1. what is manganese (mn) and what is it used for? what mineral forms does it occur in? how has the...
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What is manganese (Mn) and what is it used for?
What mineral forms does it occur in?
How has the amount of Mn changed through time?
How has the chemistry of Mn ores changed through time?
isotopes major elements trace elements rare-earth elements
Outline: What Will We See?
Molango
What do these changes tell us about the history of the Earth?
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Mn deposits come in 6 flavors, increasing in diversity with time
>2400 Ma Archean style 2400 – 1700 Paleoproterozoic style 1700 – 900 No deposits 900 – 550 Neoproterozoic style 550 – 150 Paleozoic-Jurassic style < 150 Post-Jurassic style
Each reflects changes in ocean-atmosphere chemistry or the radiation of new life forms
What Will We Learn?
Giant stromatolites
Chocolate-brown dolomites
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What is Manganese?
Element 25, manganese was isolated by Johan Gottlieb Gahn, a Swedish chemist, in 1774 by heating the mineral pyrolusite (MnO2) in the presence of charcoal.
Nearly 90% of all of the Mn produced each year is used in the production of steel to make it easier to form and to increase its strength and resistance to impact.
Manganese is also used to give glass an amethyst color and is responsible for the color of amethyst gemstones.
First, it is not magnesium!
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The fortunes of Mn follow that of Fe
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How Does it Behave?Mn is nearly identical to Fe in its geochemistry.
Manganese Iron Mn/Fe
Formula wt 54.94 55.85
Oxidation states @ 1 atm, 398K 4+, 3+, 2+ 3+, 2+
Coordination number w O 6 6
Ionic radius, Å 0.83 0.78
Crustal abundance, ppm 950 56 000 0.017
Concentration in seawater, µg/L 0.072 0.25 0.288
in Black Sea surface water 0.56 0.29 1.93
in Black Sea deep water 333 4.14 80.4
Because of its much lower crustal abundance, it is submerged by Fe except in special environments
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How Does it Behave?Mn is nearly identical to Fe in its geochemistry.
Manganese Iron Mn/Fe
Formula wt 54.94 55.85
Oxidation states @ 1 atm, 398K 4+, 3+, 2+ 3+, 2+
Coordination number w O 6 6
Ionic radius, Å 0.83 0.78
Crustal abundance, ppm 950 56 000 0.017
Concentration in seawater, µg/L 0.072 0.25 0.288
in Black Sea surface water 0.56 0.29 1.93
in Black Sea deep water 333 4.14 80.4
Because of its much lower crustal abundance, it is submerged by Fe except in special environments
Dissolves
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Unless S2- present
Sulfidic
How Does it Behave?Mn is nearly identical to Fe in its geochemistry.
Manganese Iron Mn/Fe
Formula wt 54.94 55.85
Oxidation states @ 1 atm, 398K 4+, 3+, 2+ 3+, 2+
Coordination number w O 6 6
Ionic radius, Å 0.83 0.78
Crustal abundance, ppm 950 56 000 0.017
Concentration in seawater, µg/L 0.072 0.25 0.288
in Black Sea surface water 0.56 0.29 1.93
in Black Sea deep water 333 4.14 80.4
Because of its much lower crustal abundance, it is submerged by Fe except in special environments
Precipitates
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Eh-pH Behavior: Oxides
Manganese in Solution
Oxides and hydroxides only: manganese should be mobile under reducing conditions but will precipitate as the oxidation state of the solution rises
Manganese in solution
Manganese in solids
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Eh-pH Behavior: + Carbonate
Adding carbonate reduces size of the area of solubility
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Eh-pH Behavior: + Sulfide
Upwards flux
Fe immobilized in deep water
Where Does It Come From Today?
Most Mn is ultimately volcanic, but is hosted by sediments because it is carried so much farther than Fe
90 %10 %2+ 2+
Mn4+ oxidesFeS2
2+
O2
Fe3+ oxides
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Why Is Mn Carried So Much Farther?
Fe follows relatively simple homogeneous kinetics (note strong pH dependence)
-d[Fe2+]/dt = k[Fe2+][O2][OH-]2
Mn reaction is heterogeneous, requiring a solid catalyst
-d[Mn2+]/dt = k0[Mn2+] + k1[Mn2+][MnO2][O2][OH]2
Mn is oxidized (hence precipitated) much more slowly than Fe
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How Is It Precipitated?
Mn bacteria,production wells GMA
Fe bacteria, production wells GMA
Most Mn is precipitated bacterially; Fe can be, but most is abiotic
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What are its Common Mineral Forms?
The dominant Mn mineral, percentage of land-based deposits
Rhodochrosite MnCO3 32.0
Braunite (Mn2O3)3(Mn,Fe)SiO3 24.3
Cryptomelane KMn8O16 8.7
Manganite MnOOH 7.8
Pyrolusite MnO2 4.9
Hausmannite Mn3O4 2.9
Psilomelane MnO2 3.9
amorphous oxides 1.9
Kutnahorite CaMn(CO3)2 1.9
Mn-calcite (Mn,Ca)CO3 1.9
Todorokite (Mn,Ca,Mg)Mn3O7_H2O 1.9
Others (oxides) 7.8
Mn2+
4*Mn2+ + 3*Mn4+
Mn nodules15
A Conflict Between Fact and Theory
Rhodochrosite and braunite, both of which are Mn2+ minerals, make up over half of the Mn phases in land-based ore deposits.
But we just said Mn is transported as Mn2+ and deposited as Mn4+
How then is the Mn in land-based deposits transported and precipitated?
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Primary Mn Deposits
Molango – Jurassic, Mexico
Most Mn ore is black, dominated by low-oxidation state minerals
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Primary Fe Deposits
Hamersley IF – Paleoproterozoic, Australia
Most Fe ore is red, dominated by high-oxidation state minerals
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Time as a Controlling Variable: Mn Deposit Tonnage Through
Time
Fan Delian
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How Are Deposits Distributed by Age?
Testable Hypothesis 1
Observation: Fe ores show a concentration in the Paleoproterozoic related to the advent of atmospheric oxygen and a much smaller concentration in the Neoproterozoic related to glaciations.
Hypothesis: Mn should follow the same pattern.
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How Are Deposits Distributed by Age?
Two Major Episodes of Iron Deposition
Peak 1 - Paleoproterozoic
Peak 2 - Neoproterozoic
Peak 3 - Oligocene
IFNo IF
IF
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Six Major Episodes of Mn Deposition
I IIV VI
IVIII
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Archean Mn Deposition
I II V VIIVIII
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Archean Mn Deposits
Relatively small – 3.7 million tonnes avg, Fe-rich – 14 % Fe2O3
Often produced by artisanal mining (Equivalent Fe ores = Algoma type) 24
Paleoproterozoic Mn Deposition
I II V VIIVIII
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Paleoproterozoic Mn Deposits
Very large – 99 mt, low-Fe – 8.1 %; Dominated by Mamatwan, which has 55 % of world’s reserves (Equivalent Fe ores = Lake Superior type)
Mamatwan – Paleoproterozoic, South Africa
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Mesoproterozoic Mn Deposition
I II V VIIVIII
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Mesoproterozoic Mn Deposits
Small – 3.9 mt, high-Fe – 11 %; Mostly volcanic except Wafangzi (No Fe ores from this period)
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Neoproterozoic Mn Deposition
I II V VIIVIII
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Neoproterozoic Mn Deposits
Widespread but small – 12.1 mt, low-Fe – 5.7 %; Dominated by Urucum –Mutun with 6.3 % of world reserves (Equivalent Fe ores = Rapitan type)
Tanganshan – Neoproterozoic, China
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Paleozoic Mn Deposition
I II V VIIVIII
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Cambrian-Jurassic Mn Deposits
Small – 7.8 mt, high-Fe – 12 %, high P – 0.8 %; Larger deposits are Karadzhal, 3.8 % of world reserves, and Molango, 1.3 % (Equivalent Fe ores = Clinton oolitic type)
Taojiang – Ordovician, China
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Post-Jurassic Mn Deposition
I II V VIIVIII
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Post-Jurassic Mn Deposits
Large – 20.4 mt, low-Fe – 4.4 %; Dominated by Oligocene deposits around the Black Sea with 16.2 % of world reserves (No significant Fe ores of this age)
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How Are Deposits Distributed by Age?
Conclusion –
Fe and Mn ores show some commonalities of distribution, but Mn is
(1) more evenly distributed in time
(2) less evenly distributed in space, being concentrated in a few giant deposits in each time period
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Mn Deposit Chemistry Through Time
Andrey Becker36
Mn Deposit Chemistry: Isotopes
Testable Hypothesis – None. This question was first addressed by Pat Okita for Molango as part of the exploration of the deposit
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Mn Deposit Chemistry: Isotopes in Space
Implication: Rhodochrosite incorporates significant organic-derived C. 38
Observation: Mn grade is closely associated with 13C depleted carbon
Mn Deposit Chemistry: Isotopes in Space
New hypothesis: Mn carbonate forms during early diagenesis by reaction of Mn oxide with organic carbon
Test: Redox buffering at Mn4+/Mn2+ couples should prevent pyrite formation at the sediment-water interface
S isotopes at Molango indicate late-stage FeS2
formation
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Mn Deposit Chemistry: Isotopes in Time
Observation: The spread of C isotopic values increases with time
Working hypothesis: there is an increase in diversity of environments with time 40
Mn Deposit Chemistry: Major Elements
Testable Hypotheses?
Nothing in the literature – lets see what the rocks themselves have to say
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Mn Deposit Chemistry: Major Elements - Fe
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Mn Deposit Chemistry: Major Elements - Fe
I high Fe
V low Fe
IV lower Fe
II lower Fe
III high Fe
VI very low Fe
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Mn Deposit Chemistry: Major Elements - Fe
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Observation: The Mn/Fe ratio in volcanic deposits increases dramatically at ~ the J-K boundary
Working hypothesis: ???????
Mn Deposit Chemistry: Major Elements - Si
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Mn Deposit Chemistry: Major Elements - Si
I
VIVII
III
VI
Diatoms
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Mn Deposit Chemistry: Major Elements - P
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Mn Deposit Chemistry: Major Elements - P
I
V
IVII
III
VIPhosphatized hard parts (Cloudina)
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Mn Deposit Genesis: Calvert’s Upwelling Model
Early Cambrian phosphogenesis episode/ radiation of shelly faunas – both related to increased diversity of environments and water chemistries
Mn2+
Mn4+
O2
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A Conflict Between Fact and Theory
Most Mn ores are not associated with high P
Most are associated with light C isotopes, which the OMZ model does not explain
How then is this Mn transported and precipitated?
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Mn Deposit Chemistry: Trace Elements -- Ba
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Mn Deposit Chemistry: Trace Elements -- Ba
I
V
IVII
III
VI
Observation: Ba is high in all but Mesoproterozoic deposits; spread increases in 2 cycles, highest in post-Jurassic 52
Force & Cannon’s Euxinic Basin Model of Mn Mineralization
Mn is soluble in anoxic-sulfidic bottom water; precipitates as oxide at oxic/anoxic interface
light δ13C
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Euxinic Basin Model of Ba Incorporation
Ba is also soluble in anoxic-sulfidic bottom water; precipitates as sulfate at oxic/anoxic interface when seawater has SO4
Ba++
BaSO4
SO4=
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Mn Deposit Chemistry: Conflict Resolution
Euxinic basin deposits are more numerous than OMZ deposits; all giant deposits are in euxinic category 55
Mn Deposit Chemistry: New Hypothesis
I
V
IVII
III
VI
No euxinic basins
Ba in manganese deposits and Mo in black shales indicate monotonous Mesoproterozoic with no euxinic basins 56
Mn Deposit Chemistry: Trace Elements –V
Diversity increases
Mes
opro
toer
ozoi
c ga
p
V is typical of other trace elements – Cu, Mo, Pb, Zn – in showing a trend to a widening spread of values with time
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Mn Deposit Chemistry: What Have We Learned So Far?
Fe, Si, P, V show a trend to a widening spread of values with time
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This pattern suggest a widening spread of environments with time, likely coupled with increased diversity of organisms
But we haven’t learned anything about the oxidation state of the atmosphere and ocean
Mn Deposit Chemistry: REE
REE in chemical sediments are often invoked as indicators of oxidation state:
Ce3+ Ce4+
Deviation = Ce/Ce*
Eu3+ Eu2+
Deviation = Eu/Eu*
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Mn Deposit Chemistry: REE - Ce
Ce3+ Ce4+ catalyzed by Mn oxides, produces insoluble Ce and a positive Ce anomaly on Mn nodules; residual seawater has a negative Ce/Ce*
Hypothesis = no oxygen in bottom water in Precambrian
Test = no negative Ce/Ce* in seawater no Mn nodules in deep sea
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Ce Systematics Today
Under oxic conditions, Ce is oxidized and scavenged on surface of Mn nodules in deep sea
Ce/Ce* = 1.65
Ce/Ce* = 1.03
Ce
Ce/Ce* = 0.17
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Mn Deposit Chemistry: REE – Ce
Diversity in Mn ores increases to both higher and lower oxidation state
Low O2
oceans – no deep-sea Mn nodules
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Mn Deposit Chemistry: REE - Eu
Eu3+ Eu2+ (only at high T) produces vent fluids with positive Eu anomalies, but Fe oxides remove all REE close to vents
Hypothesis = No oxygen in bottom water of restricted basin w. vents
Test = no vent-derived Fe oxides no REE scavenging positive Eu/Eu* in local seawater
or in global seawater for global anoxia
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Eu Systematics Today
Under oxic conditions, REE are scavenged at the vent by Fe oxides; seawater Eu/Eu* reflects continental sources
Eu/Eu* = 7 - 11
Eu/Eu* = 0.61
Eu
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Fe Deposit Chemistry: REE – Eu
Low O2
oceans; vent-signature Eu in BIF
O2 in surface water; REE vent-signature lost in Fe deposits
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diversification
Mn Deposit Chemistry: REE – Eu
Greater variety of Mn environments with time
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Barrie Bolton
What Do We Learn About Earth History?
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What Have We Learned about the History of Mn?
I pre –2400 Ma. No O2 in atm. No oxidative
weathering on continents; anoxic but non-sulfidic
bottom waters owing to absence of SO4
Small, Fe-rich manganese deposits,
perhaps in localized higher oxidation state
environments
II 2400 – 1700 Ma. Low O2 in atm. Anoxic deep
water, intermittently sulfidic, but low SO4 surface
water
Giant Kalahari Mn deposit, great era of
Banded Iron Formations
III 1700-900 Ma. Low O2 in atm. Minor Mo-enriched
black shales; euxinic basins absent?
Virtual disappearance of sediment-hosted
Mn
IV 900-550 Ma. Increased O2 in atm. Reappearance
of euxinic basins
Large number of glacial-associated Mn
deposits
V
VI
550 – 0 Ma. High O2. Continental break-up;
radiation of metazoans, shelly faunas
Abrupt increase in Ba, P, and V in sed-
hosted Mn ores
150-0 Ma. High O2. Continental break-up;
radiation of diatoms
Abrupt increase in Si, Cu, Mo, Pb, V, Zn in
volc-hosted Mn ores68
What Have We Learned in the Broader Sense?
Geology is a science Geology is a science with an interplay with an interplay between hypothesis-between hypothesis-driven and exploration-driven and exploration-driven inquirydriven inquiry
The diversity of life, of sedimentary environments, The diversity of life, of sedimentary environments, and of their ore deposit products increases and of their ore deposit products increases through timethrough time
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Mn Deposit Workers
Liu Tie-bing
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Mn Deposit Workers
Eric May
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Mn Deposit Workers
Pat Okita
Jessamine72
Mn Deposit Workers
Enjoy the adventure73