You may not know the name, but if you have an iron meteorite, you probably have a schreibersite. It is an iron-nickel phosphide mineral with the chemical formula (Fe,Ni)3P. It’s all over the space rocks that fall to Earth. In fact, it is the main component of most nickel-iron-bearing meteorites.
When scientists look at iron meteorites under a microscope, they often find two different shapes. First of all, it looks like a thin plates. It then forms a shell around the nodules of troilite. Pyrite is simply an iron sulfide mineral. In other words, a hard sulfide core is surrounded by a phosphide shell.
This mineral is also available in a stick version. Geologists call this variety “rhabdite”. Scientists have long believed that rhabdite is a completely separate mineral species. they are wrong. It’s just a schreibersite with a different shape. Plate crystals and rod crystals have the same basic structure. They all belong to the tetragonal system.
This is important because phosphorus is an important part of life. DNA and RNA need it. ATP uses it as energy. However, phosphorus is usually trapped in the earth’s insoluble rocks. Schreibersite was different. Soluble in water. The schreibersite melts when the meteorite falls into the sea. This releases active phosphorus into the primordial soup. Some scientists believe that this process helped initiate the chemical reactions that produced the first living cells. It turns out that a space rock may have provided the spark.
The physical properties of these crystals are the standard of natural elements. They are hard. It’s dense. It has a metallic sheen. If you want exact numbers for hardness or density, you have to look in tables of natural elements. But the story is in the chemistry. It is an alloy of iron, nickel and phosphorus. How shells form around other minerals. In fact, what were once thought to be two different minerals are actually just two forms of one mineral.
Why is this distinction important? How we interpret the history of the solar system is important. This is important to understand where our building blocks come from. This is important to the simple but profound idea that we are made of phosphide minerals dissolved in star stuff, and sometimes, that stuff is a phosphide mineral that dissolved in the rain.