On the occurrence of Mg and Fe rich carb

E

N. Jb. Miner. Abh. (J. Min. Geochem.) 192/1 (2015), 59–71
Published online October 2014; published in print January 2015

Article

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 carbonates 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, hydrotalcite, aragonite, and/or huntite were found. The mineral assemblages formed as the result of precipitation from percolating Mgrich 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

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, including weathered volcanic tuff sequences, coastal sabkhas,
karstic terrains, continental lacustrine environments,
highly alkaline carbonate playas and weathered serpentinized rocks (Faust 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 & FielDing 2003). Ultramafic rocks are considered 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-

1. Introduction
Huntite [Mg3Ca(CO3)4] and hydromagnesite [Mg5(CO3)4
(OH)2.4H2O] are classified as metastable carbonate minerals (Kinsmann 1967, sánchez-Román et al. 2011).The
formation of huntite has been attributed to several mechanisms 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 sabkha (PeRthuisot et al. 1990).
! Precipitation from Mg-bearing pore waters during
early diagenesis (Kinsman 1967), at the expense of dolomite 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

© 2014 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany
DOI: 10.1127/njma/2014/0271

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A. Eslami et al.

guished : i) atmospheric (e.g. o’neil & BaRnes 1971); ii)
decarboxylation of organic rich sediments (e.g. FallicK et
al. 1991); iii) thermal decarbonation of limestones (FallicK et al. 1991); iv) decomposition of organic material in
soil (e.g. PetRov 1967); v) regional metamorphic reactions
above 300 °C (aBu-jaBeR & KimBeRley 1992); vi) vol-

canogenic sources (ilich 1968); vii) deep-seated source

(KReulen 1980); viii) combinations of all these processes.
In the Nain ophiolite mélange, Iran, hydromagnesiterich 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).

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

61

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 km2, extends from NNW to SSE and is surrounded
by Cenozoic sedimentary rocks in the east and Cenozoic
volcanic rocks in the west (Fig. 1b) (DavouDzaDeh 1972).
The main rock units in the Nain area include a Late Cretaceous ophiolitic complex and overlying younger rocks.
From bottom to top, this ophiolitic mélange consists of
a basal metamorphic zone, peridotites (dunite, harzburgite) 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 (DavouDzaDeh 1972). Two strike-slip fault sets

as pure hydromagnesite have found applications as environmentally friendly fire retardants (Rothon 2003, lioDaKis & tsouKala 2010, hollingBeRy & hull 2010, hollingBeRy & hull 2012a).
The aim of this paper is to characterize the nodular hydromagnesite-rich assemblages and the white carbonate
samples in highly sheared ultramafic rocks from the Nain
ophiolite mélange. It will also discuss their economic potential 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 microcontinent (CIM) (Fig. 1a). This belt comprises a set of dis-

Fig. 2. Outcrop view of carbonates and the serpentinite ground. a – Mg- and Fe-rich carbonates forming nodular assemblages on a serpentinite ground (scale: 3 cm); b – magnesite nodules (scale: 15 cm); c – shallow exploration pits in serpentinized ground.

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A. Eslami et al.

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 (samples HNIR05 and HNIR09) which are very hard were collected 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).

have been identified in the Nain region. These developed
during two faulting stages (naDimi & sohRaBi 2008): one

in the early Tertiary (late Cretaceous-Miocene) and other
in the late Tertiary (after late Miocene). naDimi & sohRaBi (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 ophiolitic units. Conjugation of the NNW-SSE and NE-SWtrending fault systems made active smaller blocks with
rotation. Movement of the blocks associated with rotation has caused cutting and rotation 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 (hassaniPaK & ghazi 2000). Recently, uranium-lead zircon
dating revealed that the Nain ophiolite was emplaced
101.2 ± 0.2 Ma (shaFaii moghaDam et al. 2013).

4. Materials and analytical techniques

3. Field observations and sampling

After testing of a series of initial samples of the white
mineral assemblages, fourteen representative samples

were selected for detailed analysis. Samples were mineralogically analysed by X-ray diffraction using a Bruker
5005X-ray diffractometer in combination with the DIFFRACplus software at the National & Kapodistrian University 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

Peridotites in the Nain ophiolite mélange are mostly serpentinized 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

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

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aragonite crystals, the most distinguishable minerals are
hydromagnesite and pyroaurite (Ni(6-x)Mgx)Fe2(CO3)
(OH)16.4H2O. 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 needlelike crystals formed on the surface of hydromagnesite crystals (Fig. 3). Brugnatellite Mg6Fe3+(CO3)(OH)13.4(H2O)
and manasseite Mg6Al2(CO3)(OH)16.4(H2O) occur as disseminated on a hydromagnesite groundmass platy and fibrous very fine-grained crystals respectively.

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 microscopy (SEM-EDS). A JEOL JSM-5600LINK ISIS instrument 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 thermal changes (exotherm or endotherm) associated with
the mass losses. Twelve magnesium carbonate-rich samples (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).

5.2. 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 molecules (Fig. 4a). This suggested that approximately
80 mass% of sample HNIR01 were hydromagnesite
[Mg5(CO3)4(OH)2 · 4H2O]. At approximately 400 °C, the
release of CO2 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 CO2 (hollingBeRy & hull 2010). The remaining 20
mass% was lizardite, which exhibits overlapping endothermic 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 H2O molecules and to the release of one
CO2 molecule in huntite, respectively (Fig. 4b). This suggested that approximately 50 mass% of sample HNIR08
were hydromagnesite [Mg5(CO3)4(OH)2 · 4H2O] and 50
mass% were huntite [Mg3Ca(CO3)4]. Between 400 °C and
630 °C, the first stage of huntite thermal decomposition
occurs with an associated loss of ~24 mass% that is assigned to the release of three CO2 molecules (hollingBeRy & hull 2012a).
An endothermic peak at ~580 °C was observed for
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
[Mg3Ca(CO3)4]. An endothermic peak at ~580 °C was observed 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 magnesite (smyKatz-Kloss 1974). The remaining 90 mass%
consisted of calcite and minor dolomite exhibiting overlapping endothermic peaks. An endothermic peak at

5. Results
5.1. Carbonate mineralogy
Semi-quantitative mineral analyses of bulk samples examined by XRD are given in Table 1. Ten distinct carbonate parageneses were determined in the studied samples:
a) Hydromagnesite-manasseite (HNIR01).
b) Hydromagnesite-pyroaurite-manasseite (HNIR02).
c) Hydromagnesite-pyroaurite (HNIR03-HNIR04HNIR10-HNIR11-HNIR14).
d) Magnesite-calcite-dolomite (HNIR05).
e) Hydromagnesite-pyroaurite-brugnatellite (HNIR06).
f) Hydromagnesite-pyroaurite-aragonite (HNIR07).
g) Hydromagnesite-huntite (HNIR08).
h) Magnesite (HNIR09).
i) Huntite (HNIR12).
j) Hydromagnesite-hydrotalcite (HNIR13).
Based on the XRD analysis, hydromagnesite is the
most dominant Mg-rich carbonate in these assemblages.
SEM-EDS analysis revealed that all carbonate minerals above are microcrystalline. Out of the needle-like

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

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Table 1. Mineralogical composition determined by XRD of selected samples of greenish white to off-white carbonate assemblages,
Nain ophiolite mélange (++for major,+for medium and – for trace components; hydromagnesite = Mg5(CO3)4(OH)2 · 4H2O; huntite =
Mg3Ca(CO3)4; pyroaurite = Mg6Fe2(CO3)(OH)16 · 4H2O; manasseite = Mg6Al2(CO3)(OH)16 · 4H2O; hydrotalcite = Mg6Al2(CO3)(OH)16 ·
4H2O; brugnatellite = Mg6Fe3+(CO3)(OH)13 · 4H2O).
Sample
HNIR

HydroHuntite Pyroaurite Manasseite Hydromagnesite
talcite

–01

++

–02

++

–

–03

++

+

–04

++

+

–

–

–

–

+

–

+
–

–05
–06

Magnesite Brugnatellite Calcite Dolomite Antigorite Lizardite

+
++

–07

++

–08

++

–

++
+

–

+
–
–

++

–

–09

++

–10

++

–

+

–11

++

–

–

–12

++

–13

++

–14

++

–
+

–

–

–

~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 (smyKatzKloss 1974).
The DTA curves of the samples HNIR03, -06, -10,
-11, -13, -14 show overlapping endothermic peaks of hydromagnesite and other minor phases such as pyroaurite,
antigorite, lizardite, manasseite, mineral phases that have
also been detected by XRD (Table 1).

6. Discussion
6.1. Serpentinite weathering and origin of the Mgcarbonates
Besides in lacustrine sedimentary basins, hydromagnesite
has been reported in serpentinized ultramafic rocks affected by strong tectonic activity, in thrust and shear zones
and areas where serpentine has been weathered to an
earthy groundmass (mumPton & tomPson 1966, BRiDeau
et al. 2007). Commonly, brucite is the early-formed secondary mineral at the expense of serpentine, turning to
metastable hydromagnesite and/or pyroaurite by the reaction of brucite with CO2-bearing groundwater at shallow depths and wet surfaces (mumPton & tomPson 1966,
hostetleR et al. 1966). Laboratory measurements have
shown that hydromagnesite can precipitate directly from
Mg- and Na-HCO3-rich solutions (alDeRman 1965, BethKe 1996). Huntite and magnesite have been reported as
direct precipitates, or diagenetic products of an aragonite
and/or hydromagnesite precursor (alDeRman 1965, Kinsmann 1967, stamataKis 1995). The precipitation of the
magnesium carbonates depends on four main parameters:
a) alkalinity; b) temperature; c) the partial pressure of CO2
and d) the amount of Ca2+ and Mg2+ ions in solution.
For pyroaurite formation the concentration of Fe3+
ions is also important. If the Ca/Mg ratio is low, arago-

5.3. Whiteness and screening tests
The samples HNIR01, HNIR02 and HNIR03 were analyzed to determine their whiteness, as all commercial hydromagnesite-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 mechanically the nodules from the greenish groundmass, preliminary tests were performed by using laboratory sieves of
200 mesh (Laboratory of Sedimentology, Geology Department 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 estimated at about 50 %.

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Paragenesis

Possible reactions
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

Hydromagnesite-mannaseite
and
hydromagnesite-hydrotalcite

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
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

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hydromagnesite-pyroauritemanasseite

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

5MgCO3+CaCO3+4H2O+Mg3Si2O5(OH)4+5/2O2 → Mg3Ca(OH)2(CO3)4+Mg5(CO3)4(OH)2 · 4H2O +2SiO2
5 magnesite+calcite +4H2O+serpentine +5/2O2 → huntite+hydromagnesite +2SiO2

huntite

3MgCO3+CaCO3+H2O +1/2O2 → Mg3Ca(OH)2(CO3)4
3 magnesite+calcite+H2O +1/2O2 → huntite

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

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67

written (see Table 2). However, the absence of thermodynamic and stability data for these minerals makes it difficult 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 hydromagnesite 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 direct precipitation from Mg-rich solutions or by interaction
of Mg-rich waters with preexisting carbonate minerals
(Dollase & ReeDeR 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 comparison to the hydrotalcite mineral associated with hydromagnesite, huntite does not occur as nodular assemblages but
as lumps and coatings in fissures of the weathered serpentinite immediately below the soil profile.

nite precipitates, followed by huntite and finally hydromagnesite (stamataKis 1995). If the concentration of Mg
and the partial pressure of CO2 are both high, magnesite
forms, whereas low partial pressure of CO2 favours the
formation of hydromagnesite for a constant Mg/Ca ratio
(stamataKis 1995). We assume several phases of evolution, weathering and tectonics of the ultramafic rocks.
Hence, at periods with high extensional stresses, pCO2
was high and hence magnesite was formed. At a later
stage, when the pCO2 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 formation of the Mg-rich carbonates. Most of the nodular occurrences are hosted in a wet earthy serpentinite, indicating 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 Mg2+ leaching from its ultramafic host rocks. The experimental work of eDelstein et
al. (1982) and relatively high solubility of serpentine minerals in pure water at normal pressures and temperatures
(lesKo 1972) supports this interpretation. The carbonate
may originate from a variety of sources. For determining
all possible sources of the CO2, stable isotope analyses by
the authors are in progress.

6.3. Thermogravimetry
Both hydromagnesite and huntite decompose endothermically. This endothermic decomposition and release of
inert gases H2O and CO2 gives them their fire retardant
properties (hollingBeRy & hull 2012b). During thermal
decomposition hydromagnesite may undergo an exothermic structural rearrangement (hollingBeRy & hull
2012a, sawaDa et al. 1978 a and b, sawaDa et al. 1979a, b,
c). The rearrangement of the crystal structure results in a
thermally more stable form, which releases CO2 at a higher temperature than the initial arrangement. Hydromagnesite thermally decomposes by first losing H2O, followed
by release of OH and finally release of CO2. The thermal
decomposition of hydromagnesite has been proposed to
occur via the following reactions (e.g. haliKia et al. 1998,
inglethoRPe & stamataKis 2003):

6.2. Mineral paragenesis
Hyrdotalcite-group minerals are layered double hydroxides or anion clays, which are natural lamellar mixed
hydroxides with interlayer spaces containing exchangeable anions (mills et al. 2012). On the basis of an X-ray
investigation, aminoF & BRoome (1930) recognized two
polytypes of hydrotalcite: pyroaurite and manasseite as
rhombohedral polytypes and hydrotalcite as the hexagonal polytype. In the Nain ophiolite mélange these minerals 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

Mg5(CO3)4(OH)2 · 4H2O → Mg5(CO3)4(OH)2 +4H2O
Mg5(CO3)4(OH)2 → 2MgCO3+3MgO +2CO2+H2O
2MgCO3 → 2MgO +2CO2

(< 250 °C)
(250 – 350 °C)
(> 350 – 550 °C).

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Fig. 5. Thermal decomposition of selected Nain hydromagnesite-rich samples in comparison with commercial Turkish hydromagnesite
(hollingBeRy & hull 2012a)

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 carbonate ions. These three decompositions would result in
losses of 15.45 mass%, 3.86 mass%, and 37.77 mass% respectively 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 H2O molecules account
for a loss of 15.40 mass%. The loss of a further H2O molecule from the decomposition of OH – and loss of four CO2
molecules would account for a further 41.50 mass% (3.85
mass% and 37.65 mass% respectively) loss (hollingBeRy
& hull 2012a).
It has also been shown that in the third stage of decomposition, the release of four CO2 molecules is strongly affected by the partial pressure of CO2 in the atmosphere and
the rate of heating (hollingBeRy & hull 2012a, sawaDa
et al. 1978 a and b, sawaDa et al. 1979a, b, c). An exothermic 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 CO2.
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 almost 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, hollingeRy & hull 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 CO2 molecules) and the second stage occurs between about 630 °C and 750 °C with a further loss
of ~12.5 mass% (further release of one CO2 molecule,
hollingBeRy & hull 2012a).
The three decomposition stages detected in the
reevesite-pyroaurite series (FRost & eRicKson 2004) are:

(Ni(6-x)Mgx) Fe2(CO3)(OH)16 · 4H2O → (Ni(6-x)Mgx) Fe2(CO3)(OH)16+4H2O
(Ni(6-x)Mgx) Fe2(CO3)(OH)16 →(Ni(6-x)Mgx)O5Fe2O3+CO2+8H2O
(Ni(6-x)Mgx)O5Fe2O3 →(Ni(6-x)Mgx)Fe2O3+O2

(150 –165 °C)
(245 – 340 °C)
(341– 455 °C)

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

The first step represents the dehydration step with the
consequent loss of H2O. This step occurs generally in the
150 °C to 165 °C temperature range. Since most thermoplastic polymers are processed within this temperature
range or higher, the presence of this mineral will be a limiting factor in their use as fire retardants in thermoplastic
polymers. The second step involves the simultaneous loss
of CO2 and H2O. It is assumed that oxides are formed. The
third mass loss step involves oxygen loss and the reduction 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 occurs in small amounts, as shown by the overlapping weak
endothermic peaks of pyroaurite with those of hydromagnesite and of other minor phases such as antigorite, lizardite and manasseite.

69

be processed below this temperature. One of the largest
areas of application for mineral fire retardants is in polyolefin 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 retardants 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 University of Athens, Greece, National Technical University
of Athens, Greece and LKAB Minerals Ltd. is gratefully acknowledged. The authors are especially obliged to
Dr. M.A. macKizaDeh from Geology Department of the
University of Isfahan for his fruitful and logistical support during fieldwork. Thanks are also expressed to Prof.
jose-PeDRo calvo (Complutense University, Madrid) and
an anonymous reviewer for their fruitful commends and
suggestions.

7. Conclusions
The white mineral assemblages occurring in the Nain
ophiolite mélange are varied and consist mainly of hydromagnesite 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 observed that the Nain Mg-rich carbonates show an initial
mass loss between about 200 °C and 400 °C which is
associated with loss of H2O. Between about 400 °C and
600 °C, the mass loss associated with the loss of CO2 varies. When heated at high heating rates, the mixed Mgrich carbonates from Nain are decomposed in three steps,
similar to that shown by Turkish sedimentary pure hydromagnesite:

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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.
luKe hollingBeRy, LKAB Minerals, Raynesway, Derby, DE21 7BE, England.

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