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?

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

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!

The fortunes of Mn follow that of Fe

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

Unless S2- present

Sulfidic

Precipitates

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

Eh-pH Behavior: + Carbonate

Adding carbonate reduces size of the area of solubility

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

Fe3+ oxides

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

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

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

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?

Primary Mn Deposits

Molango – Jurassic, Mexico

Most Mn ore is black, dominated by low-oxidation state minerals

Primary Fe Deposits

Hamersley IF – Paleoproterozoic, Australia

Most Fe ore is red, dominated by high-oxidation state minerals

Time as a Controlling Variable: Mn Deposit Tonnage Through

Fan Delian

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.

Two Major Episodes of Iron Deposition

Peak 1 - Paleoproterozoic

Peak 2 - Neoproterozoic

Peak 3 - Oligocene

IFNo IF

Six Major Episodes of Mn Deposition

I IIV VI

Archean Mn Deposition

I II V VIIVIII

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

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

Mesoproterozoic Mn Deposition

I II V VIIVIII

Mesoproterozoic Mn Deposits

Small – 3.9 mt, high-Fe – 11 %; Mostly volcanic except Wafangzi (No Fe ores from this period)

Neoproterozoic Mn Deposition

I II V VIIVIII

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

Paleozoic Mn Deposition

I II V VIIVIII

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

Post-Jurassic Mn Deposition

I II V VIIVIII

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)

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

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

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

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

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

Mn Deposit Chemistry: Major Elements - Fe

Observation: The Mn/Fe ratio in volcanic deposits increases dramatically at ~ the J-K boundary

Working hypothesis: ???????

Mn Deposit Chemistry: Major Elements - Si

Diatoms

Mn Deposit Chemistry: Major Elements - P

VIPhosphatized hard parts (Cloudina)

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

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?

Mn Deposit Chemistry: Trace Elements -- Ba

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

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

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

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

V is typical of other trace elements – Cu, Mo, Pb, Zn – in showing a trend to a widening spread of values with time

Mn Deposit Chemistry: What Have We Learned So Far?

Fe, Si, P, V show a trend to a widening spread of values with time

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*

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

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* = 0.17

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

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

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

Fe Deposit Chemistry: REE – Eu

Low O2

oceans; vent-signature Eu in BIF

O2 in surface water; REE vent-signature lost in Fe deposits

diversification

Mn Deposit Chemistry: REE – Eu

Greater variety of Mn environments with time

Barrie Bolton

What Do We Learn About Earth History?

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

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

IV 900-550 Ma. Increased O2 in atm. Reappearance

of euxinic basins

Large number of glacial-associated Mn

deposits

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

Mn Deposit Workers

Liu Tie-bing

Mn Deposit Workers

Eric May

Mn Deposit Workers

Pat Okita

Jessamine72

Mn Deposit Workers

Enjoy the adventure73

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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