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The Mineral pyrrhotite

Stacked Pyrrhotite Crystals

Pyrrhotite is unusual in that it maintains a variable iron content, as reflected in its odd chemical formula. The variability is formed by deficiencies of iron in its chemical structure, which is also responsible for its magnetic properties. Greater deficiency of iron is responsible for stronger attraction to magnetic fields. If the mineral lacks any iron deficiencies, it is no longer Pyrrhotite, but a rare, non-magnetic mineral called Troilite. (Troilite usually originates from meteoric sources.) 


The variation of iron in Pyrrhotite's structure is also responsible for more than one crystal symmetry type, allowing it to form in both hexagonal and monoclinic symmetries. Multiple crystal symmetries should technically classify Pyrrhotite as a mineral group rather than a single mineral, but a sub-classification is rarely made due to polytypes appearing similar and perhaps even combined within individual specimens. Though most Pyrrhotite specimens are stable, some tend to crumble in collections.

Pyrrhotite is commonly pseudomorphed by Pyrite, with complete replacement of the Pyrhottite by sparkling Pyrite crystals. Pyrrhotite is named from the Greek word pyrrhos, which means "color of fire". This is despite the fact that the usual color of Pyrrhotite is a bronze color rather than red; however its tarnish may have hints of sparkling red. 

Chemical Formula

Fe1-xS
(where x ranges from 0.0 to 0.2)

Color

Metallic yellow, bronze, dark silvery gray. Crystals may tarnish on exposed surfaces, usually darkening, and sometimes with a mild red or blue iridescence.

Crystal System

Monoclinic

Properties

Streak
Dark gray to black
Hardness
3.5 - 4.5
Transparency
Opaque
Specific Gravity
4.5 - 4.6
Luster
Metallic
Cleavage
None, though exhibits basal parting
Fracture
Uneven
Tenacity
Brittle
Other ID Marks
Strongly or moderately attracted to magnetic fields.

Crystal Habits

Pyrrhotite forms in several crystal polytypes with possible symmetry in more than one crystal group. Crystals are usually pseudohexagonal in shape, and they may be flat or wider midsections. A well-known habit is as stacked prismatic barrel-shaped crystals. Also tabular, in platy aggregates, bladed, in hexagonal rosettes, and in groups of integrown tabular hexagons and barrels. Commonly grainy or massive. Crystals are usually horizontally striated, and frequently have cracks in them.

3D Crystal Atlas

Additional Information

Composition
Iron sulfide, sometimes with nickel and cobalt
In Group
Sulfides; Simple Sulfides
Striking Features
Crystal habits and magnetism.
Environment
Pyrhottite occurs in a host of environments, including hydrothermal replacement deposits, pegmatites, marbles, nepheline syenites, carbonatites, and skarns.
Rock Type
Igneous, Sedimentary, Metamorphic

Other Names

Magnetic Pyrites

Uses

Pyrrhotite is an ore of iron. Well crystallized Pyrrhotite specimens are important collectors minerals. The aesthetic prismatic crystal forms are especially treasured by collectors and in high demand.

Noteworthy Localities

Several localities worldwide have produced outstanding examples of Pyrrhottite in attractive crystals and aggregates. Well crystallized and lustrous platelets are well known from Dal'negorsk, Primorskiy Kray, Russia; and highly lustrous, intergrown crystals clusters have been recently coming from the Yaogangxian Mine, Hunan Province, China. Etched crystals are classics at the Herja Mine, Chiuzbaia (Kisbánya), Baia Mare, Maramureș Co., Romania; and well-formed flattened crystals and rosettes have come from Trepča, Kosovo.

In Brazil, flattened bronze Pyrrhottite crystals have come from the Morro Velho mine, Nova Lima, Minas Gerais. Thick, prismatic crystals, representing some of the best forms of this mineral, are classics at the Potosí Mine, Santa Eulalia District, Chihuahua, Mexico.

Pyrrhottite is common in many ore deposits in the U.S., but rarely in good crystals. American localities of note include Trumbull, Fairfield Co., Connecticut; and Franklin, Ogdensburg, and Sparta, Sussex Co., New Jersey. In Canada, thick, often stacked crystals have come from the Bluebell Mine, Riondel, British Columbia; and microcrystals are known from Mont Saint-Hilaire, Québec.

Common Mineral Associations

Albite, Scapolite, Andradite, Pyrite, Galena, Titanite, Pentlandite, Quartz

Distingushing Similar Minerals

Pentlandite, Pyrite, and Chalcopyrite - Lack any attraction to magnets.
Hematite and Magnetite - Different color and crystal habits.


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