Journal of Minerals & Materials Characterization & Engineering, Vol. 3, No.2, pp 99-103, 2004 Printed in the USA. All rights reserved
Investigation Of Hardness Anisotropy In Tourmaline
*M.O. Adeoye and O.O. Adewoye
*Department of Metallurgical and Materials Engineering,
.mi Awolo
. University, Ile-Ife
.. Nigeria
Engineering Materials Development Institute, Akure, Nigeria.
Tourmaline is a ring silicate material with a hexagonal crystal structure.
Tourmaline crystal is made use of as an electronic component, e.g, as a transducer,
mainly because of the anisotropy it exhibits in its properties. Microindentation technique
was employed in the research reported in this paper, using a Knoop indenter, to
investigate the anisotropy in the hardness of the tourmaline crystal on its two major
crystallographic planes: (0001) and {1010}. The material was found to exhibit hardness
anisotropy in conformity with its rotary symmetry elements.
The material was identified and analysed using various x-ray techniques, and was
found to contain some impurities as expected of natural crystals. Tourmaline was found
to have a Si/Al ratio of 1.4. The orientations of the crystal samples were determined by
obtaining and indexing the Laue x-ray back-reflection patterns of the crystal samples.
Keywords: Tourmaline, Hardness anisotropy, Microhardness, Knoop.
1.0 Introduction
1.1 Hardness
Hardness in solid materials is a deformation parameter; it is the resistance of a
material to indentation. Hardness on the micro- scale is known as microhardness and is
obtained by the application of very small indenters under low loads on the surface of a
material e.g. Vickers pyramid tests and Knoop tests. Similar to other crystalline materials,
the variation of microhardness for tourmaline is dependent on orientation in single
crystals. Such crystals are said to exhibit anisotropy in hardness, i.e., hardness varying
from one direction to the other on the same test plane [1,2]. In this work the aim was not
to determine the absolute hardness value of tourmaline but rather to study its variation
with crystallographic direction on the most prominent crystallographic planes of the
single crystal. The Knoop rhombohedral pyramid indenter is used for this study because
there is less ambiguity in aligning it with respect to any given crystallographic directions
due to its long diagonal being seven times longer than the short diagonal. That is the
indenter has a parallelogram base with the short diagonal related to the long diagonal by a
factor of 7.1114. The Knoop hardness HKd under a load L is then given by
100 M.O. Adeoye and O.O. Adewoye Vol. 3, No.2
where d is the length of the long diagonal of the indent.
1.2 Materials
Tourmaline is a group name applied to the natural silicate minerals of the general
formula XY3Z6(BO3)3Si6O18(OH)4 where X can be Na or Ca; Y can be substitutions of
monovalent, divalent, trivalent or quadrivalent cations (Li, Mg, Mn, Fe, Al etc.); and Z
can be occupied by Al, Mg, Cr, Fe3+, Fe2+, etc. [3,4]. F- or O2- can substitute for OH-. The
commonest end-members of tourmaline are: buergerite, dravite, elbaite, liddicoatite,
schorl and uvite. Tourmaline crystal shows parallel grouping (or growth), and has a
rhombohedral (trigonal) crystal structure with a space group of R3m. [5]. Although this
crystal class (3m) is based on the rhombohedral lattice [6], it can also be based on the
hexagonal lattice [7]. In general, crystals in the rhombohedral system can be referred to
as possessing a hexagonal lattice. Detail information on rhombohedral-hexagonal
transformation has been given in the literature (e.g., [8,9,10]). It is categorized as a
crystal symmetry class known as ditrigonal pyramidal [6,7]. It cleaves very poorly on
planes {1120} and {1011}. Tourmaline varies in colour from transparent to opaque as
in black schorl.
Due to its polarity, a charge of electricity may be induced in a tourmaline crystal
when pressure is applied parallel to the c-axis. In other words, not only does the ele ctrical
conductivity of tourmaline crystal change with crystallographic directions, but also with
the pressure applied to the crystal, thus, tourmaline is a piezoelectric material, and
exhibits a linear piezoelectric effect. This property together with high strength, chemical
stability, and high frequenc y, account for its use in many instruments such as transducers
for measuring hydrostatic pressure and in depth-recording devices, and also in
instruments for detecting submarines and underwater obstructions [ 11,12]. Tourmaline is
also used for calibration of piezoelectric manometers and for testing the possible
applicability of a device or procedure for use on materials having complex structures and
compositions. Because of its light-polarizing effect, tourmaline is also used in the
polariscope utilized especially by jewelers. The mineral tourmaline is one of the best
known naturally occurring dichroic materials. The refractive index of tourmaline for the
ordinary beam is no = 1.64, and for the extraordinary beam is ne = 1.62 [13].
From the foregoing, the anisotropic nature of tourmaline crystal is apparent. The
anisotropy in its hardness on the crystallographic planes {0001} i.e., the basal plan e, and
the prismatic planes {1010} are therefore investigated in this work. The species of
tourmaline used here was buergerite - NaFe3Al6(BO3)3Si6O18(O,F)4. The crystals were
euhedral. They were of prismatic habit which is the usual habit of these minerals. The
crystals were prismatic in shape. The prism faces had natural striations parallel to their
length, which are evidences of a particular form: hexagonal prism in these cases.
f (HKd ) = 14230L / d2 (1)
Vol. 3, No.2 Investigation of Hardness Anisotropy in Tourmaline101
2.0 Experimentals
X-ray powder diffra ction technique
was used to identify the crystals on a
Philips X-ray Diffractometer PW 1390
with PW 1050 goniometer incorporating a
graphite monochromator. The radiation
was CuK1 (8 = 1.54060Å). Natural
minerals usually contain wide ranges of
impurities in varying proportions. The
materials used in this work were not
exceptions, being natural minerals. Hence,
the elemental chemical analysis of the
minerals specimens was carried out using
x-ray photoelectron spectroscopy (XPS)
which is sensitive to only the first 10nm of
the surface on a Physical Electronics 5400
system with a MgK (8 = 9.8900Å)
radiation at 15kV and 325W, and energy
dispersive x-ray analysis (EDXA) in a
JOEL 850 scanning electron microscope
(SEM) which can probe deeper into the
Crystal orientations were
determined by carrying out Laue x-ray
back-reflection on the crystals using a
Philips PW 1729 back-reflection camera. Specimens of the tourmaline crystals were cut
parallel to the prismatic (1010) plane and others parallel to the basal (0001) planes and
then polished down to ¼ :m finish.
The anisotropy investigation was carried out using a Knoop indenter on a
Shimadzu Microhardness Tester, T ype M. (See [14] for details). A load o f 100g wa s used
throughout. Tests were performed at room temperature with a dwell time of 15 seconds.
The angle of orientation of the long diagonal of the Knoop indenter was varied on each of
the specimen surfaces (0001) and {1010} between 0° and 180°. For each indentation the
long diagonal was measured. Six indentations were made for each orientation. Thus each
long diagonal value was an average of six values. The Knoop hardness number (KHN)
was then obtained for each orient ation using equation (1) and plotted as shown in Figure
2.0 Discussion
Figure 1: Hardness, KHN, anisotropy in
tourmaline as exhibited on (a) the basal plane
(0001), 0° corresponds to [1010] while 90°
corresponds to [1210], and (b) the prismatic
planes {1010}, 0° corresponds to [1210] while
90° corresponds to [0001]
102 M.O. Adeoye and O.O. Adewoye Vol. 3, No.2
The x-ray analyses identified the crystals to be the ring-silicate mineral
tourmaline speci es buer gerite - NaFe3Al 6(BO3)3Si6O18(O,F)4. They were found to contain
a low level of some impurities such as calcium and phosphorus or their compounds. The
Si/Al ratio was found to be 1.4. [14]
Figure 1 shows the hardness anisotropy obtained from measurements of planes
(0001) and {1010} of tourmaline. In Figure 1a, 0°corresponds to [1010] while
90°corresponds to [1210]. The curve displa ys several peaks (hard dire ctions) each about
50° from the next, and several troughs (soft directions) also about 50° from one another.
The symmetry displayed is found to be consistent with the operation of (0001) 1210ƒ
slip system, in other words, flow on the basal plane of tourmaline is controlled by
(0001)1210ƒ system. The picture for the prismatic plane (1010), Figure 1b, is a “bell
shape” symmetry over a range of 0° to 180° starting with [1210] direction as 0°. Only
one peak is exhibited here at 90° which is the direction [0001]. Thus [0001] represents
the highest hardness direction on this plane with a Knoop hardness (K HN) of about 1540
kg mm-2, and [1210] is the softest direction with about 1000 kg mm-2 KHN. This also
presents a flow consistent with (0001)1210ƒ system. The di fference between the soft est
and the hardest directions is about 500 kgf mm-2 (Knoop hardness).
4.0 Conclusion
The material, tourmaline crystals, were found to exhibit anisotropy in Knoop
hardness in conformity with their rotary symmetry elements of 3 on the basal planes. On
this plane the probable operating slip system controlling the indentation process was
found to be (0001)1120ƒ. On the prismatic planes {1010} the slip system thought to be
in operation during indentation was also (0001)1120ƒ.
The authors wish to thank Professor T.F. Page, the he ad of the Materials Division
and the Ceramics Tribology Research Group at the University of Newcastle Upon Tyne,
England, for allowing the use his laboratory. And Dr. J. J. Weimer of Chemistry
Department/Chemical & Materials Engineering Department, University of Alabama in
Huntsville, Huntsville, Alabama, U.S.A. for his assistance on the work on XPS and
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