4 þ Mg
2þ -sepiolite
Volcanic glass þ Mg
2þ þ H 4 SiO 4 -palygorskite
Smectite þ Mg
2þ þ H 4 SiO 4 - palygorskite
Clinoptilolite þ palygorskite þ calcite- K-feldspar þ dolomite þ quartz
Clinoptilolite þ sepiolite þ calcite- K-feldspar þ dolomite þ quartz
Amorphous hydroxides (mainly Fe) þ H 4 SiO 4 þ Mg
2þ -Fe-montmorillonite
Nontronite þ Mg
2þ þ reduced sulfur-saponite þ FeS 2
Dissolved silica adsorption by clay minerals
Amorphous aluminosilicate reconstitution?
Smectite-mixed-layer illite–smectite?
After Kastner (1981).
AUTHIGENIC DEPOSITS 333