Torshab Mine Stone

KEYWORDS
Actinolite, Amphibole, Asbestiform, asbestos,
Chrysotile, EPA, FESEM, Fibers, Libby, Mineral, Mon-
tana, Richterite, TEM, Tremolite, Winchite

ABSTRACT
This study involved the development and appli-
cation of an analysis protocol using transmission elec-
tron microscopy (TEM) and field emission scanning
electron microscopy (FESEM) for the particle-by-par-
ticle characterization (i.e., chemistry, crystallography,
and morphology) of mixed mineral dust. This proto-
col was developed to aid in the differentiation between
asbestos and non-asbestos amphibole particles that
occur as accessory minerals around the former ver-
miculite mine located near Libby, Montana. Using
microscopical methods to distinguish between asbes-
tos and non-asbestos minerals is critical in regulatory
settings. A standard TEM asbestos protocol was
supplemented with FESEM by relocating and imaging
each potential asbestos particle identified by TEM.
Features key to distinguishing between asbestos and
non-asbestos amphibole particles, such as overall par-
ticle shape and surface roughness, are readily appar-
ent in the FESEM images, but cannot be observed in a
standard TEM image alone. FESEM imaging also dem-
onstrates that amphibole cleavage fragments and
single crystals have dimensions and morphological
features very different than true asbestos. The FESEM
results have helped the geological and microscopy
communities better understand the morphology of the
minerals found in and around Libby and show that
Complementary TEM and FESEM Characterization of
Amphibole Particles in Mixed Mineral Dust from Libby,
Montana, U.S.A.
B. R. Strohmeier, K. L. Bunker, K. E. Harris, R. Hoch, and R. J. Lee
RJ Lee Group, Inc.*
* 350 Hochberg Road, Monroeville, PA, 15146, USA
MICROSCOPE Vol 55:4 173-188 (2007)
FESEM is a valuable complementary tool to TEM for
characterizing the morphology and surface character-
istics of particles suspected to be asbestos.

INTRODUCTION
“Asbestos” is a commercial term applied to a group
of naturally occurring silicate minerals that occur in
the asbestiform variety. The term “asbestiform” is
used to describe an unusual crystallization habit (the
actual shape assumed by a crystal or aggregate of crys-
tals) of certain minerals that form as thin, hair-like
fibers. Asbestiform fibers share several properties,
including the occurrence of bundles in which the indi-
vidual fibers can be easily separated, high tensile
strengths, high aspect ratios (i.e., length:width from
20:1 to greater than 1000:1), high flexibility, and a mac-
roscopic resemblance to organic fibers [1,2].
Asbestiform describes a special type of fibrosity – not
all fibrous minerals are asbestiform. The six minerals
specifically regulated as asbestos by the U.S. Federal
government are[3,4,5]: chrysotile (fibrous serpentine)
and five varieties of amphibole fibers: crocidolite
(riebeckite asbestos), amosite (cummingtonite-
grunerite asbestos), anthophyllite asbestos, tremolite
asbestos, and actinolite asbestos (see Table 1). In addi-
tion to the six regulated asbestos minerals, 388 miner-
als (including 92 silicate and aluminosilicate species)
are known to occur, at least occasionally, in fibrous
form, some of which are asbestiform [6]. The vast
majority of amphibole minerals occur as massive
rocks. Amphibole minerals generally have prismatic
cleavage planes parallel to the c-axis which intersect
at an angle of approximately 56° [1]. Amphibole frag

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