CRYPTOCRYSTALLINE MAGNESITE IN WEATHERING CRUST
OF ULTRABASIC ROCKS
Yu. A. Afonin
and I. G. Maryasev
Translated from Novye Ogneupory, No. 10, pp. 11 – 17, October, 2006.
Original article submitted July 24, 2006.
Specifics of the morphology, qualitative composition, and spatial extent of cryptocrystalline (pelitomorphic)
magnesite deposits and manifestations confined in the weathering crust of ultrabasic rocks are described. It is
noted that despite not very big resources, the magnesite material extracted from these deposits can be used for
products with high technological properties that are in great demand. In this context, in prospecting minerals
associated with ultrabasic rocks (chromites, nickel silicate), it seems expedient to pay special attention to the
possibility of discovering industrial-scale deposits of magnesite in the weathering crust of these rocks, espe
cially in vein rocks.
According to the industrial-genetic classification ,
cryptocrystalline magnesite belongs to the residual super-
gene (infiltration) type. It is formed under chemical weather-
ing of ultrabasic rocks due to the effect of vadose carbonated
waters on the lower strata of the weathering crust, which is
compiled by serpentinites over dunites and periditites. Mor-
phologically, the deposits of magnetite constitute lenses, ve-
neer bodies, pockets, stockworks, and veins. Their bedding
depth, as a rule, is not more than 50 m from the surface but
occasionally reaches 100 m or more.
Magnesite in these deposits is pelitomorphic (cryptocrys
talline) and nodular. Its color is usually white, greenish-
white, or yellowish. The mineral composition of the ore mass
is simple: magnesite, dolomite, chalcedony, quartz, calcite,
serpentine, and sepiolite. A typical feature is the presence of
free silica, which is formed in chemical decomposition of
magnesium silicates. Indications of boron-bearing minerals
are sometimes registered (with B
content up to 0.1%).
Deposits of cryptocrystalline magnesite are known in the
Russian Federation (Orenburg Region, Republic of Bashkor
tostan), Kazakhstan, Mongolia, Greece, Serbia, Turkey, Po
land, India, Saudi Arabia, Guatemala, and Cuba. Magnesite
resources in individual deposits are usually not large (in total
not more than 10 million tons, even considering contiguous
deposits). These amounts are incomparable with crystalline
magnetite deposits , which supply most of the world’s
production of magnesian refractories. At the same time,
powders from the specified materials sinter well. The prod-
ucts obtained in sintering these powders have high density
(up to 3.5 g/ton), a small pore volume (3 – 5%), increased
basicity (the ratio of CaO to SiO
), and a well-decrystallized
periclase phase (the sizes of periclase single crystals are
within the limits of 70 – 200 ìm). That is why powders
based on Balkan cryptocrystalline magnesites, which have
been worked for many decades in Greece and Serbia, are al-
ways in demand in European countries.
Below, we give brief characteristics of some typical de
posits and manifestations of cryptocrystalline magnetite.
The Khalilovskii deposit is located near the Khalilovo
railway station in the Orenburg Region (Russia). Magnesite
in this deposit has the form of diversely directed veinlets and
veins of thickness from 0.05 to 0.50 m, as well as pockets in
the southwest part of the serpentinized ultrabasic rock mass.
In some places magnesite is siliceous. The external appear
ance of this magnesite is shown in Fig. 1a. The total reserves
of the deposit before the beginning of extraction was esti
mated as 4.1 million tons. It was worked in the 1920s –
1930s by the Orenburgmagnezit company using opencast
mining and manual ore sorting. The average yield of lump
magnesite was ~12%.
The weathering crust of the Millionnoe chromite de
posit in the Kempirsaiskii ultrabasic rock mass (Aktyu
binsk Region, Kazakhstan). Cryptocrystalline (amorphous)
magnesite is confined in the zone of leached serpentinites
(saturation 10 – 12%) and has the form of veinlets of thick
ness 0.05 – 0.20 m of diverse orientation and, less frequently,
pockets. Manifestations of magnesite are registered up to
Refractories and Industrial Ceramics Vol. 47, No. 5, 2006
1083-4877/06/4705-0269 © 2006 Springer Science+Business Media, Inc.
Magnezit Works JSC, Russia.
Magnezit Group JSC, Russia.