Parting

Parting is the way some mineral crystals break along flat planes of weakness that are not part of the crystal's normal cleavage, such as twin planes or thin layers of a second mineral. In amphiboles, parting can produce long, thin fragments that look like fibers under a microscope even though the mineral did not grow as asbestos.

Origin

Parting is a standard term in descriptive mineralogy, used alongside cleavage and fracture to describe how a mineral breaks. Unlike cleavage, which every crystal of a mineral shares, parting appears only in specimens that have the particular planes of weakness, so it varies from one deposit to another.

Significance in Asbestos Context

Parting matters in disputes over whether elongated particles in talc, vermiculite, rock or dust are true asbestos fibers or cleavage fragments. Defense experts sometimes argue that long, thin amphibole particles came from crushing ordinary crystals that split along cleavage or parting planes, and so should not be counted as asbestos. Plaintiffs' experts respond that the body reacts to a particle's size and shape, and that particles of the same dimensions can cause disease whatever the geology says about how they formed. Parting can make this harder to sort out, because fragments broken along parting planes can be thinner and straighter than ordinary cleavage fragments. Analysts look at features such as parallel sides, split ends, bundles and curvature to tell asbestiform fibers from fragments, but the call is not always clean. For a family, the practical point is that a lab report calling particles "cleavage fragments" or "non-asbestiform" is an interpretation, and it can be tested by another qualified analyst looking at the same data.

Example Sentence

The defense mineralogist attributed the long tremolite particles to parting along twin planes, while the plaintiff's expert counted them as asbestiform fibers.

Related Terms

Cleavage Fragment, Cleaving, Asbestiform, Non-Asbestiform, Habit, elongate mineral particles (EMPs), Amphibole Group

Notes

• If a report dismisses particles as fragments, ask what features the analyst relied on.
• Ask whether the images and count sheets behind the report are available for a second review.
• The same sample can be described differently by different experts; the underlying measurements are what matter.