Physiochemical properties are features evident as a result of chemical composition, crystallography, or special arangment of molecules, ions, electron bond features, or excitation effects that can be made visible. Illumination systems include polarized light, phase contrast, interference systems, fluorescence, etc. The example immediately below is a thin section of dinosaur bone. The first image shows the structure and color of the fossil bone. The second image is the same view showing the crystallographic orientation of the minerals making up the fossil by using a red plate compensator. These are just a few examples of the feature that are part of this section.

The refractive index of a particle is a fundamental property of the particle and is intimately associated with the mechanical properties of the material. it is the result of the electron density per unit volume and the nature of the molecular and crystalline bonds holding the material together. With the particle in a fixed mounting medium the ability to measure its refractive index as a function of wavelength is limited to the Duc de Chaulnes method. More information on the particles refractive index may be available from the effects at the interface between the particle and the mounting medium. That is covered in the section on "Interface Properties".

The color and brightness of a particle viewed with transmitted light is the result of the electron density and bond energies of the molecules that make up the particle. Morphology caused light scatter has been discussed above and affects the light that is transmitted. Here we are considering the net effect of scatter, reflection, and absorption. Surface scatter and reflection are affected by the medium around the particle. In a fixed mounting medium, particles will behave in a fixed way

Absorption is the light lost in the particle that is converted from electromagnetic energy into thermal energy. Absorption is often wavelength dependent and contributes to the apparent color of the particle.

Hematite has a complex refractive index of 2.937 + i(0.24268) for epsilon. The omega refractive index is about 3.2 + i(0.1) at the same wavelength. When using transmitted light hematite appears nearly opaque. With its high birefringence, approximately 0.28, even small particles appear red between crossed polarizing filters. It appears red because it transmits red wavelengths much more efficiently than the shorter wavelengths (blue, green, yellow). With Brightfield illumination the background is too bright to see the small amount of red light transmitted. With crossed polarizers the background is dark and the light transmitted as a result of the high birefringence of hematite and its absorption properties is red.

Polished sections of tourmaline were the first linear polarizing filters.

The reflectivity of the particle is evident in reflected light. It is a result of the electron density difference between the particle and the mounting medium and the electrical conductivity and magnetic permeability of the molecules that make up the particle.

The transmitted color of the particle consists of the wavelengths that are least affected by reflection and absorption.

The reflected color is the result of the wavelengths at which the refractive indices of the mounting medium and the particle are most different and the loss of the wavelengths that are absorbed.

Birefringence is the property of showing more than one refractive index as a function of particle orientation and wavelength. Such a particle will exhibit interference colors when viewed between crossed circular polarized filters. They will typically show extinction positions (see below) with rotation of the stage when viewed between crossed linear polarizing filters.

Anomalous birefringence is the result of birefringence varying by wavelength. The result is anomalous interference colors. Silicon carbide and crocidolite asbestos are two common examples. Crocidolite has higher birefringence in red light (longer Wavelengths) than in blue. As a result, very thin fibers of crocidolite appear red between crossed polarizing filters. Thicker fibers appear blue because of the strong blue color of the mineral.

Silicon carbide has higher birefringence in blue light (Shorter Wavelenghts) than in red. As a result, blue wavelengths cycle more rapidly than red wavelengths. Yellow interference color begins for thinner particles and first order red appears purple because blue is increasing well before red significantly decreases. This effect changes the color sequence through the whole range of microscopic silicon carbide particles.

When a material is placed under stress the distribution of the electrons in the material is changed. The amount of change is different for each material and is a characteristic of the material. The photoelastic constant of the matrial is a measure of the electron displacement (strain) as a function of the load (stress) applied as long as the deformation is elastic, springs back when the load is removed. If the Young's Modulus of the matrial is exceeded, then some of the deformation becomes permenant. In some materials the applied load can be "frozen" in place, as in the case of high stress glass sheet. Polarized light can make the displacement visible. Both plastic deformation and elastic deformation result in an anisotropic distribution of electrons in the material that becomes visible as interference colors when the object is viewed between crossed linear or crossed circular polarizing filters. Click on the photographs below for more information.

Polarized light is depolarized at the interface between a conductive particle and a non-conductive mounting medium. This light halo effect with transmitted crossed polarized light indicates an opaque particle is a wear metal particle or at least is conductive. Graphite is sufficiently conductive to produce this effect. Pencil debris can be distinguished from combustion residue by this effect.

If the refractive index of a transparent particle is much different than the medium in contact with it, then the polarized beam can be changed at the interface as a result of reflection. If the interface is aligned with the polarizer or analyzer then the beam is not affected. In other orientations reflection at the interface results in rotation of the polarized beam and the interface appears to show a first order white interference color.

Bireflection is the property of having different reflectivities in different directions of linear polarized light. It is most evident when viewed with reflected crossed polarized light. The object appears bright on a black background.

Pleochrism is the property of changing color on rotation when viewed with linear polarized light. Many colored materials show this property.

The optical sign of a birefringent or bireflective particle is either positive or negative depending on the relative value of its refractive indices.

The sign of elongation is positive or negative depending on the orientation of its highest refractive index to the long axis of the partical.

Interference colors may result from birefringence or from thin film effects.

As an anisotropic particle between crossed linear polarizing filters is rotated in the plane of the stage by rotating the stage it generally goes dark every 90 degrees, but not always. If the particle goes dark (extincts) and has a characteristic morphology then its "extinction position" can be said to be parallel to a characteristic morphology (parallel extiction), bisect a characteristic angle (symmetric extinction), or, if neither of those conditions are satisfied, then it has oblique extinction. Some anisotropic materials don't change on rotation of the stage. Mature cotton fibers are an example. They have no extiction position. Other materials my darken but not go extinct. Peristerites are an example. Some materials shade into reddish, then bluish darker positions on rotation because the extinction position for red and blue wavelengths are not aligned. Wood fibers are a common example.

Twinning in a crystal is the result of a change in crystallograph orientation along a plane. It may be evident by the crystal having different extinction position along different planes, as with the feldspar crystal below, or by showing alternating pleochroism, as with the olivine crystal below.