Hematite

To a remarkable degree, the color red is a gift of hematite. From lipstick to fire trucks and rusted scrap iron, most red pigments in our world and society are composed of hematite. Its importance isn't limited to color, however. Hematite is also the most abundant and economically important source of iron in our world. Since the beginning of the Iron Age, hematite has played an integral role in human society as our primary source of iron for weapons, railroad lines, and sky scrappers. Rust is simply another form of hematite and hematite dust is responsible for the reddish color of many soils and the Martian landscape.

Description and Identifying Characteristics

Hematite occurs in a range of forms, and its color can range from black to gray, or from red to brown depending on which variety is present. Regardless of their different appearances, all varieties of hematite exhibit a distinctive reddish-brown streak that serves to distinguish it from most common minerals. The mineral’s most common varieties are metallic and earthy hematite. Metallic hematite, also called specular hematite, has a shiny luster and may exhibit a micaceous habit, which means that it is easy to break small flakes off a sample. The flakes are quite hard, but are easily separated from the sample, making it difficult to recognize the mineral’s hardness. Some metallic hematite samples have a rounded bumpy surface that formed as fibrous crystals of hematite grew out from a surface into a fluid-filled space. Earthy forms of hematite are typically red to reddish brown and often called ‘Red Ochre’. These earthy varieties are also very soft and can be scratched by a fingernail. Occasionally, hematite may even occur as small rounded oolites. Oolites are small, sand-sized spherical or oval grains that form as hematite precipitates from fluids. Typically oolites consist of thin concentric crystal layers that, on broken surfaces, appear as nested spheres or ovals. When not broken, hematite oolites appear to be red, well-rounded sand grains or may be mistaken for lithified fish eggs.

Hematite occurs in a variety of igneous and metamorphic rocks, but is most abundant in sedimentary settings. In sedimentary rocks, hematite can either have formed from have originally formed directly from direct precipitation out of marine waters, or as a concentration and enrichment deposit formed from groundwater.

Regardless of its setting, hematite is usually found with other iron-bearing minerals, especially magnetite, goethite and siderite. In Early Proterozoic (2.5 to 1.6 billion year ago) iron ore deposits, layers of hematite and other iron oxides alternate with bands of chert (microcrystalline quartz) to form distinctively layered deposits known as Banded Iron Formation. These Banded Iron deposits formed from marine precipitation at a time when the chemistry of the Earth’s atmosphere and ocean systems was fundamentally different than they presently are. As photosynthetic organisms increased the concentration of free oxygen in Earth’s systems, such banded iron precipitation ceased.

With few exceptions, the economic uses of hematite revolve about its iron-rich composition and characteristic red color. Its name comes from ‘haima’, the Greek words for blood, a reference to hematite’s distinctive color. Other minerals, such as magnetite, contain higher concentrations of iron, but hematite is so much more abundant that it is the most economically important iron ore. In North America, over 90% of our iron comes from hematite deposits and without hematite our steel-based society could not exist. The use of iron and steel throughout the industrial world is so prevalent that it is difficult for many people to imagine a world without them.

Aside from iron and steel production, the most important historical and economic use of hematite has been as a pigment. Hematite gets its name from the Greek word ‘hamatitis’, which means blood-red, after the color of the mineral in its powdered form. Nearly every human society has used the earthy red ochre variety of hematite as a pigment for paints, glazes, facial and body decorations. Because of hematite’s abundance, red has always been one of the cheapest paint hues to produce, making it a logical choice for painting large structures. The low cost of hematite-stained red paint led to an American icon — the red barn.

In addition, the specular variety of metallic hematite is used as an ornamental stone, for engravings and in some jewelry, typically as beaded necklaces or rings. Collectors also prize some rare hematite varieties as mineral samples.

The Lake Superior region iron ranges of northern Minnesota, Wisconsin and Michigan held tremendous amounts of hematite, together with other iron ores such as magnetite, goethite and siderite. Although these iron ore deposits have been metamorphosed and enriched by groundwater leaching away more soluble materials, they all originated as marine sedimentary deposits in shallow Early Proterozoic (2.5 to 1.6 billion years ago) basins. The rise and fall of Iron Range mining has remained the economic heartbeat of the area’s regional economy but these deposits also played a more crucial historical role on the world stage. During WWII ore from the Iron Range provided most of the steel for the tanks, planes and ships of the Allied war effort. Without these deposits, world history may have taken a tragically different path.

Finely dispersed hematite also occurs throughout the Upper Midwest in a variety of red colored sedimentary rocks, soil layers and glacial tills, with the most famous example occurring in the southwest corner of Minnesota. Pipestone National Monument is centered about an outcrop of thin, weakly metamorphosed claystone, whose distinctive red color is due to finely dispersed hematite. This ‘pipestone’ was ideally suited for carving into pipes and other ornaments. It was quarried by native tribes for centuries and traded across much of the North American continent. According to Dakota legends, the deep red color of the pipestone, and much of its spiritual strength, came from the blood of many people killed in ancient battles. Finally the Great Spirit brought the pipe, and the peace it represented, to the warring tribes. Other cultures have also attributed the color of hematite deposits to ancient battles and blood flowing into the ground, but the American Indian legends appear to be the only ones in which the color has come to symbolize peace.

With all of hematite’s many varieties, it is natural that some of them can be easily confused with other metallic minerals. All of the hematite varieties though, exhibit the same distinctive red to red-brown streak when scratched across a rough hard surface, which should serve to distinguish hematite from similar-appearing minerals.

Magnetite:

Magnetite may be mistaken for the more metallic varieties of hematite, and to make matters worse, the two commonly occur mixed together. Pure samples of the two can be distinguished because magnetite is magnetic, while hematite is not.  Some hematite samples, however, do contain enough magnetite that they are weakly magnetic, so it is always a good idea to check a magnetic sample’s streak by scratching the sample across a white ceramic plate. Hematite will leave a red to red-brown streak on the plate, while magnetite will leave a black streak.

Ilmenite:

Ilmenite is a dense, black metallic mineral that can be confused with metallic hematite, but again the two mineral’s streak should serve to distinguish them. Ilmenite exhibits a black streak similar to that of magnetite, while hematite’s streak is red to reddish brown.

Graphite:

Graphite has a shiny metallic appearance and dark color so it may initially be confused with metallic hematite, but the two minerals’ other properties easily distinguish them. Metallic varieties of hematite are much harder than graphite, and have a higher specific gravity (they feel heavier for their size than graphite does). Graphite also has a distinctive greasy feel and is soft enough that it will leave marks on paper, as well as your fingers!

Goethite (a.k.a. Limonite):

Goethite is a hydrous (water-bearing) iron oxide that forms as a direct precipitate in marine water and bogs, and as an alteration of other iron-bearing minerals. Along with hematite, goethite is the other mineral that makes up most of the rust on natural and artificial iron exposures. As goethite’s origins can be similar to those of hematite, the two minerals often occur together and may be confused for one another. However, the two can be distinguished by their streak. Scratching a sample of Goethite across a white porcelain plate will leave a yellow to yellowish brown streak on the plate, while a hematite sample will leave a red to reddish brown streak.