Download Chapter 22 Hoofstuk 22

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Carbonates and other minerals with
triangular anion groups.
Sedimentary origins
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Triangular anion group: (XO3)n
Carbonates: CO32-
 Simple salts of carbonic acid H2CO3: calcite CaCO3; dolomite
CaMg(CO3)2
 With additional anions: malachite Cu2(CO3)(OH)2
 Mixed compounds: sulfate-carbonates; phosphatecarbonates, etc.
Nitrates: NO3 Borates: BO33- (some borates also tetrahedra BO45-)
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Study info for NB carbonates, nitrates and borates
from Table 22.1
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Forms by bonding of carbonic acid
(H2CO3) to:
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Na, Ca, Mg, Fe, Mn, Zn, Sr, Ba
 low ionization potential
 large cations
 For Na+, H2O is incorporated into structure
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Bi2+, Cu2+and rare earth elements (REEs) only
when OH-, F- or O2- present to weaken the
CO32- complex
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Rhombohedral (trigonal), orthorhombic or
monoclinic
Calcite and dolomite:
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CO32- assymetrical; occupies parallel planes in
calcite and dolomite in same orientation
 Therefore highly anisotropic – high birefringence:
large difference between properties parallel and
perpendicular to c
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Structure analogy with NaCl
 Replace Na+ with Ca2+; Cl- with CO32-; compress
structure along body diagonal to give rhombohedral
symmetry
 Calcite: cubic close-packing of CO32- with Ca+ in
interstices
 Aragonite: hexagonal close-packing of CO32- with
Ca+ in interstices, but with distorted structure
 Dolomite: same structure as calcite but layers of Ca2+
are alternated with layers of Mg2+
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Carbonates display polymorphism and isomorphism
 Isomorphy:
 Small cations: isomorphy within calcite structure (calcite,
magnesite, siderite, rhodochrosite, smithsonite)
 Large cations: isomorphy within aragonite structure
(aragonite, witherite, strontianite, cerussite)
Polymorphy of CaCO3: High T; low P: calcite; Low
T; high P: aragonite
Sheet and chain like structures common in carbonates
Polymerization of triangular groups common in borates:
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BO33- isolated
Combined into B2O54- or B2O42- pairs, B4O84- rings or B2O54chains
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Majority has rhombohedral symmetry
Growth form:
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Commonly as cleavage rhombohedron
Many other forms
Symmetry decreases with increasing
complexness of composition
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Colorless to white
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Reaction with HCl
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Mn: pink (rhodochrosite)
Cu: green (malachite) or blue (azurite)
Fe: yellow (ankerite, siderite) or brown
2HCl + Ca(Pb,Zn,Mg…)CO3  H2O + CO2 +
Ca(Pb,Zn,Mg…)Cl2
Some only when concentrated acid or mineral heated
High birefringence & high order interference colors
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A result of strong anisotropy
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Associations:

Al-Mg-(Na)
association
 Magnesite Group
 Hydrotalcite Group

Na(K)-Ca-Ba
association
 Calcite-Aragonite
Group
 Dolomite-Barytocalcite
Group
 Bastnaesite Group

Zn-Cu-Pb(U)
association
 Smithsonite Group
 Malachite Group
 Cerussite Group
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Structural type
Calcite group
 Dolomite group
 Aragonite group
 Soda carbonate
minerals
 Other carbonate
minerals
 Nitrates
 Borates
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Often with minor Mg, Fe, Mn
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Very diverse habit
Limestone: chemical or biological origin
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Limited at room T
Usually high in Mg when precipitate from seawater
Forms marble when metamorphosed
Skarns and hydrothermal deposits
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Endmember of magnesite-siderite isomorphic
series
Gray-white solid masses of granular texture
Occur in:
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Dolostones affected by hydrothermal solutions –
recrystallisation
Veins formed during weathering of serpentinites
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Brown crystals, highly vitreous luster
Granular aggregates in hydrothermal veins
Sedimentary rocks: hydrothermal alteration
Chemical origin
Fe ore
Rusty color (white when nonoxidized); high
density
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Pale pink color (but also white, grey, greengrey)
Hydrothermal deposits
Sedimentary Mn deposits
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Limited solid solution, with Zn replaced by:
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Occurs as:
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Fe, Mn, Ca, Pb, Mg, Cd, Co
Colloidal, botryoidal, earthy masses OR
White, green, brown crystals
Oxidation product of sphalerite (ZnS) ores
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Double salt of Ca and Mg
Calcite structure with alternating Mg and Ca
layers
Solid solution with calcite at high T
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Used as geotermometer when coexisting
Solid solution with ankerite: CaFe(CO3)2
Hydrothermal deposits and sedimentary rocks
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In sedimentary rocks: usually result of secondary
diagenetic transforming process
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Double salt of Ca and Fe
Calcite structure with alternating Fe and Ca
layers – therefore very similar to dolomite –
solid solution
Also in this group:
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Kutnahorite - CaMn(CO3)2