difference between FUNCTIONAL MINERAL and FILLER?

I am basing my answer on my experience in a number of industries, paint, plastics and papermaking. Other industries might have different explanations and definitions.

Functional minerals or pigments are normally used in small proportions to give a special property usually a colour to the end product. Examples of this would be titanium dioxide for whiteness and opacity, phthalocyanate blues or greens, iron oxide, synthetic or natural to give a brown, red , orange or yellow colour. These functional minerals are usually expensive, often much more so than the base plastic are other paint ingredients.

Fillers (which are also called extenders) are added to impart physical properties to the end product, such as increasing the viscosity of the paint, adding body to the paint so that a thicker film can be applied. They can increase the rigidity of plastics. They are much cheaper than the other ingredients. Typical fillers are clay, calcium carbonate,kaolin, etc. Fillers are almost universally natural inerals and synthetic fillers are rare.