FASSAITE

“Fassaite” – (Ca,Na)(Mg,Fe²⁺,Al,Fe³⁺,Ti)[(Si,Al)₂O₆] – is a discredited term referring to a variety of augite characterized by high Fe³⁺ and Al contents and low Na content. The name originates from the Fassa Valley, in the Province of Trento, Italy.

Another term used for the same material is “pyrogom.”

There is a contradiction in the published data: while several references (DHZ, Nesse, Stosch) describe fassaite as having a high Fe content, other sources (Mindat, Wikipedia) classify it as having a low Fe content. The correct characterization is a high Fe content.

1. Characteristics

Macroscopically, fassaite ranges from light green to dark green—sometimes appearing almost black—and forms short, prismatic monoclinic crystals, as is typical for pyroxenes. It tends to be more translucent than common augites.

2. Geology and Deposits

Fassaite forms at high temperatures (>650ºC?) in skarns, within the contact zone between magmatic rocks and limestones. At the type locality, it is found at the contact between a syenite and limestones. It also occurs in eclogites (associated with pyrope garnet) and in meteorites.

3. Mineral Associations

In skarns, it is associated with vesuvianite, garnets (andradite or grossular), diopside, wollastonite, actinolite, hornblende, biotite, albite, calcite, epidote, humite, spinel, and titanite.

Bibliography:

Rao, A.T. & Rao, M.V. 1970. Fassaite form a calc-silicate skarn vein near Gondivalasa, Orissa, India. The American Mineralogist, vol. 55, May June 1970, p. 975-980