Triphylite

Triphylite crystal in matrix, collected from Smith Quarry (Chandler Mills Quarry), Newport, New Hampshire 43°21′28″N 72°15′8″W[1]
General
Category	Phosphate minerals
Formula (repeating unit)	LiFePO4
IMA symbol	Trp[2]
Strunz classification	8.AB.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pmnb
Unit cell	a = 6.0285(6) Å, b = 10.3586(9) Å, c = 4.7031(3) Å, Z = 4
Identification
Color	Green–blue gray, brown–black
Crystal habit	Massive, granular, prismatic
Cleavage	{100} perfect, {010} imperfect, {011} poor
Fracture	Uneven–subconchoidal
Mohs scale hardness	4–5
Luster	Vitreous–subresinous
Streak	White–grayish white
Diaphaneity	Transparent–translucent
Optical properties	Biaxial (+), 2V = 0°–65°
Refractive index	nα=1.675–1.694, nβ=1.684–1.695, nγ=1.685–1.700
Birefringence	δ = 0.0060–0.0080
References	[3][4][5]

Triphylite is a lithium iron(II) phosphate mineral with the chemical formula LiFePO₄.^[6] It is a member of the triphylite group and forms a complete solid solution series with the lithium manganese(II) phosphate, lithiophilite. Triphylite crystallizes in the orthorhombic crystal system. It rarely forms prismatic crystals and is more frequently found in hypidiomorphic rock. It is bluish- to greenish-gray in color, but upon alteration becomes brown to black.

Etymology and history

The mineral was first discovered and examined in 1834 by German mineralogist Johann Nepomuk von Fuchs at Hennenkobel Mine in the Bavarian Forest.^[7][8] The name derives from the Greek words tri ("three") and phulon ("family"), referring to the three cations found in natural samples of triphylite (Li⁺, Fe²⁺, Mn²⁺).^[5]

Crystal structure

Crystal structure of triphylite

Triphylite crystallizes in an orthorhombic crystal system. The lithium coordinates to six oxygen atoms in a distorted octahedron. Likewise, the iron centers are octahedrally coordinated. The structure contains isolated phosphate tetrahedra.^[9]

Properties

Triphylite is soluble in hydrochloric and sulfuric acid. Under a blowpipe, it melts to form a dark gray, magnetic ball.^[7] Over time, the mineral undergoes alteration by oxidation, increasing the oxidation state of iron from +2 to +3 and allowing the lithium to escape, forming heterosite, FePO₄.^[8]

Triphylite forms a complex solution series with lithiophilite, LiMnPO₄, so that natural sources of triphylite usually contain manganese. The structures of members within this series are similar to olivine-type silicates.^[10]

References

1. "Chandlers Mill Quarry, Newport, Sullivan Co., New Hampshire, USA". MinDat.org.
2. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
3. "Triphylite". WebMineral.com. Retrieved July 15, 2015.
4. "Triphylite". Mindat.org.
5. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). "Triphylite" (PDF). Handbook of Mineralogy. Chantilly, VA: Mineralogical Society of America.
6. IMA-CNMNC List of Mineral Names (May 2015), International Mineralogical Association
7. von Fuchs, Johann Nepomuk (1834). "Ueber ein neues Mineral (Triphylin)". Journal für Praktische Chemie. 3: 98–104. doi:10.1002/prac.18340030120..
8. von Fuchs, Johann Nepomuk (1835). "Vermischte Notizen, 3. Triphylin". Journal für Praktische Chemie. 5: 319. doi:10.1002/prac.18350050138..
9. "Triphylin". Römpp Online. Retrieved 2020-06-13.
10. "Lithiophite-Triphylite Series". Mindat.org.