Balangeroite

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Balangeroite
General
Category Inosilicate
Formula
(repeating unit)
(Mg,Fe,Fe,Mn)42Si16O54(OH)40
Strunz classification 09.DH.35
Crystal symmetry Monoclinic prismatic
H-M symbol: 2m
space group: P2n
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
Crystal system Monoclinic
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. It is considered an asbestiform in an assemblage of other mineral phases like chrysotile, magnetite and Fe-Ni alloys. In addition to its fibrous occurrence, balangeroite's association with chrysotile raises concerns about its potential toxicity when its fibers are inhaled.

Introduction[edit]

Balangeroite is classified as belonging to one of the two asbestiform silicate groups, the serpentine group. It is intergrown with the dominant member of the serpentinite, chrysotile,[4] often associated with tremolite (a contaminant of chrysotile), which is classified as part of the amphibole group, the other asbestiform silicate. Massive serpentines are economically important for providing building material. The fibrous nature of chrysotile is particularly valuable for thermal insulation purposes, fireproofing etc.[5] Tremolite contaminated chrysotile shows that the toxicity of the asbestos is due to the presence of tremolite and not the entire mass of the chrysotile.[6] Recent publications by Turci [6] have drawn some conclusions that balangeroite contaminated chrysotile does have some areas of concern and can be attributed to the overall toxicity of the airborne fibers in the Balangero mine. Therefore, it cannot be compared to tremolite or croidolite in level of pathogenicity as the two have been proven by autopsies and biopsies to be present in the bodies of the people exposed to their fibers.

Composition[edit]

The chemical formula for balangeroite is (Mg, Fe2+, Fe3+, Mn2+)42Si16O54(OH)40 [4] and it has been calculated as shown in the diagram below by Compagnoni as follows:

Table 1a. Chemical analysis of balangeroite [4]
SiO2 28.37
TiO2 0.03
Al2O3 0.27
Fe2O3 8.89
Cr2O3 0.03
FeO 16.95
MnO 3.59
MgO 31.81
CaO 0.13
H2O 9.93
Total 100.00

Wet chemical, X-ray fluorescence and electron microprobe analyses were used to deduce the composition of balangeroite.[4] 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.[4] 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.[4] 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 [4] (Mg 25.70 Fe2+7.69 Fe3+3.63 Mn2+1.65 Al0.17 Ca0.07 Cr0.01 Ti0.01) total= 38.93 Si15.38O53.66(OH)35.92.

Structure[edit]

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.[6] Balangeroite is isostructural to gageite.[4]

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.[6] The surrounding fluid takes in a large number of the cations which are octahedrally coordinated, which unlike amphiboles, may be easily removed.[6] As a consequence, the Mg and Fe are released forcing the silicate structure to become loosely bound and therefore pass into solution.[6] Further tests have been conducted on Balangeroite's ecopersistence and it showed fairly low eco-persistence at neutral pH.[6] Further studies were conducted by imitating weathering in an experiment to predict if weathered fibers retain the toxic potential present in freshly extracted fibers.[5] The tests proved that balangeroite showed removal of Mg and Si which shows a continuous structural severance which extends far beyond the surface.[5]

Physical properties[edit]

Balangeroite can develop as loose fibers or compact when in large volumes which can be prismatic.[4] Antigorite flakes are included in relict prismatic balangeroite, while transmission electron microscopy observation shows that fibrous balangeroite is partially replaced by chrysotile.[7] The fibers run a couple of centimeters in the [001].

Geologic occurrences[edit]

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.[7] 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.[7] 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.[7] 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.[7]

Balangeroite was named after the location in which it was discovered.[4] 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.[4] This new mineral, balangeroite, was tested and found to be completely different from xylotile and metaxite in composition as well as optical properties.[4] 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".[4]

References[edit]

  1. ^ Mindat.org
  2. ^ Webmineral.com
  3. ^ Handbook of Mineralogy
  4. ^ a b c d e f g h i j k l m Compagnoni, Roberto; Ferraris G and Fiora L (1983). "Balangeroite, a new fibrous silicate related to Gageite from Balangero, Italy". American Mineralogist 68: 214–29. 
  5. ^ a b c Klein, Cornelius; Barbara Duttrow (2008). "19". In Ryan Flahive. The Manual of Mineral Science (23rd ed.). John Wiley & Sons, Inc. pp. 515–520. ISBN 978-0-471-72157-4. 
  6. ^ a b c d e f g Turci, Francesco; Tomatis M and Compagnoni R, et al (2009). "The Role of Associated Mineral Fibers in Chrysotile Asbestos Health effects: The Case of Balangeroite". Annals of Occupational Hygiene 53: 491–497. doi:10.1093/annhyg/mep028. 
  7. ^ a b c d e Groppo, C; Tomatis M; Turci F; et al (2005). "The Potential Toxicity of Non-Regulated Asbestiform Minerals: Balangeroite from the Western Alps Part 1: Identification and Characterization.". Journal of Toxicology and Environmental Health- Part A- Current Issues 68: 1–19. doi:10.1080/15287390590523867. 

External links[edit]