Download Boron in magmatic processes

Boron, aluminium,
carbon, silicon
Boron (B)
Universe: 0.001 ppm (by weight)
Sun: 0.002 ppm (by weight)
Carbonaceous meteorite: 1.6 ppm
Earth's Crust: 950 ppm
Seawater: 4.4 ppm
Introduction of boron
One of Goldschmidt early study (in 1923) showed that
boron was much more abundant in sedimentary rocks (300
ppm) than in igneous rocks (3 ppm). The large variation in
boron concentration of different rock types is a
consequence of three characteristics: its preferential
partitioning into melts, its high mobility in the aqueous
phase and its strong affinity for clay minerals.
Boron in magmatic processes
Boron concentrations relatively low in basalts (6-0.1 ppm),
and higher in more evolved rocks such as granites (85
ppm). Partitioning is not the only cause for high boron
concentrations in granites; some S-type granites contain
high concentrations because they are derived from boronrich sedimentary material. The final residual melt phases
often form pegmatites which may contain up to 1360 ppm
boron with the boron concentrated within tourmalines
which can contain up to 4% boron. The tourmalines appear
from pegmatitic to hydrothermal stages.
Well-known boron-silicates in basic magmatites are:
danburite, datolite, axinite-group minerals.
Boron in magmatic processes
Submarine igneous rocks that have been hydrothermally
altered are often altered to clay minerals such as smectite.
Adsorption of boron onto these clays can account for
elevated boron concentrations in altered basalts. Boron is
thus progressively leached from the host-rock in high
temperature in geothermal systems.
Detail the order in which boron substitutes into secondary
minerals; in decreasing order: biotite, pyroxenes,
plagioclase, amphiboles.
Boron frequently emanates from volcanic vents. For
example, deposits of sassolite - B(OH)3 - commonly occur
around the vents of fumaroles. The presence of sassolite
suggests that boron is transported in the vapor phase.
Boron in weathering and sedimentary
processes
The boron content of sedimentary rocks is partly related to
the amount and type of clay minerals present. Clay
minerals, such as illite, smectite and montmorillonite
incorporate boron from water both by surface adsorption
and as structurally bound boron. In contrary, the Aloxides(hydroxides) and kaolinite-containing clays show
lower boron concentrations.
Marine sedimentary rocks tend to contain more boron than
fluvial and lacustrine sedimentary rocks, as the seawater
interacting with the marine rock contains more boron.
Boron in marine environment
The amount of boron in marine sediments is greater than
the amount of boron in the ocean waters. Marine sediments
contain about 100 ppm boron.
Seawater contains 4.6 ppm boron, and concentrations do
not vary significantly throughout the oceans. Boron is
strongly adsorbed onto secondary minerals (e.g. clay
minerals), and these may buffer the seawater boron
concentration. Seawater is the source of virtually all of the
boron in altered oceanic crust and much of the boron in
sediments and island arc environments.
Boron in evaporites
The major source of boron is in borates from evaporite
deposits, most commonly in desert playas, as borax and
colemanite. Borax is by far the most significant mined
source of boron. These deposits are in connection with
thermal activity (boron-bearing thermal water).
Because of the differences of boron-concentrations of
freshwater and seewater, the marine borates contain
Na-Mg-K cations, while freshwater/lacustrine borates
contain mainly Ca-Mg cations.
Boron in the biosphere
Boron is essential element of many plants. However, the
soil has negative anomaly about boron (except of soil
which lies in see-side).
Some marine animals show boron enrichment boron (e.g.
corals, silica-spongiae). Marine sapropels have higher
boron contains, than freshwater sapropels.
Aluminium (Al)
Universe: 50 ppm (by weight)
Sun: 60 ppm (by weight)
Carbonaceous meteorite: 9300 ppm
Earth's Crust: 82000 ppm
Seawater: Atlantic surface: 9.7 x 10-4 ppm
Pacific surface: 1.3 x 10-4 ppm
Aluminium in magmatic processes
Aluminium is the most abundant metal (8.3%) in the Earth's
crust. It is a major constituent of many igneous minerals
such as feldspars. The Al-concentration shows enrichment
from ultrabasic to acidic magmatics.
It has low abundance in early magmatic differentiates,
except anorthosite (it main mineral is anorthite). Calcalkaline magmatic rocks contain the most important rockforming silicates with Al (feldspars, amphiboles, micas,
epidote, quartz). There are characteristic Si4+ Al3+
substitution in these silicates. The Al has a range of
coordination numbers ( 4, 5 and 6) in solids / silicates.
Aluminium in magmatic and
metamorphic processes
There are other compounds in magmatic and metamorphic
processes, than corundum, spinel, topaz, chrysoberyl etc.
The latter rather in post-magmatic stage.
Some aluminosilicates can be used as indicators of PT
conditions in metamorphism: kyanite, andalusite,
sillimanite, which are polymorphs.
Aluminium in weathering and
sedimentary processes
Aluminium has a low solubility at Earth's surface. This
solubility is pH dependent and increases at low pH or in
alkaline solution. A second important behavior concerns
the increase of solubility in presence of organic ligands:
chelating complexes with oxalic acids are responsible for a
net increase of AI solubility. That accounts for the
mobilization of AI in acid soils and the increase of AI
concentration in natural acid solutions.
Disordered aluminosilicates (allophane and imogolite)
are frequently described in soils developed on ash parent
materials and as stream precipitates in volcanic areas.
Aluminium in weathering and
sedimentary processes
During weathering of crustal rocks, aluminium is
accumulated in clay minerals (kaolinite, smectite, illite,
vermiculite), or oxyhydroxides: böhmite AIO(OH) and
hydroxide gibbsite, Al(OH)3). The type of secondary Alcontaining minerals depend on the degree of chemical
weathering.
Bauxites mainly formed under tropic or subtropic climates.
Lateritic bauxites contain mainly gibbsite, however böhmite
can predominate in karstic bauxites. In tropical soils (and
occassionally bauxites, too) AI may substitute for Fe in
goethite and hematite. Diaspore, AIO(OH) and corundum
(Al203) are high-temperature phases which are present in
metamorphic bauxites.
Aluminium in the biosphere
Al widespread in all organics, but not essential element. It
was detected up to 20 % Al-oxide some ash of plants,
and of coal ash.
There is a tendency, that low-class animals contain more
Al, than high-class animals.
Carbon (C)
Universe: 5000 ppm (by weight)
Sun: 3000 ppm (by weight)
Carbonaceous meteorite: 15000 ppm
Atmosphere: 350 ppm
Earth's Crust: 480 ppm
Seawater: 23 ppm
Introduction of carbon
Unique characteristic: it forms in more compounds than the
all element together.
High abundance in the Universe, the Sun, and the Earths
crust, too.
It has siderophil character in meteorites, lithophil in the
Earths crust, atmophil in the atmosphere, and biophil in
the biosphere. Essential element for the life.
Carbon in magmatic and metamorphic
processes
Prominent carbon-containing natural materials include
diamond, the hardest of the minerals, with a threedimensional structure; graphite, often used as a lubricant
because its two dimensional structure allows planes to slip
laterally. Diamond is high-pressure polymorph of carbon,
origin from the Earth mantle, and moves to the crusts quick
magmatic processes. Graphite is a high-temperature
polymorph of carbon, it forms mainly metamorph
processes, or it has pegmatitic origin. However, graphite
can form on Earth from sedimentary organic carbon
subjected to high pressures at great depths of burial, high
temperatures or both.
Carbon in magmatic, metamorphic and
sedimentary processes
Carbon is a constituent of carbonate anion. The simple
carbonates (minerals of calcite and aragonite groups) occur
in post-magmatic origin (except carbonatites, which are
early differentiates), and in sedimentary environments.
Carbonate anion can build in silicates structures, e.g. in
scapolites, cancrinites. Similar appearance is known in
apatite structure, too.
A relatively recently-discovered three-dimensional structure
group, called fullerenes, forms a near-spherical cage of
carbon atoms. The first identified fullerene structure
consisted of 60 carbon atoms. Fullerenes occur naturally
on Earth in some metamorphic rocks and meteorite impact
crater debris.
Carbon in weathering and sedimentary
processes
Carbonate sediments comprise a major portion of the
Earth's sedimentary rocks. Carbonate rocks consist almost
entirely of calcite (CaCO3), which is predominantly biogenic
in origin, and dolomite - CaMg(CO3)2 - which is formed by
diagenic alteration of calcite (so-called Mg-metasomatism).
Another major type of sedimentary rock, shale, contains
widely varying amounts and types of sedimentary organic
carbon.
The high-carbon end member of sedimentary organic
carbon (mixture of carbon-containing compounds) is coal.
Oil-shale has a wide range of carbon content, and may
play a major role as an energy source in the future.
Organic minerals – carbon in the
lithosphere-biosphere interaction
Organic minerals are natural organic compounds with both
a well-defined chemical composition and crystallographic
properties; their occurrences reveal traces of the high
concentration of certain organic compounds in natural
environments.
The organic minerals are divided into two groups: (1) ionic
organic minerals, in which organic anions (it can contains
carbon) and various cations are held together by ionic
bonds, and (2) molecular organic minerals, in which
electroneutral organic molecules (mainly carboncontaining) are bonded by weak intermolecular
interactions.
Organic minerals – carbon in the
lithosphere-biosphere ineraction
1) Ionic organic minerals, oxalates, formates, acetates,
mellitates, citrates etc. The most widespread are the
oxaletes, e.g. whewellite, weddelite.
2) Molecular organic minerals, amides, purines, polycyclic
aromatic organic minerals (e.g. kárpátite), alkanes
(evenkite). Molecular mixing occurs in molecular organic
minerals and leads to molecular solid-solutions.
In natural environments, the distinction between inorganic
and organic materials is obscure, and thus the interactions
between inorganic and organic materials are ubiquitous.
Lichens and fungi living on mineral surfaces produce
organic acids and facilitate the dissolution of heavy metals
such as Cd, Pb etc.
Organic minerals
Organic minerals are expected to be promising biomarkers
in the detection of life and in the recognition of biological
activity in the geological records of extraterrestrial
material, such as Mars. If we consider such products of
organic–inorganic interactions and living organisms as
members of the mineral kingdom, we could make unlimited
contributions to many branches of Earth and planetary
sciences.
Silicon (Si)
Universe: 700 ppm (by weight)
Sun: 900 ppm (by weight)
Carbonaceous meteorite: 1.4 x 105 ppm
Earth's Crust: 2.771 x 105 ppm
Seawater:
Atlantic surface: 0.03 ppm
Atlantic deep: 0.82 ppm
Pacific surface: 0.03 ppm
Pacific deep: 4.09 ppm
Silicon in magmatic processes
Silica refers to silicon bonded with two oxygen atoms and is
one of the solid forms in which silicon is found in the Earths
crust. Silicate is silicon complexed with four oxygen atoms
in an anionic complex in aqueous solution or is the other
solid form in which silicon is found.
The small ionic radius of silicon (0.42 A) and ease of 4-way
covalent bonding ensures that it is found mainly in
tetrahedral coordination. Some substitution of aluminum
(ionic radius 0.51 A) occurs causing an accompanying
charge imbalance. This is the most often substitution of
silicates.
Silicon in magmatic processes
It is found on the Earth's surface in coordination with
oxygen as silica or silicate, SiO2 or SiO44-, respectively,
although it can form octahedrally coordinated rarely, SiO68in a few minerals, notably stishovite (SiO2), or in organic
chelates. In solution, silicon is present as a monomer or
polymer of silicic acid, H4SiO4. Silicon is a major
component of most rock-forming minerals and hence can
reach high concentrations in igneous, metamorphic and
sedimentary rocks. The ability of silicate tetrahedra to
polymerize in this manner is largely responsible for its
importance in rock types which form under a wide range of
physical and chemical conditions.
Silicon in magmatic processes
Silica phase relationships are complex and controlled
mainly by the degree of condensation of silicate monomers
and the extent of substitution of aluminum for silicon. The
classical Bowen reaction series has two branches
describing the order in which the major silicate minerals
crystallize from magma as the temperature of the melt
decreases.
The Si-concentration of magmas show enrichment from
ultrabasic to acidic range (from olivine – pyroxene –
feldspar – to quartz).
Silica polymorphs
There are several polymorphs of silica of which the
dominant form is low or -quartz at Earth's surface
conditions. Evidence of the occurrence of high or -quartz
can be found as -quartz pseudomorphs. Cristobalite is
commonly intergrown with K-feldspar in devitrification
textures. Cristobalite and tridymite are observed rarely in
cavities of siliceous volcanic rocks. Cristobalite and
cristobalite-tridymite intergrowths known as opal-CT are
observed in young cherts, volcanic tuffs, and marine rocks
as metastable diagenetic phases between opal-A,
amorphous silica with varying amounts of water, and the
stable phase, -quartz. Coesite and stishovite are
observed in high pressure environments such as
impact craters.
Silica polymorphs
The diagenesis of silica in marine rocks is of considerable
interest because of the high organic carbon contents, and
hence petroleum-generating potential, of many highly
siliceous sediments, and because of the effect of silica
phase on rock properties which then affect the ability of the
rock to transmit or store petroleum. Planktonic marine
diatoms and radiolaria extract silicic acid from seawater to
build their frustules or tests. The product is highly porous
amorphous silica with a variable but high water content (up
to 30%) and high surface area. This amorphous silica,
opal-A, undergoes diagenetic reactions usually through an
intermediate phase, opal-CT, to the thermodynamically
stable phase, quartz.
Silicon in weathering and sedimentary
processes
Silicate minerals formed upon the cooling of magma or
during metamorphic processes react in three ways upon
exposure to water and oxygen at Earth surface conditions.
Minerals in which all or most of the resulting elements are
soluble dissolve congruently. Olivine and pyroxene are
examples of this type of silicate mineral. Minerals in which
at least some of the resulting constituents are only slightly
soluble weather incongruently. Feldspars are examples of
this type of silicate mineral. In particular, minerals
containing aluminum tend to weather incongruently
producing clay minerals, the precise type of which depends
upon the climate and parent material.
Silicon in weathering and sedimentary
processes
The highly resistant minerals have very low aqueous
solubilities, they are unreactive. Quartz is an example of
this type of mineral and it is found in soil profiles after other
minerals have altered or dissolved. In fact, quartz is so
highly resistant to both chemical weathering and
mechanical weathering that it is the main constituent of the
highly evolved sedimentary rocks, sandstones.
Dissolved silica concentrations in surface waters show
different values (river water: 6-16 ppm, seewater in surface:
under 1 ppm, in subsurface: 4-6 ppm). This range in
concentration reflects the reactivity of the phases involved
as well as the effects of other processes such as
adsorption.
Silicon in weathering and sedimentary
processes
Quartz is relatively slow to reach equilibrium while
amorphous silica dissolves and precipitates quickly. Hence
waters are often greatly oversaturated with respect to
quartz and under to nearly saturated with respect to
amorphous silica. The exception are thermal springs in
which quartz-saturated waters rising rapidly through near
surface rocks are emitted on the surface. At this
temperature ( ~ 100°C), the solubilities of quartz and
amorphous silica are nearly the same.
The solubility of silica strongly depends of pH, it easily
dissolved in high pH. If the pH values decrease, the silica
precipitate again.