There are a few minerals that are used in plastics PP as fillers, wollastonite is one of them.
According to Wikipedia, Wollastonite is a calcium inosilicate mineral (CaSiO3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium.
Wollastonite improves tensile and flexural strength for plastics, it reduces resin consumption, and improves thermal and dimensional stability at elevated temperatures. Surface treatments are used to improve the adhesion between the wollastonite and the polymers to which it is added. As a substitute for asbestos(toxic) in floor tiles, friction products, insulating board and panels, paint, plastics, and roofing products, wollastonite is resistant to chemical attack, inert, stable at high temperatures, and improves flexural and tensile strength.
The uniqueness of wollastonite:
*Needle-like shape contributing aspect ratio
*Naturally white color
*Moderate Mohs’ hardness
*High melt temperature
*Surface chemistry compatible with silane and titanate treatments as well as coupling agents