Fluocerite

Fluocerite, also known as tysonite, is a mineral consisting of cerium and lanthanum fluorides, with the chemical formula (Ce,La)F₃.^[1][2] The end members are classified as two different mineral types depending on the cation, fluocerite-(Ce) and fluocerite-(La), corresponding respectively to lanthanum trifluoride and cerium trifluoride. Both crystallize in the trigonal system.^[3]

Fluocerite-(La)
General
Category	Minerals
Formula	(La,Ce)F3
Strunz classification	03.AC.15 (03)
Dana classification	09.03.04.02 (09)
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral
Space group	P3c1 (No. 165)
Unit cell	328.80 ų (Calculated from Unit Cell)
Identification
Formula mass	196.02
Colour	Greenish-yellow
Crystal habit	Platy, tabular
Cleavage	Indistinct, Imperfect
Mohs scale hardness	4-5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Density	5.93
Birefringence	0.006

Fluocerite-(Ce)
General
Category	Minerals
Formula	(La,Ce)F3
Strunz classification	3/A.10-20
Dana classification	9.3.4.1
Crystal system	Trigonal
Crystal class	Hexagonal Scalenohedral
Unit cell	320.86 ų (Calculated from Unit Cell)
Identification
Formula mass	196.99
Colour	Light yellow; darkening to yellow- and red-brown; colourless to pale pink (transmitted light)
Crystal habit	Massive, Primsatic
Cleavage	Distinct/Good
Fracture	Irregular/Uneven, Splintery, Sub-Conchoidal
Tenacity	Brittle
Mohs scale hardness	4.5-5
Luster	Vitreous, Resinous, Pearly
Streak	Yellow-white
Diaphaneity	Transparent to translucent
Specific gravity	5.93 - 6.14
Density	5.93 - 6.14 g/cm3 (Measured)
Birefringence	0.005 - 0.007

Fluocerite-(Ce) was first described (without the Ce) in 1845 from hydrothermal veins in granite in Sweden.^[4] Fluocerite-(La) was first described in 1969 from the type locality in central Kazakhstan.^[3] The name tysonite was given in 1880 to the same type of mineral found in Colorado.^[5][6] Tysonite-type structure is used for rare-earth fluorides with the P3c1 space group structure.^[7]