Phosphosiderite
Red phosphosiderite between violet strengite. Picture width 4 mm.
General
Category	Phosphate minerals
Formula (repeating unit)	Hydrated iron phosphate FePO4•2H2O
IMA symbol	Phsd[1]
Crystal system	Monoclinic
Crystal class	Prismatic 2/m
Space group	P21/n (no. 14)
Unit cell	454.76 ų
Identification
Formula mass	186.85
Color	Usually red to pink to purple, sometimes green, usually yellow veined
Crystal habit	Tabular {010} or stout prismatic [001]
Twinning	Common on {101}, typically as interpenetration
Cleavage	{010} Distinct, {001} Indistinct
Fracture	Uneven
Mohs scale hardness	3.5–4
Luster	Vitreous
Streak	White
Diaphaneity	Transparent, translucent
Specific gravity	2.74-2.76
Density	2.74 - 2.76 measured, 2.76 calculated
Optical properties	Biaxial (-)
Refractive index	nα = 1.692 nβ = 1.725 nγ = 1.738
Birefringence	0.046
Pleochroism	Visible
2V angle	Measured: 62°, Calculated: 62°
Dispersion	Very strong r > v
Solubility	Totally soluble in hydrochloric acid, nearly insoluble in nitric acid

Phosphosiderite is a rare mineral named for its main components, phosphate and iron. The siderite at the end of phosphosiderite comes from the word "sideros", the Greek word for iron. It was published in 1890, and has been a valid species since pre-IMA. It is an IMA approved mineral which got grandfathered, meaning its name is still believed to refer to an existing species.^[2]

Properties

Phosphosiderite is a member of the metavariscite group,^[2] and probably forms a complete series with metavariscite. It is the dimorph of strengite.^[3] It is totally soluble in Hydrochloric acid and nearly insoluble in nitric acid.^[4] It is usually cut into a cabochon shape for jewelry, and used as an ornamental stone.^[3] It mainly consists of oxygen (51.38%), iron (29.89%) and phosphorus (16.58%), but contains hydrogen (2.16%) as well.^[5]

Colors

Phosphosiderite is often found in a bright orchid-purple tint with yellow-colored streaks. The vein-like streaks found in the stone are known as cacoxenite. Four other rare color variants are rose red, brown-reddish yellow, mossy green, and dark shades of purple. Phosphosiderite can also be found as a colorless mineral.^[6] It is pleochroic; on the X axis around 4° it is light rose, on the Y axis it is carmine red, and on Z it is colorless.^[6]

Occurrences and localities

Phosphosiderite is mined in some parts of Chile, Argentina, Germany, Portugal, and the United States. It occurs in association with strengite, turquoise, triphylite, leucophosphite, huréaulite, barbosalite and laueite.^[7] In zoned granitic pegmatites, it usually occurs as an alteration product of primary phosphates. This makes it a secondary phosphate.^[3] It may be replacing shells and bones, and can occur in the soil as a component of it as well.^[7] However, the latter is rarer.^[3]

References