Magnetite is one of several types of iron oxide and its official (IUPAC) name is Siderite, aka chalybite, is a carbonate of iron, FeCO3.    From Wordnik.com.  [Latest Articles] Reference

All carbonates are soluble with effervescence in dilute acids; some, such as chalybite and magnesite, require the aid of heat.    From Wordnik.com.  [A Text-book of Assaying: For the Use of Those Connected with Mines.] Reference

It is rhombohedral in crystallization and isomorphous with calcite and chalybite.    From Wordnik.com.  [Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"] Reference

Carbonates of many of the metals, such as carbonate of lead (cerussite), carbonate of iron (chalybite), carbonates of copper.    From Wordnik.com.  [A Text-book of Assaying: For the Use of Those Connected with Mines.] Reference

In the red and brown iron ores and ochres ferric iron is present; in chalybite the iron is in the ferrous state; and in magnetite it is present in both forms.    From Wordnik.com.  [A Text-book of Assaying: For the Use of Those Connected with Mines.] Reference