Balangeroite

Balangeroite
Balangeroite
General
Category	Inosilicate
Formula (repeating unit)	(Mg,Fe,Fe,Mn)42Si16O54(OH)40
Strunz classification	9.DH.35
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P2/n
Unit cell	a = 19.4 Å, b = 9.65 Å, c = 19.4 Å; β = 91.1°; Z = 2
Identification
Color	Brown
Crystal habit	Fibrous
Cleavage	Very good in two directions
Tenacity	Brittle
Luster	Vitreous to greasy
Streak	Brownish white
Diaphaneity	Subtranslucent to opaque; transparent in thin section
Specific gravity	2.96 - 3.10
Optical properties	Biaxial -
Refractive index	nα = 1.680 nγ = 1.680
Birefringence	δ = 0.000
Pleochroism	Dark brown and yellow brown parallel and perpendicular to [001]
References	[1][2][3]

Balangeroite is found in one of the most important chrysotile mines in Europe, the Balangero Serpentinite. Hence, it is usually mistaken as an asbestiform in an assemblage of other mineral phases like chrysotile, magnetite and Fe-Ni alloys. However, Balangeroite does not lead to serious health problems caused by asbestos fibers.

Introduction

Balangeroite is classified as an Inosilicates with 4-periodic single chains, Si₄O_12. It is a completely separate mineral from any true asbestos. It is economically important for providing building material, especially for thermal insulation purposes, fireproofing etc.^[4] Recent publications by Turci^[5] have drawn some conclusions that balangeroite was not asbestos, had poor ecopersistence and biopersistence. This study also pointed out that it was the obvious chrysotile exposures there, not balangeroite, that caused the incidence of mesotheliomas.

Composition

The chemical formula for balangeroite is (Mg, Fe²⁺, Fe³⁺, Mn²⁺)₄₂Si₁₆O₅₄(OH)₄₀^[6] and it has been calculated as shown in the diagram below by Compagnoni as follows:

Wet chemical, X-ray fluorescence and electron microprobe analyses were used to deduce the composition of balangeroite.^[6] The common intergrowth with chrysotile proved to be valuable in providing better chemical resolution as portrayed in table 1. The results varied due to submicroscopic intergrowths or zoning. From the wet chemical analysis, there was 9.5% average weight loss after calcination at 1000 °C, due to the presence of water.^[6] This was calculated as the difference from 100% of microprobe results, with the assumption that large quantities of material usually contain some impurities, and the possible oxidation of Fe²⁺ under heating.^[6] A ratio of Fe²⁺/Fe³⁺ = 2.12 was obtained and on the basis of the known volume and density, the empirical formula for the unit cell was derived^[6] (Mg _25.70 Fe²⁺_7.69 Fe³⁺_3.63 Mn²⁺_1.65 Al_0.17 Ca_0.07 Cr_0.01 Ti_0.01) total= 38.93 Si_15.38O_53.66(OH)_35.92.

Structure

Balangeroite is based on an octahedral build that consists of channels that are filled by chains of silicate tetrahedra grouped in three and 4 rows running along the fiber axis.^[5] Balangeroite is isostructural to gageite.^[6]

In contrast to chrysotile, however, balangeroite has more metal ions than silicon ions and might be in some cases seen as complex iron oxide containing some type of silicate structure in its framework.^[5] The surrounding fluid takes in a large number of the cations which are octahedrally coordinated, which unlike amphiboles, may be easily removed.^[5] As a consequence, the Mg and Fe are released forcing the silicate structure to become loosely bound and therefore pass into solution.^[5] Further tests have been conducted on Balangeroite's ecopersistence and it showed fairly low eco-persistence at neutral pH.^[5] Further studies were conducted by imitating weathering in an experiment to predict if weathered fibers retain the toxic potential present in freshly extracted fibers.^[7] The tests proved that balangeroite showed removal of Mg and Si which shows a continuous structural severance which extends far beyond the surface.^[4]

Physical properties

Balangeroite can develop as loose fibers or compact when in large volumes which can be prismatic.^[6] Antigorite flakes are included in relict prismatic balangeroite, while transmission electron microscopy observation shows that fibrous balangeroite is partially replaced by chrysotile.^[8] The fibers run a couple of centimeters in the [001].

Geologic occurrences

The piemonte zone, remnant of the Piemontese Ocean from the Late Jurassic, is home to the majority of the serpentines of the Western Alps. The Balangero mine is located in the Lanzu Ultramafic Massif which is in the inner part of the piemonte zone.^[8] The Lanzu Ultramafic Massif is believed to have been involved in the subduction processes that were affiliated with the closure of the Piemontese Ocean in the late Jurassic.^[8] The earliest generation of metamorphic veins and in particular type 1 Vein that constitute relict prismatic balangeroite (often includes antigorite flakes) were formed during prograde high pressure metamorphism.^[8] Fibrous balangeroite is limited to the serpentine-infested rim of the northern Lanzu Ultramafic Massif, with its abundance in the inactive Balangero asbestos mine, where it was discovered.^[8]

Balangeroite was named after the location in which it was discovered.^[6] Mine workers at the Balangero mine had first discovered it and named it, based on its overall color and fibrous nature of other minerals present in the mine, xylotile or metaxite.^[6] This new mineral, balangeroite, was tested and found to be completely different from xylotile and metaxite in composition as well as optical properties.^[6] Balangeroite was already discovered and a somewhat pure specimen was in the Turin University Mineralogy institute's museum since 1925, inventory no. 14873, labeled as "fibrous serpentine (asbestos)- San Vittore, Balangero".^[6]

References

External links

• Entry on Mineraldata.com