Feldspathic igneous rock includes all common rock that is formed from the melting of existing rock by volcanic eruptions and lava flows. This includes limestone and granites and any combination of common minerals that are formed of magma. In 2008, a study by B. Ronald Frost and Carol D. Frost released in the Journal of Petrology attempted to put these rocks in five separate classifications. This method groups certain rocks by the type of magma they come from and the concentration of one mineral over another.
Hematite is commonly mined from both limestone and granite. Hematite forms after the magma that melted the original rock solidifies. It is removed from the other minerals making up the rock by spraying the rock with pressurized water. This separates the hematite from the rock so it can be harvested for processing. But the runoff water from this process often contains aluminum and iron sulfates that can pollute the environment.
Large hematite deposits have formed beneath the Earth's surface. They are made of magma that is especially rich with the hematite mineral. When the magma solidifies it creates a layer of bedded hematite between other rock types underneath the soil, such as granite. These deposits provide the necessary resources to make steel. But it takes millions of years for a hematite deposit to form, so excessive mining will deplete these deposits.
A large deposit of hematite was discovered on Mars in 1998. The Mars Global Surveyor detected the deposit after strong winds eroded the dirt on top. The deposit is a concentrated area of hematite formed by igneous rock types from volcanoes that are now inactive. This discovery supports the theory that Mars once sustained life because water must be present for hematite to form from cooling magma.