Phosphosiderite Properties, Facts and Photos

What is Phosphosiderite?

Phosphosiderite is a naturally occurring form of iron phosphate found only in a handful of locations worldwide.

A relatively rare mineral, it was discovered in 1858 and named two years later after its main constituents, phosphate and iron.

'Phosphosiderite' comes from 'phospho' (from phosphate) and 'siderite', which comes from the Latin 'sídēros', meaning 'iron'.

Most phosphosiderite comes from Germany, the USA, Portugal, Chile and Argentina.

Although once known as metastrengite, this name is now mostly obsolete.

Crystals of phosphosiderite occur in shades of red and pink and tend to be extremely small. Phosphosiderite occurs more widely with a massive or botryoidal crystal habit.  This material is mainly used for lapidary purposes.

In mineralogy, the term crystal habit describes the external shape of a crystal or group of crystals and how well they have formed. The habit described as 'massive' means the mineral's crystals grow tightly together as one large, shapeless mass. Unlike individual well-formed crystals, they have no visible internal structure and no distinguishable external shape.

An example of a mineral with a massive crystal habit is turquoise.

The crystal habit described as botryoidal means crystals have a rounded shape. The word comes from the ancient Greek word 'botrys' meaning 'bunch of grapes'.

Phosphosiderite that occurs in massive form can be found in striking shades of lilac, lavender and purple. It often features yellow spidery veins, which are inclusions of cacoxenite (ca~cox~enite). This iron aluminium phosphate mineral usually, but not always, occurs within other minerals.

Phosphosiderite is believed to have started as the abundant phosphate mineral triphylite. Over long periods of time and through various geological processes, triphylite turns into phosphosiderite.

Phosphosiderite is typically found in pegmatite, an igneous rock known for its unusually large crystals.

Pegmatites form when magma rich in water and volatiles, cools slowly underground. These conditions allow for the growth of large crystals, often of minerals that are rare in other types of rocks.
On the Mohs scale of mineral hardness, phosphosiderite grades 3.5 to 4. Being quite soft means that it must be handled carefully. Phosphosiderite is also brittle and can break easily.

Article Pictures

The mineral turquoise in rock matrix is courtesy of Stan Celestian.

The malachite with a botryoidal crystal habit is courtesy of Ron Wolf.

The phosphosiderite tumbled stones are from our collection.

Pictures are clickable and redirect to the original image.

Pop-up photos: Botryoidal hematite - Courtesy of James St. John. Cacoxenite - Courtesy of Ron Wolf.