Foitite

Foitite is an unusual, iron-rich member of the tourmaline supergroup. Unlike common black tourmaline, or schorl, foitite is alkali-deficient: the structural position normally occupied by sodium or calcium is largely vacant. This subtle chemical difference cannot normally be confirmed by appearance alone, which is why laboratory analysis is often needed to distinguish foitite from schorl. Foitite was formally named in 1993 in honour of American mineralogist Franklin F. Foit Jr Foitite generally forms during the later stages of mineral growth in boron-rich granitic pegmatites and associated veins.

As the remaining mineral-forming fluids become depleted in sodium, the growing tourmaline may become increasingly vacancy- dominant, producing foitite or foitite-rich zones. It commonly appears as bluish-black, indigo-black or very dark greenish crystals, fibrous aggregates, fine sprays or dense coatings. Individual crystals may be elongated and strongly striated, sometimes with the triangular cross-section characteristic of tourmaline. Foitite from the Erongo Mountains The Erongo Mountains of Namibia are internationally known for complex granitic pegmatites containing tourmaline, quartz, feldspar, beryl, fluorite and many other collectible minerals. Foitite has been reported from the Erongo Mountains and has been studied as part of the region’s chemically complex tourmalines.

Recorded associations include schorl, quartz and smoky quartz, beryl varieties such as aquamarine and goshenite, fluorite, feldspar, muscovite, siderite and hematite. Some Erongo material forms fibrous or finely crystalline growths over feldspar and earlier tourmaline crystals, producing the shimmering blue-grey to black textured surfaces seen on many specimens. However, the boundary between schorl and foitite can be complicated: a single Erongo tourmaline may contain chemically zoned areas that range from schorl-rich to foitite-rich. For that reason, a specimen should ideally have analytical confirmation before being represented as pure foitite rather than foitite-rich tourmaline or a foitite–schorl association.

At Davib Ost Farm in the Erongo Region, foitite occurring as fibrous structures and aggregates within schorl has been confirmed by electron-probe analysis. This material does not come from a large commercial mine. It is recovered from small pegmatite workings in the rugged Erongo Mountains, where artisanal miners search individual mineral pockets. Foitite is an uncommon member of the tourmaline group, and well-formed blue-grey specimen material from Erongo is produced only intermittently. Mineral group: Tourmaline supergroup Composition: Iron- and aluminum-rich borosilicate Did you know?

Some specimens historically labelled simply as black tourmaline or schorl may contain foitite-rich zones. Its vacancy- dominant chemistry was not easily recognized until modern analytical methods allowed mineralogists to measure the sodium deficiency and distribution of iron and aluminum within individual crystals.