Mineral
Rutilo
TiO₂ · Óxidos
Rutile is the mineral titanium dioxide (TiO₂).
It forms in chemically austere parageneses within hydrothermal systems of low elemental diversity. In these settings, iron, titanium and silicon crystallise independently, linked only to oxygen. Rutile crystallises as the titanium compositional unit within that association.
The star structures on haematite develop in two stages. First, tabular haematite (Fe₂O₃) crystallises as plates. Then a new episode of hydrothermal fluids deposits titanium onto them. Rutile does not grow at random: the crystal lattices of both minerals fit better at certain angles. Hence the needles appear as straight, repeated, geometric bundles.
Etymology
From the Latin "rutilus", reddish or shining.