Francolite is a carbonate-rich variety of the mineral fluorapatite and is present in most sedimentary phosphorites. It is a variety of apatite. It is the phosphorus-bearing mineral of nearly all sedimentary phosphorites, has a variable chemical composition. It has a variable chemical composition which can be represented by (Ca, Mg, Sr, Na)10(PO4, SO4, CO3)6F2-3. The mineral was named after its occurrence at Wheal Franco, Whitchurch, Tavistock District, Devon, England.
Fig: Francolite
Properties
- Color: Colorless, Whitish.
- Density: 3.12
- Diaphaneity: Transparent to Translucent
- Habit: Earthy – Dull, clay-like texture with no visible crystalline affinities, (e.g. howlite).
- Habit: Massive – Uniformly indistinguishable crystals forming large masses.
- Habit: Pulverulent – Forms a loose, poorly-coherent powdery mass.
- Hardness: 5 – Apatite
- Luster: Vitreous – Resinous
- Streak: white
It is now well established that this mineral can form authigenically in organic-rich muds1 or by replacement of precursor carbonate but the origin of some phosphorite deposits is still in dispute or unknown.
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