On the occurrence of Mg and Fe rich carb

1. Introduction

Huntite [Mg

3

Ca(CO

3

)

4

] and hydromagnesite [Mg

5

(CO

3

)

4

(OH)

2

.4H

2

O] are classified as metastable carbonate

min-erals (

Kinsmann

_1967,

sánchez-Román

_{et al. 2011).The}

formation of huntite has been attributed to several

mecha-nisms and environments some of which are listed below:

!

Precipitation from percolating waters moving through

magnesium-rich rocks such as magnesite, dolomite

and/or hydromagnesite deposits (e.g.

Faust

_1953,

sKinneR

1958,

tRailKill

1965,

zachmann

1989).

!

Bacterial activities during the initial formation of

sab-kha (

PeRthuisot

_{et al. 1990).}

!

Precipitation from Mg-bearing pore waters during

early diagenesis (

Kinsman

_{1967), at the expense of}

do-lomite which becomes unstable when the Mg/Ca ratio

is higher than required for dolomitization (e.g.

iRion

_&

mülleR

_1968,

mülleR

_{et al. 1972).}

!

Near-surface weathering of serpentinite and highly

serpentinized rocks (e.g.

DamoDaRan

&

somaseKaR

1975,

nemec

1981,

stangeR

&

neal

1994,

BashiR

et al. 2009,

eslamizaDeh

_{et al. 2014,}

losos

_{et al.}

2013).

Although huntite is a rare carbonate mineral, it has

been found in a wide range of geological settings,

includ-ing weathered volcanic tuff sequences, coastal sabkhas,

karstic terrains, continental lacustrine environments,

highly alkaline carbonate playas and weathered

serpenti-nized rocks (

F

aust

1953,

cole

&

lanchucKi

1975,

calvo

et al. 1995,

zeDeF

_{et al. 2000,}

BashiR

_{et al. 2009).}

Similarly, hydromagnesite and magnesite occur in a

variety of geological settings and environments. Several

interactive mechanisms such as supergene, hypogene,

and combined supergene-hypogene processes have been

proposed to explain the origin of magnesite in ultramafic

settings (

zachmann

1989,

stamataKis

1995,

Russell

et al.

1999,

FRanK

&

F

ielDing

2003). Ultramafic rocks are

con-sidered to be the source of the Mg for the fluid (

o’neil

&

BaRnes

_1971,

zeDeF

_{et al. 2000,}

miRnejaD

_{et al. 2008).}

In these settings, several carbonate sources can be

distin-On the occurrence of Mg- and Fe-rich carbonate mineral

assemblages hosted in the Nain ophiolite mélange, Central

Iran and their industrial potential

Alireza Eslami, Michael G. Stamatakis, Maria Perraki, Charalampos Vasilatos and

Luke Hollingbery

With 5 figures and 2 tables

Abstract: In the Nain ophiolite mélange, central Iran, off-white mineral assemblages occur as nodular magnesium rich carbon-ates and thin veinlets disseminated within an earthy serpentinite groundmass. They are related to tectonically disturbed, strongly weathered zones of the ultramafic rocks. Combined XRD, SEM and TG/DTA analysis revealed that the mineralogy of the Mg-rich carbonate is varied. Ten distinct paragenetic assemblages containing hydromagnesite, pyroaurite, manasseite, brugnatellite, hydro-talcite, aragonite, and/or huntite were found. The mineral assemblages formed as the result of precipitation from percolating Mg-rich meteoric waters through brecciated serpentinites. The source of Mg in excess in the groundwater is attributed to the hydrolysis of Mg-rich minerals in the predominant serpentinized ultramafic rocks. Selected hydromagnesite-rich samples were tested as fire retardants. Even though hydromagnesite is the predominant mineral phase, the economic importance of the mineral assemblages in total is limited mainly because of the insufficient whiteness and the presence of Fe-rich minerals that cause undesirable thermal reactions.

Key words: nodular Mg-rich carbonates, serpentinite, hydromagnesite, huntite, pyroaurite, fire retardants, economic importance, Nain ophiolite mélange, Iran

Published online October 2014; published in print January 2015

60

A. Eslami et al.

guished : i) atmospheric (e.g.

o’

neil

&

B

aRnes

1971); ii)

decarboxylation of organic rich sediments (e.g.

F

allicK

et

al. 1991); iii) thermal decarbonation of limestones (

F

al

-licK

et al. 1991); iv) decomposition of organic material in

soil (e.g.

P

etRov

1967); v) regional metamorphic reactions

above 300 °C (

a

Bu

-

j

aBeR

&

K

imBeRley

1992); vi)

vol-canogenic sources (

i

lich

1968); vii) deep-seated source

(

K

Reulen

1980); viii) combinations of all these processes.

In the Nain ophiolite mélange, Iran,

hydromagnesite-rich mineral assemblages in close relation with brecciated

zones within serpentinites have been reported recently.

Natural mixtures of huntite and hydromagnesite, as well

Fig. 1. a – Distribution of different ophiolite complexes in Iran; b – Simplified geological map of north of Nain town (modified after

DavouDzaDeh 1972).

F-0271_njma_192_1_0059_0071_Eslami_0271.indd 60 11.11.2014 15:01:50

as pure hydromagnesite have found applications as

envi-ronmentally friendly fire retardants (

R

othon

2003,

l

ioDa

-Kis

&

t

souKala

2010,

h

ollingBeRy

&

h

ull

2010,

h

ol

-lingBeRy

&

h

ull

2012a).

The aim of this paper is to characterize the nodular

hy-dromagnesite-rich assemblages and the white carbonate

samples in highly sheared ultramafic rocks from the Nain

ophiolite mélange. It will also discuss their economic

po-tential as fire retardants.

2. Geological setting

Ophiolites in Iran represent the remnants of the Tethyan

ophiolite belt in the Anatolian segment of the Alpine –

Himalayan orogene. The Nain-Dehshir-Baft ophiolitic

belt marks the boundaries of the Central Iranian

micro-continent (CIM) (Fig. 1a). This belt comprises a set of

dis-membered ultramafic, plutonic, volcanic and sedimentary

mélange complexes that crop out along the Nain-Dehshir

fault (Fig. 1a). The northern most part of this ophiolitic

belt is known as the Nain ophiolite mélange, which

is one of the highly dismembered oceanic lithosphere

fragments formed throughout the Mesozoic. It covers

~600 km

2

_{, extends from NNW to SSE and is surrounded}

by Cenozoic sedimentary rocks in the east and Cenozoic

volcanic rocks in the west (Fig. 1b) (

D

avouDzaDeh

1972).

The main rock units in the Nain area include a Late

Cre-taceous ophiolitic complex and overlying younger rocks.

From bottom to top, this ophiolitic mélange consists of

a basal metamorphic zone, peridotites (dunite,

harzbur-gite) and serpentinized peridotite, layered, isotropic and

pegmatite gabbros, sheeted dykes, pillowed to massive

basalts, pelagic limestone and radiolarite. The ophiolite

sequence is overlain by Turonian – Maastrichtian pelagic

limestones (

D

avouDzaDeh

1972). Two strike-slip fault sets

62

A. Eslami et al.

have been identified in the Nain region. These developed

during two faulting stages (

n

aDimi

&

s

ohRaBi

2008): one

in the early Tertiary (late Cretaceous-Miocene) and other

in the late Tertiary (after late Miocene).

n

aDimi

&

s

ohRa

-Bi

(2008), reported strike-slip movements in this area

changed the motion of older faults from reverse or thrust

into oblique-slip or strike-slip. Due to strong deformation,

ophiolitic rocks especially serpentinites are highly folded

and sheared within thrust sheets of the study area. These

tectonic structures have facilitated alteration of the

ophi-olitic units. Conjugation of the NNW-SSE and

NE-SW-trending fault systems made active smaller blocks with

rotation. Movement of the blocks associated with

rota-tion has caused cutting and rotarota-tion of this ophiolite as

well as younger Quaternary sediments; it has also caused

uplift in this region. The Late Albian age (

∼

100 Ma) has

been reported for the genesis of the Nain ophiolite (

h

as

-saniPaK

&

g

hazi

2000). Recently, uranium-lead zircon

dating revealed that the Nain ophiolite was emplaced

101.2 ± 0.2 Ma (

s

haFaii

m

oghaDam

et al. 2013).

3. Field observations and sampling

Peridotites in the Nain ophiolite mélange are mostly

ser-pentinized and occur as massive serpentinite, sheared

and fractured serpentinite, and/or serpentinite veins.

The sheared and fractured serpentinite is the commonest

in the Nain ophiolite mélange. It resulted from slight to

strong deformation of massive serpentinite. In general,

serpentinites are mostly weathered into greenish white to

off-white assemblages of Mg-and Fe-rich carbonates. In

some cases tiny spherical aggregates are distributed on

the surface and in the fractures of these weathered rocks

(Fig. 2a). These nodular Mg-rich carbonates (samples

HNIR01, HNIR02, HNIR03, HNIR04, HNIR06, HNIR07,

HNIR10, HNIR11, IHNIR13 and HNIR14) were collected

in four outcrops of the Nain ophiolite mélange (Fig. 1b).

a)

Northeast of the Separo village (53° 01′ 57″ E,

33° 07′ 07″ N).

b)

Southwest of Soheil-Pakuh village (53° 01′ 32″ E,

33° 09′ 42″ N).

c)

North of the Khugachow village (53° 03′ 33″ E,

33° 02′ 59″ N).

d)

East of Sarar (53° 043′ 29″ E, 33° 01′ 36″ N).

Fractured cauliflower-shaped aggregates ranging in

diameter from 0.5 to 1 cm or pseudo-oolitic masses

(sam-ples HNIR05 and HNIR09) which are very hard were

col-lected from the weathered crust that covers serpentinites

(Fig. 2b). Two samples (sample IRNCH08 and IRNCH12)

were collected from shallow exploration pits excavated

in the serpentinite grounds in the Nain ophiolite mélange

(Fig. 2c).

4. Materials and analytical techniques

After testing of a series of initial samples of the white

mineral assemblages, fourteen representative samples

were selected for detailed analysis. Samples were

min-eralogically analysed by X-ray diffraction using a Bruker

5005X-ray diffractometer in combination with the

DIF-FRACplus software at the National & Kapodistrian

Uni-versity of Athens (UoA), Greece. The diffractometer

was operated using Cu Kα-radiation at 40 kV and 40 mA

scanned with 0.020° step size and 1.0 s step time. The raw

Fig. 3. SEM of blade crystals of hydromagnesite, partially covered by fine-grained pyroauriteaggregates (pyraourite aggregates are shown by arrows).

F-0271_njma_192_1_0059_0071_Eslami_0271.indd 62 11.11.2014 15:01:51

files (XRD diagrams) were evaluated for mineralogical

identifications using the EVA 10.0 program of the Bruker

DIFFRACplus-D5005 software. Samples were examined

micro-chemically and visually by scanning electron

mi-croscopy (SEM-EDS). A JEOL JSM-5600LINK ISIS

in-strument at the University of Athens was used. This was

combined with a microanalyzer energy dispersive system

OXFORD LINK ISIS 300, with software ZAF correction

quantitative analysis. The system was operating at 20 kV,

0.5 nA and 50 second time of analysis.

To study the thermal stability, thermogravimetry (TG)

was used to measure the magntitude of the mass losses

against temperature and differential thermal analysis

(DTA) was used to measure the magnitude of the

ther-mal changes (exotherm or endotherm) associated with

the mass losses. Twelve magnesium carbonate-rich

sam-ples (HNIR01, HNIR02, HNIR03, HNIR05, HNIR06,

HNIR08, HNIR09, HNIR10, HNIR11, HNIR12, HNIR13

and HNIR14) were characterised using a Mettler Toledo

TGA/SDTA 851 instrument at the School of Mining and

Metallurgical Engineering of The National Technical

University of Athens (NTUA), Greece. The temperature

was raised at a constant rate (10 °C/min) from ambient to

1200 °C. Sample sizes of approximately 40 – 80 mg were

used and the mass constantly monitored alongside the

thermal heat flow.

The whiteness of selected samples was measured by a

LANGE instrument, using barium sulfate as a reference

material for 100 % whiteness (UoA).

5. Results

5.1. Carbonate mineralogy

Semi-quantitative mineral analyses of bulk samples

ex-amined by XRD are given in Table 1. Ten distinct

carbon-ate parageneses were determined in the studied samples:

a) Hydromagnesite-manasseite (HNIR01).

most dominant Mg-rich carbonate in these assemblages.

SEM-EDS analysis revealed that all carbonate

min-erals above are microcrystalline. Out of the needle-like

aragonite crystals, the most distinguishable minerals are

hydromagnesite and pyroaurite (Ni(6-x)Mgx)Fe

2

(CO

3

)

(OH)

16

.4H

2O. Hydromagnesite occurs as platy 5 – 20 µm

large laths and/or blades arranged in parallel (Fig. 3). In

contrast, pyroaurite occur as fine-grained platy and

needle-like crystals formed on the surface of hydromagnesite

crys-tals (Fig. 3). Brugnatellite Mg

6

Fe

3+

(CO

3

)(OH)

13.

4(H

2

O)

and manasseite Mg

6

Al

2

(CO

3

)(OH)

16.

4(H

2

O) occur as

dis-seminated on a hydromagnesite groundmass platy and

fi-brous very fine-grained crystals respectively.

5.2. TG/DTA

An endothermic peak at ~330 °C was observed for the

DTA analysis of sample HNIR01, which corresponds

to a mass loss of ~12 % or the mass of four water

mol-ecules (Fig. 4a). This suggested that approximately

80 mass% of sample HNIR01 were hydromagnesite

[Mg

5

(CO

3

)

4

(OH)

2

· 4H

2

O]. At approximately 400 °C, the

release of CO

2

initiates. At around 520 °C, a mass loss

occurs which is associated with an exothermic reaction,

followed by an endothermic reaction. It has been shown

that this exothermic reaction is due to the formation of

crystalline magnesium carbonate after the initial loss of

some CO

2

(

hollingBeRy

&

hull

2010). The remaining 20

mass% was lizardite, which exhibits overlapping

endo-thermic peaks. Sample HNIR02 shows a similar thermal

behavior.

Endothermic peaks at ~330 °C and at ~730 °C were

observed for the DTA analysis of sample HNIR08, which

correspond to a loss of ~7.5 mass% or the mass of the four

hydromagnesite H

2

O molecules and to the release of one

CO

2

molecule in huntite, respectively (Fig. 4b). This

sug-gested that approximately 50 mass% of sample HNIR08

were hydromagnesite [Mg

5

(CO

3

)

4

(OH)

2

· 4H

2

O] and 50

the DTA analysis of sample HNIR12, which corresponds

to a loss of ~37 mass% (Fig. 4c). This suggested that

more than 98 mass% of Sample HNIR12 were huntite

[Mg

3

Ca(CO

3

)

4

]. An endothermic peak at ~580 °C was

ob-served for the DTA analysis of sample HNIR05, which

corresponds to a loss of ~6 mass%. This suggested that

approximately 10 mass% of sample HNIR05 were

mag-nesite (

smyKatz-Kloss

1974). The remaining 90 mass%

64

A. Eslami et al.

Fig. 4. TG (black line)/DTG (blue line)/DTA (red line) curves of (a) sample HNIR01 (hydromagnesite), (b) sample HNIR08 (hydromagne-site-huntite) and (c) sample HNIR12 (huntite).

F-0271_njma_192_1_0059_0071_Eslami_0271.indd 64 11.11.2014 15:01:52

~590 °C was observed for the DTA analysis of sample

HNIR09, which corresponds to a loss of 49 mass%. This

suggests that approximately 95 % magnesite (

s

myKatz

-K

loss

1974).

The DTA curves of the samples HNIR03, -06, -10,

-11, -13, -14 show overlapping endothermic peaks of

hy-dromagnesite and other minor phases such as pyroaurite,

antigorite, lizardite, manasseite, mineral phases that have

also been detected by XRD (Table 1).

5.3. Whiteness and screening tests

The samples HNIR01, HNIR02 and HNIR03 were

ana-lyzed to determine their whiteness, as all commercial

hy-dromagnesite-based fire retardants require a brightness of

> 95 %. All three samples have undesirable low whiteness

values, ranging between 85 and 90 %.

In order to identify the possibility to separate

mechani-cally the nodules from the greenish groundmass,

prelimi-nary tests were performed by using laboratory sieves of

200 mesh (Laboratory of Sedimentology, Geology

De-partment of University of Isfahan, Iran). The separation

of the nodules was poor because they broke easily and

mixed with the earthy serpentine phases as they passed

through the sieve. The recovery of the nodules was

esti-mated at about 50 %.

6. Discussion

6.1. Serpentinite weathering and origin of the

Mg-carbonates

Besides in lacustrine sedimentary basins, hydromagnesite

has been reported in serpentinized ultramafic rocks

affect-ed by strong tectonic activity, in thrust and shear zones

and areas where serpentine has been weathered to an

earthy groundmass (

m

umPton

&

t

omPson

1966,

B

RiDeau

et al. 2007). Commonly, brucite is the early-formed

sec-ondary mineral at the expense of serpentine, turning to

metastable hydromagnesite and/or pyroaurite by the

re-action of brucite with CO

2

-bearing groundwater at

shal-low depths and wet surfaces (

m

umPton

&

t

omPson

1966,

h

ostetleR

et al. 1966). Laboratory measurements have

shown that hydromagnesite can precipitate directly from

Mg- and Na-HCO

3

-rich solutions (

a

lDeRman

1965,

B

eth

-Ke

1996). Huntite and magnesite have been reported as

direct precipitates, or diagenetic products of an aragonite

and/or hydromagnesite precursor (

a

lDeRman

1965,

K

ins

-mann

1967,

s

tamataKis

1995). The precipitation of the

magnesium carbonates depends on four main parameters:

a) alkalinity; b) temperature; c) the partial pressure of CO

2

and d) the amount of Ca

2+

_{and Mg}

2+

_{ions in solution.}

For pyroaurite formation the concentration of Fe

3+

66

A.

Eslam

i et al.

Table 2. Suggested reactions for the formation of carbonate minerals in the Nain ophiolite mélange.

Paragenesis Possible reactions

Hydromagnesite-mannaseite and

hydromagnesite-hydrotalcite

a) MgAl2O4+8H2O +9CO2+5Mg3Si2O5(OH)4 → Mg6Al2(CO3)(OH)16 · 4H2O +2Mg5(CO3)4(OH)2 · 4H2O +10SiO2

spinel +8H2O +9CO2+5 serpentine → hydrotalcite or manasseite +2 hydromagnesite +10SiO2

b) 10MgCO3+17H2O+MgAl2O4 → Mg6Al2(CO3)(OH)16 · 4H2O+Mg5(CO3)4(OH)2 · 4H2O +5CO2

10 magnesite +17H2O+spinel→hydrotalcite or manasseite+hydromagnesite +5CO2

c) 4MgCO3 +2Mg3Si2O5(OH)4 +13H2O+MgAl2O4 +CO2→Mg6Al2(CO3)(OH)16 · 4H2O+Mg5(CO3)4(OH)2 · 4H2O +4SiO2

4 magnesite +2 serpentine +13H2O+spinel+CO2 → hydrotalcite or manasseite+hydromagnesite +4SiO2

hydromagnesite-pyroaurite-manasseite

a) 7Mg3Si2O5(OH)4+16H2O +10CO2+Fe2SiO4+1/2O2+MgAl2O4 → Mg6Fe2(CO3)(OH)16 · 4H2O +2Mg5(CO3)4(OH)2 · 4H2O+Mg6Al2(CO3)

(OH)16 · 4H2O +15SiO2

7 serpentine +16H2O +10CO2+fayalite +1/2O2+spinel → pyroaurite +2 hydromagnesite+manasseite +15 SiO2

b) 16MgCO3 +29H2O+MgAl2O4+Fe2SiO4+1/2O2 → Mg6Al2(CO3)(OH)16 · 4H2O+Mg6Fe2(CO3)(OH)16 · 4H2O+Mg5(CO3)4(OH)2 · 4H2O +10CO2 +SiO2

16 magnesite +29H2O+spinel+fayalite +1/2O2 → manasseite+pyroaurite+hydromagnesite +10CO2+SiO2

c) MgCO3+19H2O +5CO2 +5Mg3Si2O5(OH)4+Fe2SiO4+1/2O2+MgAl2O4 → Mg6Al2(CO3)(OH)16 · 4H2O+Mg6Fe2(CO3)

(OH)16.4H2O+Mg5(CO3)4(OH)2 · 4H2O +11SiO2

magnesite +19H2O +5CO2+5serpentine+fayalite +1/2O2+spinel → hydrotalcite or manasseite+pyroaurite+hydromagnesite +11SiO2

hydromagnesite-pyroaurite

a) Fe2SiO4+7Mg3Si2O5(OH)4+5H2O +13CO2+1/2O2 → Mg6Fe2(CO3)(OH)16 · 4H2O +3Mg5(CO3)4(OH)2 · 4H2O +15SiO2

fayalite +7serpentine +5H2O +13CO2+1/2O2 → pyroaurite +3hydromagnesite +15 SiO2

b) 11MgCO3+17H2O +1/2O2+Fe2SiO4 → Mg6Fe2(CO3)(OH)16 · 4H2O+Mg5(CO3)4(OH)2 · 4H2O+SiO2+6CO2

11 magnesite +17H2O +1/2O2+fayalite → pyroaurite+hydromagnesite+SiO2+6CO2

hydromagnesite-huntite 5MgCO_{5 magnesite+calcite +4H}3+CaCO3+4H2O+Mg3Si2O5(OH)4+5/2O2 → Mg3Ca(OH)2(CO3)4+Mg5(CO3)4(OH)2 · 4H2O +2SiO2

2O+serpentine +5/2O2 → huntite+hydromagnesite +2SiO2

huntite 3MgCO_{3 magnesite+calcite+H}3+CaCO3+H2O +1/2O2 → Mg3Ca(OH)2(CO3)4

2O +1/2O2 → huntite

F-0271_njma_192_1_0059_0071_Eslami_0271.indd 66

11.11.2014 15:01:52

nite precipitates, followed by huntite and finally

hydro-magnesite (

s

tamataKis

1995). If the concentration of Mg

and the partial pressure of CO

2

are both high, magnesite

forms, whereas low partial pressure of CO

2

favours the

formation of hydromagnesite for a constant Mg/Ca ratio

(

s

tamataKis

1995). We assume several phases of

evolu-tion, weathering and tectonics of the ultramafic rocks.

Hence, at periods with high extensional stresses,

pCO

2

was high and hence magnesite was formed. At a later

stage, when the pCO

2

was low, hydromagnesite formed.

The area studied is tectonically affected as shown by the

presence of shear zones and detachments. These tectonic

structures facilitated alteration of ultramafic host rocks.

Hence, we assume a similar mechanism for the

forma-tion of the Mg-rich carbonates. Most of the nodular

oc-currences are hosted in a wet earthy serpentinite,

indicat-ing their direct relationship with the groundwater. The

Mg source in excess in the groundwater for the Mg-rich

carbonates is associated with the presence of vast masses

of serpentinized ultrabasic rocks in this area. Hydrolysis

of Mg-rich minerals caused Mg

2+

_{leaching from its}

ultra-mafic host rocks. The experimental work of

e

Delstein

et

al. (1982) and relatively high solubility of serpentine

min-erals in pure water at normal pressures and temperatures

(

l

esKo

1972) supports this interpretation. The carbonate

may originate from a variety of sources. For determining

all possible sources of the CO

2

, stable isotope analyses by

the authors are in progress.

6.2. Mineral paragenesis

Hyrdotalcite-group minerals are layered double

hydrox-ides or anion clays, which are natural lamellar mixed

hydroxides with interlayer spaces containing

exchange-able anions (

m

ills

et al. 2012). On the basis of an X-ray

investigation,

a

minoF

&

B

Roome

(1930) recognized two

polytypes of hydrotalcite: pyroaurite and manasseite as

rhombohedral polytypes and hydrotalcite as the

hexago-nal polytype. In the Nain ophiolite mélange these

min-erals are associated with hydromagnesite and occur as

nodular assemblages and thin, discontinuous veinlets in

heavily sheared serpentinized rocks. For the formation of

nodular assemblages, several balanced reactions can be

written (see Table 2). However, the absence of

thermody-namic and stability data for these minerals makes it

dif-ficult to specify the exact reaction that took place. These

equations assume the involvement of the precursor phases

of serpentine and magnesite, as a source of Mg. Olivine

(fayalite) and spinel are assumed as sources of Fe and Al,

respectively. This is necessary for the formation of

hy-dromagnesite and hydrotalcite-like minerals. The absence

of quartz, talc or other silica-rich phases revealed that Si

must have been transported away from body.

Huntite is a soft and fine-grained chemical precipitate,

which has been re-dispersed in water. It can form at low

temperature at surface and near-surface conditions by

di-rect precipitation from Mg-rich solutions or by interaction

of Mg-rich waters with preexisting carbonate minerals

(

D

ollase

&

R

eeDeR

1986). It has been mined as pigment

in low depth pits, since the ancient times. Commercially,

a natural mixture of huntite and hydromagnesite is sold

under the trade name “UltraCarb” which has an industrial

economic value as an industrial fire retardant. In

compari-son to the hydrotalcite mineral associated with

hydromag-nesite, huntite does not occur as nodular assemblages but

as lumps and coatings in fissures of the weathered

serpen-tinite immediately below the soil profile.

6.3. Thermogravimetry

Both hydromagnesite and huntite decompose

endother-mically. This endothermic decomposition and release of

inert gases H

2

O and CO

2

gives them their fire retardant

properties (

h

ollingBeRy

&

h

ull

2012b). During thermal

decomposition hydromagnesite may undergo an

exo-thermic structural rearrangement (

h

ollingBeRy

&

h

ull

2012a,

s

awaDa

et al. 1978 a and b,

s

awaDa

et al. 1979a, b,

c). The rearrangement of the crystal structure results in a

thermally more stable form, which releases CO

2

at a

high-er temphigh-erature than the initial arrangement.

Hydromagne-site thermally decomposes by first losing H

2

O, followed

by release of OH and finally release of CO

2

. The thermal

decomposition of hydromagnesite has been proposed to

occur via the following reactions (e.g.

h

aliKia

et al. 1998,

i

nglethoRPe

&

s

tamataKis

2003):

Mg

5

(CO

3

)

4

(OH)

2

· 4H

2

O → Mg

5

(CO

3

)

4

(OH)

2

+4H

2

O

(< 250 °C)

Mg

5

(CO

3

)

4

(OH)

2

→ 2MgCO

3

+3MgO +2CO

2

+H

2

O

(250 – 350 °C)

68

A. Eslami et al.

The mass loss associated with the decomposition of

the hydroxide ion probably occurs somewhere between

330 °C and 430 °C but is overshadowed by the larger

mass loss associated with the decomposition of the

car-bonate ions. These three decompositions would result in

losses of 15.45 mass%, 3.86 mass%, and 37.77 mass%

re-spectively and result in a total loss of 57.08 mass%. Using

hydromagnesite’s molecular mass of 467.5 g mol

–1

_{it can}

be calculated that loss of the four H

2

O molecules account

for a loss of 15.40 mass%. The loss of a further H

2

O

mole-cule from the decomposition of OH

–

_{and loss of four CO}

2

molecules would account for a further 41.50 mass% (3.85

mass% and 37.65 mass% respectively) loss (

h

ollingBeRy

&

h

ull

2012a).

It has also been shown that in the third stage of

decom-position, the release of four CO

2

molecules is strongly

af-fected by the partial pressure of CO

2

in the atmosphere and

the rate of heating (

h

ollingBeRy

&

h

ull

2012a,

s

awaDa

et al. 1978 a and b,

s

awaDa

et al. 1979a, b, c). An

exo-thermic rearrangement of the crystal structure to a more

thermally stable form may occur under certain conditions

such as high heating rates or high partial pressures of CO

2

.

As mentioned above (see Fig. 4a), TG/DTA analyses

of the hydromagnesite-rich samples of this area clearly

show three decomposition steps (most evident in the

al-most pure hydromagnesite sample HNIR01), compared

to only two steps of the commercial hydromagnesite of

Turkish origin. However, under high rates of heating the

Nain hydromagnesite shows a decomposition profile very

similar to that of Turkish hydromagnesite of sedimentary

origin (Fig. 5,

h

ollingeRy

& h

ull

2012a).

Huntite decomposes through two stages as clearly

shown in Figure 4 c. The first stage occurs between about

400 °C and 630 °C with an associated loss of ~37 mass%

(release of three CO

2

molecules) and the second stage

oc-curs between about 630 °C and 750 °C with a further loss

of ~12.5 mass% (further release of one CO

2

molecule,

h

ollingBeRy

& h

ull

2012a).

The three decomposition stages detected in the

reevesite-pyroaurite series (

F

Rost

& e

RicKson

2004) are:

Fig. 5. Thermal decomposition of selected Nain hydromagnesite-rich samples in comparison with commercial Turkish hydromagnesite (hollingBeRy & hull2012a)

(Ni

(6-x)

Mg

x

) Fe

2

(CO

3

)(OH)

16

· 4H

2

O → (Ni

(6-x)

Mg

x

) Fe

2

(CO

3

)(OH)

16

+4H

2

O

(150 –165 °C)

(Ni

(6-x)

Mg

x

) Fe

2

(CO

3

)(OH)

16

→(Ni

(6-x)

Mg

x

)O

5

Fe

2

O

3

+CO

2

+8H

2

O

(245 – 340 °C)

(Ni

(6-x)

Mg

x

)O

5

Fe

2

O

3

→(Ni

(6-x)

Mg

x

)Fe

2

O

3

+O

2

(341– 455 °C)

F-0271_njma_192_1_0059_0071_Eslami_0271.indd 68 11.11.2014 15:01:53

The first step represents the dehydration step with the

consequent loss of H

2

O. This step occurs generally in the

150 °C to 165 °C temperature range. Since most

thermo-plastic polymers are processed within this temperature

range or higher, the presence of this mineral will be a

lim-iting factor in their use as fire retardants in thermoplastic

polymers. The second step involves the simultaneous loss

of CO

2

and H

2

O. It is assumed that oxides are formed. The

third mass loss step involves oxygen loss and the

reduc-tion in the moles of oxygen in the mixed metal oxide. The

release of oxygen is obviously a negative effect in terms

of fire retardancy. In the samples studied, pyroaurite

oc-curs in small amounts, as shown by the overlapping weak

endothermic peaks of pyroaurite with those of

hydromag-nesite and of other minor phases such as antigorite,

lizar-dite and manasseite.

7. Conclusions

The white mineral assemblages occurring in the Nain

ophiolite mélange are varied and consist mainly of

hy-dromagnesite and/or huntite as well as minor quantities

of pyroaurite, manasseite, hydrotalcite and brugnatellite.

The Mg-rich carbonates formed by weathering of highly

tectonised ultramafic rocks. From TG/DTA data, we

ob-served that the Nain Mg-rich carbonates show an initial

mass loss between about 200 °C and 400 °C which is

associated with loss of H

2

O. Between about 400 °C and

600 °C, the mass loss associated with the loss of CO

2

var-ies. When heated at high heating rates, the mixed

Mg-rich carbonates from Nain are decomposed in three steps,

similar to that shown by Turkish sedimentary pure

hydro-magnesite:

1. Endothermic loss of water of crystallization.

2. Dehydroxylation and formation of amorphous MgCO

3

.

3. Endothermic MgCO

3

decarbonation.

The presence of minor amounts of hydrotalcite-group

minerals associated with the hydromagnesite may cause

undesirable thermal reactions.

Possible industrial uses

Hydromagnesite and huntite have already been

character-ized as fire retardant additives for polymers (

h

ollingBeRy

& h

ull

2012b). However, in the deposits studied here,

hydromagnesite is accompanied by hydrotalcite-group

minerals as unusual minor Mg- and Fe-rich

carbon-ate phases. The low decomposition temperature of the

reevesite-pyroaurite will limit the use of mixtures

con-taining hydromagnesite and huntite to polymers that can

be processed below this temperature. One of the largest

areas of application for mineral fire retardants is in

poly-olefin and PVC wire and cable sheathing, but these types

of compounds are typically processed at temperatures

higher than the 150 °C decomposition temperature of the

reevesite-pyroaurite minerals. Thus, the presence of these

minerals will be a limiting factor in their use as fire

retard-ants in thermoplastic polymers. In addition, the economic

importance of these mineral assemblages is limited due to

the undesirable color (brightness ranging from 85-90 %)

and the difficulty to mechanically separate the cotton

balls from the earthy serpentinite groundmass.

Acknowledgments

Analytical support of the National & Kapodistrian

Uni-versity of Athens, Greece, National Technical UniUni-versity

of Athens, Greece and LKAB Minerals Ltd. is

grateful-ly acknowledged. The authors are especialgrateful-ly obliged to

Dr. M.A.

m

acKizaDeh

from Geology Department of the

University of Isfahan for his fruitful and logistical

sup-port during fieldwork. Thanks are also expressed to Prof.

j

ose

-P

eDRo

c

alvo

(Complutense University, Madrid) and

an anonymous reviewer for their fruitful commends and

suggestions.

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Manuscript received: April 1, 2014; accepted: August 20, 2014. Responsible editor: G. Franz

Authors’ addresses:

aliReza eslami (corresponding author), Department of Economic Geology, Faculty of Basic Sciences, Tarbiat Modares University, Tehran 14115-175, Iran.

michael g. stamataKis, chaRalamPos vasilatos, Department of Geology and Geoenvironment, Section of Economic Geology &

Geochemistry, National & Kapodistrian University of Athens, Panepistimiopolis, Ano Ilissia, 157 84 Athens, Greece.

maRia PeRRaKi, School of Mining and Metallurgical Engineering, National Technical University of Athens, 9 Heroon Politechniou St., Zografou, 15780, Greece.