Phosphosiderite: Properties, Facts and Photos
What is Phosphosiderite?
Phosphosiderite is a naturally occurring form of iron phosphate found in a small number 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', from the Greek sideros, 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, although they're extremely rare. They also tend to be very 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 formed from another phosphate mineral called triphylite. Over long periods of time, triphylite turns into phosphosiderite.
Triphylite 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: Courtesy of Stan Celestian.
The malachite with a botryoidal crystal habit: Courtesy of Ron Wolf.
The phosphosiderite tumbled stones are from our collection.
Pop-up photos
Botryoidal hematite: Courtesy of James St. John. Cacoxenite: Courtesy of Ron Wolf.
