Stichtite

Stichtite is a hydrated magnesium-chromium carbonate (Mg₆Cr₂(OH)₁₆CO₃·4H₂O), a soft purple mineral that forms by the alteration of chromium-bearing serpentinite. Stichtite forms as a secondary mineral through the chemical weathering of chromite-bearing serpentinite. The process begins with ultramafic rocks, primarily peridotite composed of olivine and pyroxene, that have already undergone serpentinization at temperatures between 200 and 500 degrees Celsius. During serpentinization, the original silicate minerals react with water to produce serpentine group minerals, and any chromium-bearing spinel (chromite, FeCr₂O₄) is left as a residual phase within the new serpentine matrix.

As carbonate-rich groundwater percolates through the serpentinite at much lower, near-surface temperatures (below 100 degrees Celsius), it attacks the exposed chromite grains. The chromium is released as Cr³⁺ ions and combined with magnesium from the surrounding serpentine, carbonate from the groundwater, and hydroxyl groups to precipitate stichtite. The vivid purple to lilac-pink color comes directly from the Cr³⁺ in octahedral coordination within the mineral's layered hydroxide structure, the same chromophore responsible for the color of ruby and emerald, but in a completely different crystal architecture.

Stichtite belongs to the hydrotalcite supergroup, a family of layered double hydroxides with positively charged brucite-like sheets separated by interlayers of carbonate anions and water molecules. This layered structure makes the mineral extremely soft (1.5 to 2 on the Mohs scale) and gives it a greasy, waxy feel. When stichtite occurs intermixed with its green serpentine host rock, the combination is marketed as "atlantisite," a striking purple-and-green ornamental stone found almost exclusively in Tasmania.

Stichtite is recognizable by its vivid purple to lilac-pink color, waxy to pearly luster, and extreme softness. It can be scratched easily with a fingernail. The mineral typically occurs as fibrous masses, botryoidal crusts, or foliated aggregates within green serpentine host rock. When cut or polished as part of atlantisite, the contrast between vivid purple stichtite and deep green serpentine is unmistakable.

Distinguish from sugilite, which is much harder (5.5 to 6.5) and has a vitreous luster. Purple lepidolite mica is also soft but has a distinctly micaceous, platy habit and peels into thin sheets. Charoite has a fibrous, chatoyant appearance and is considerably harder (5 to 6). The combination of extreme softness, waxy feel, purple color, and association with serpentine is diagnostic for stichtite.