tremolitegroup

The tremolite group, also referred to as the tremolite-actinolite group, is a subset of the amphibole mineral family characterized by calcic chemistry and a continuous solid-solution series between the magnesium-rich end-member tremolite and iron-rich members such as actinolite and ferroactinolite. In this group, calcium is a dominant cation in the A-site of the amphibole structure, while magnesium and iron substitute for each other in the M-sites, producing a range of compositions along the series.

Physically, tremolite-group minerals crystallize in the monoclinic system and typically occur as prismatic, elongated crystals with two good cleavage directions at about 56 and 124 degrees. Tremolite itself is usually colorless to white or pale green, whereas actinolite tends to be green due to iron content; ferroactinolite and related members are darker. The general amphibole formula accommodates substitutions that yield the tremolite-actinolite-ferroactinolite series, with end-members defined by predominant Mg, mixed Mg-Fe, or predominant Fe.

Geologically, tremolite-group minerals form under metamorphic conditions and are common in metamorphosed carbonate rocks (marbles and calcsilicate rocks), ultramafic and serpentinite sequences, and talc-tremolite schists. They can appear during regional and contact metamorphism across a range of grades. Associated minerals include calcite, dolomite, diopside, chlorite, talc, and serpentine. Tremolite asbestos is a fiber-form of tremolite and is a health concern; asbestos-grade varieties were widely used in the past but are now heavily regulated due to health risks.

In geology, the presence and chemistry of tremolite-group minerals provide information about metamorphic conditions and rock composition, aiding interpretations of metamorphic facies and tectonic history.