Francolite is a carbonate rich variety of the mineral fluorapatite and is present in most sedimentary phosphorites. 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.
References
Francolite Wikipedia(Text) CC BY-SA