s content.
Physical Properties … Additional Reading
Additional Reading:
Physical Properties

About This Document
NYCO has the benefit of many years of experience in the production, handling and marketing of
wollastonite products. The purpose of this document is to share some of this accumulated
knowledge and experience with NYCOs customers, associates, agents and distributors with a
view towards the successful usage and application of NYCOs wollastonite products.
The document provides a basis for understanding and for obtaining more definitive information or
advice. This guide in no way represents a definitive reference for wollastonite. In consideration of
the peculiarities of wollastonite, NYCO strongly recommends that the appropriate professionals
carry out any design of products, processes, installations or modifications. NYCO also
recommends that any test work deemed necessary by those involved in these endeavors be
carried out on the appropriate NYCO products. NYCO can provide contacts that have had past
experience working with wollastonite.
Where specific information is given, this is based on NYCOs experience and as such is offered
as an example only. Situations and conditions in others facilities will be different enough to
warrant that a qualified professional makes specific recommendations. NYCO can provide
assistance in contacting individuals with past experience.
Feel free to contact NYCO for additional information.
*** NYCO believes the information and recommendations herein to be accurate and reliable. However, since any assistance
furnished by NYCO with reference to proper use and disposal of its products is provided without charge, and since use conditions
and disposal are not within its control, NYCO assumes no obligation or liability for such assistance and does not guarantee results
from use of such products or other information herein, no warranty, express or implied is given. Information herein concerning
laws and regulations is based on U.S. federal laws and regulations except where specific reference is made to those of other
jurisdictions. Since use conditions and governmental regulations may differ from one location to another and may change within
time, it is the buyers responsibility to determine whether NYCOs products are appropriate for buyers use, and to assure buyers
workplace and disposal practices are in compliance with laws, regulations, ordinances, and other governmental enactment
applicable in the jurisdiction(s) having authority over the buyers operations. ***

Physical Properties
Specific Gravity
The specific gravity of pure wollastonite (triclinic) can be calculated based on unit cell parameters
to be 2.96. Measured specific gravities are typically fall in the following range: 2.87-3.09. This
variation is due to trace or minor amounts of various impurity ions such as aluminum, iron,
magnesium, manganese, potassium and sodium, which substitute for calcium and distort the
crystal lattice. The specific gravity of commercial wollastonite products is also affected by the
content of impurity minerals such as calcite (s.g. of 2.70-2.95), garnet (s.g. of 3.5-3.8), diopside
(s.g. of 3.2-3.3), etc.


Bulk Density Mixing, compounding, storing and shipping ground materials requires knowledge of their apparent
bulk densities. The bulk density of commercial wollastonite products depends primarily on their
fineness and aspect-ratio however, specific gravity, moisture content and test method can also
play a role. The volume-weight test consists of measuring the volume of a given weight of
material under specified conditions, and then calculating the weight per unit volume. This is done
in the loose condition (aerated) and in the tapped condition (compacted). Typical measured
bulk densities for NYCO products are as follows:
Loose Bulk Density, kg/m
3
(lbs./cu.ft.)

products vary from 220 to 1360 (14 to 85)

Tapped Bulk Density, kg/m
3
(lbs./cu.ft.)

products vary from 420 to 1440 (26 to 90)

In a related test, wollastonite is dispersed in water and the resulting suspension is then allowed to
stand for a fixed period of time in a glass column. The height of the sediment is recorded. This is
commonly referred to as wet volume and gives an indication of particle size, aspect-ratio and
settling characteristics.
Bulk density can be important in applications where the powder performs certain functions such
as in steel casting mold powders, where the powder itself must be thermally insulating.
Color
When pure, the mineral is brilliantly white, but impurities even in trace amounts may color it
cream, grey, pink, brown, or red. This color change is related to the presence of iron and other
coloring ions. Color may be imparted by impurities on the crystal surface (deposited by the
passage of groundwater) or by impurities actually contained in the crystal structure.
The luster is glassy to silky (vitreous to pearly). Luster is important in applications such as
plastics, paints and coatings as it in turn imparts luster to surface finishes.
Brightness
The dry brightness and whiteness of wollastonite are important in determining its suitability for
certain filler and ceramic applications. Brightness is determined by measuring the reflectance of
finely ground powder against a standard that is assigned a brightness of 100. Magnesium oxide
and barium sulphate are the two standards used. G.E. brightness, a term used in North America,
refers to brightness measured with a General Electric reflectometer. Commercial wollastonite
products usually have a G.E. brightness ranging from 80 to 95. The Hunter method is also used
to measure the brightness of wollastonite.
Optical Properties
Typical values for pure polymorphs of wollastonite are shown below.
Optical Properties of Pure CaO SiO
2
Structure


:z :y
Sign

a wollastonite - triclinic

1.618

1.630

1.632

0.014

39
O
4
O
(-)

a Parawollastonite - monoclinic

1.618

1.630

1.632

0.014

38
O
0
O
(-)

b Pseudowollastonite

1.610

1.611

1.654

0.044

9
O
-

(+)

The introduction of iron increases the refractive indices and the optic axial angle of wollastonite.
In general, the effect of the entry of manganese is similar to that of iron. Melting Point
The melting point for pure wollastonite is generally accepted as being 1540
O
C. The fluid
temperature for commercially produced wollastonite is generally somewhat lower than this value.
NYCO has found that for NYCOs wollastonite products, the fluid temperature can be as low as
1380
O
C. Melting point is important for end-uses such as ceramics, metallurgical fluxes, and heat
or fire-resistance. The actual subject of melting point appears straightforward, however, it can be
rather involved and complex including such things as Softening Point, Upper Plastic Deformation
Point, Fluid Point, Fusion Range, Melt Rate, etc. Sometimes crystallization characteristics are just
as important with a corresponding set of parameters (but generally different values) reached upon
cooling. Unfortunately there are few standardized test procedures and test results can be easily
influenced by such things as test method, particle size, bulk density, atmosphere, etc.
Thermal Conductivity Thermal conductivity measurements are specific to the application.
Wollastonite is normally considered to have low thermal conductivity. Functional fillers for plastics
typically have low thermal conductivity however, wollastonite may be advantageous in these
applications since relative to other fillers, it can increase thermal conductivity of the polymer
matrix without deterioration of electrical insulating properties. In the continuous casting of steel,
the steel is very near solidification and the mold powder must have good insulating properties. As
liquid flux and partially molten flux are poor insulators, the unmelted layer of powder must provide
the majority of the heat retention. Insulating properties of the powder are dependent upon thermal
conductivity, bulk density, etc.
Thermal Expansion
A characteristically low coefficient of thermal expansion combined with aspect-ratio, impart high
thermal shock resistance and dimensional stability in high temperature applications such as fire-
resistant board or refractory linings. The coefficient of linear expansion is generally accepted as
being 6.5x10-6 mm/mm/
O
C.
Mechanical Properties
The limited information regarding the mechanical properties of wollastonite is indicated below.
Mechanical Properties of Wollastonite
Form

Elastic Modulus, GPa Tensile Strength, MPa
Wollastonite fibers (3-150 m m
dia.)

303-530

2700-4100

Electrical Properties
Wollastonite can be considered to be an insulator or non-conductor of electricity. For pure
wollastonite, the commonly accepted value for DC electrical conductivity is 1.5E-11 mho/m. The
dielectric permittivity of pure wollastonite is generally accepted as 8.60 @ 1MHz. Specific
measurements of wollastonite rock samples are as follows:
Relative Dielectric Permittivity & Loss Tangent
Description

1 kHz
10 kHz
100 kHz
1 MHz
Wollastonite (Mexico)

6.9 0.0152

6.8 0.0046

6.8 0.0078

6.9 0.0046

The dielectric value of wollastonite in an epoxy formulation is 4.6, loss factor 10? = 1.0. For use in
electrical ceramics, wollastonite provides a low loss dielectric crystalline phase which has proved
useful in the production of high frequency electronic equipment.