Pyrite and pyrrhotite are both iron sulfide minerals, but they differ significantly in their physical and chemical properties. Below is a detailed comparison between the two:

### 1. **Chemical Composition**:
– **Pyrite (FeS₂)**: Pyrite consists of iron (Fe) and sulfur (S) in a fixed ratio of 1:2. It has a highly ordered crystal structure, where each iron atom is bonded to two sulfur atoms.
– **Pyrrhotite (Fe₁₋ₓS)**: Pyrrhotite has a variable composition, with iron deficiency (ₓ) in its structure. Its chemical formula is often written as Fe₁₋ₓS, meaning it has less iron compared to pyrite. This variation gives pyrrhotite different magnetic and physical properties.

### 2. **Crystal Structure**:
– **Pyrite**: Pyrite forms in the **isometric crystal system**, typically exhibiting cubic or octahedral crystals. This crystal symmetry contributes to its nickname “fool’s gold” due to its shiny, metallic luster and well-defined shape.
– **Pyrrhotite**: Pyrrhotite forms in the **monoclinic or hexagonal crystal system** and typically appears in more massive or granular forms, rather than the well-defined cubic structures seen in pyrite.

### 3. **Color and Appearance**:
– **Pyrite**: Pyrite is a pale, brassy-yellow color with a metallic luster, often resembling gold. It is hard and brittle.
– **Pyrrhotite**: Pyrrhotite tends to be darker, ranging from bronze to brownish-black, with a less shiny, more matte metallic luster. It can also appear tarnished or have a reddish hue due to oxidation.

### 4. **Hardness**:
– **Pyrite**: Pyrite has a Mohs hardness of **6 to 6.5**, making it harder than pyrrhotite.
– **Pyrrhotite**: Pyrrhotite is softer, with a Mohs hardness of **3.5 to 4.5**, which means it can be scratched more easily than pyrite.

5. Magnetic Properties:
Pyrite: Pyrite is non-magnetic.
Pyrrhotite: Pyrrhotite is weakly to strongly magnetic due to the iron deficiency in its structure. The more iron-deficient the mineral is, the stronger its magnetic properties. This is a key distinguishing feature between the two minerals.

6. Occurrence and Associations:
Pyrite: Pyrite is very common and found in various geological environments, from sedimentary deposits to hydrothermal veins. It is often associated with quartz and other sulfide minerals.
Pyrrhotite: Pyrrhotite is less common than pyrite and is typically found in mafic igneous rocks and high-temperature ore deposits. It is often associated with nickel, platinum, and copper deposits.

7. Tarnish and Weathering:
Pyrite: Pyrite is more stable in surface conditions but can oxidize over time, forming a yellowish tarnish. In humid conditions, it can form sulfuric acid, leading to acid mine drainage.
Pyrrhotite: Pyrrhotite is less stable and oxidizes more easily, often developing a reddish-brown tarnish. This makes it more prone to weathering in the presence of air and water.

8. Economic Importance:
Pyrite: Pyrite is primarily mined for sulfur and sulfuric acid production, although it has little direct economic value for gold despite its appearance.
Pyrrhotite: Pyrrhotite is important in the mining of **nickel** and other metals, often found in sulfide-rich ore bodies. Its magnetic properties make it useful for identifying ore deposits in geophysical surveys.