THE MINERAL MAGNETITE

  • Chemical Formula: Fe3O4, Iron Oxide
  • Class: Oxides and Hydroxides
  • Group: Spinel
  • Uses: Major ore of iron and as mineral specimens
  • Specimens

Magnetite is an oxide of iron (as is hematite). It is not a component of ordinary rust, although it can form as iron oxidizes in a dry environment. When you see sparks from welding operations or from iron striking a hard surface (as when it is held against a spinning grinder), the sparks are due to the rapid (and exothermic) oxidation of tiny particles of iron into magnetite.

Magnetite is a natural magnet, hence the name, giving it a very nice distinguishing characteristic. Explaining the magnetism is not easy but here is a go at it. Remember, electricity produces magnetic fields just as magnetism produces electric fields. Magnetite is a member of the spinel group which has the standard formula A(B)2O4. The A and B represent usually different metal ions that occupy specific sites in the crystal structure. In the case of magnetite, Fe3O4, the A metal is Fe +2 and the B metal is Fe +3; two different metal ions in two specific sites. This arrangement causes a transfer of electrons between the different irons in a structured path or vector. This electric vector generates the magnetic field.

Magnetite is the primary oxide of iron found in meteorites (and thus presumably in asteroids).

Thermite is a mixture of powdered magnetite and powdered aluminum. When ignited, the aluminum fuel burns using the magnetite as an oxidizer, resulting in much heat, aluminum oxide (corundum), and molten iron.

PHYSICAL CHARACTERISTICS:

  • Color is black.
  • Luster is metallic to dull.
  • Transparency: Crystals are opaque.
  • Crystal System is isometric; 4/m bar 3 2/m
  • Crystal Habits are typically octahedrons but rarely rhombododecahedron and other isometric forms, most commonly found massive or granular. Twinning of octahedrons into spinel law twins is seen occasionally.
  • Cleavage is absent although octahedral parting can be seen on some specimens.
  • Fracture is conchoidal.
  • Hardness is 5.5 - 6.5
  • Specific Gravity is 5.1+ (average for metallic minerals)
  • Streak is black.
  • Associated Minerals are talc and chlorite (schists), pyrite and hematite.
  • Other Characteristics: Magnetism stronger in massive examples than in crystals, striations on crystal faces (not always seen).
  • Notable Occurrences include South Africa, Germany, Russia and many localities in the USA.
  • Best Field Indicators are magnetism, crystal habit and streak.
MAGNETITE specimens:
(hover for more info)
MAGNETITE specimen mag-2
$ 125.00
Dims: 2-3/4" x 2-1/2" x 1-1/2"
Wt: 7.1 oz
Republic Mine, Hanover, Grant Co., New Mexico, U.S.A.
There are many, many crystals of Magnetite on this specimen! As a matter of fact, apart from some surface rust, the specimen is made up entirely of Magnetite. The largest complete crystal measures 1/2" x 1/2" x 11/16", and all complete crystals have relatively clean faces with minimal damage and some interesting growth patterns. They are black, needless to say, and exhibit a submetallic luster. There is a lot of material here-- almost a half a pound, or about 1/5 kilo.
no photo
mag-2 ($125.00)
Republic Mine, Hanover, Grant Co., New Mexico, U.S.A.
MAGNETITE specimen mag-3
$ 180.00
Dims: 4-1/4" x 3-1/4" x 2-1/2"
Wt: 1 lb., 13.1 oz
Republic Mine, Hanover, Grant Co., New Mexico, U.S.A.
This particular specimen has more Magnetite crystals on it than any that I have seen; I don't think that I could count them all and still remain sane! The entire specimen is made up of Magnetite; only a small portion occurrs in a massive form, whereas the bulk of the specimen consists of intergrown octahedral crystals of varying size(1/16 to over 3/8" along an edge). There is noticeable damage to some of the crystals, but these tend to be small ones. Amidst the crystals are rusty patches of oxidation. They have good form as a rule and a rather dull luster and the inevitable black color. There are LOTS of crystals on both the "top" and "bottom" of this specimen, so a rotating stand may be the best way to expose all of them to the viewer.
no photo
mag-3 ($180.00)
Republic Mine, Hanover, Grant Co., New Mexico, U.S.A.
MAGNETITE specimen mag-4
$ 85.00
Dims: 3-1/4" x 2" x 1-1/2"
Wt: 6.26 oz
Republic Mine, Hanover, Grant Co., New Mexico, U.S.A.
I consider this to be a "tasteful" specimen. A few dozen octahedral Magnetite crystals line a shallow crevice in the specimen, which itself is made up almost entirely of Magnetite that ranges from crystalline to massive in form. The color is basic black with a low, waxy luster. There are a few small rusty patches on the piece, along with a patch of a white powdery substance that doesn't look like it belongs, but seems to, anyway. Crystals range in size from about 1/16 to 1/2 inch, with many(including the largest) showing damage, especially at points where the specimen was separated from the surrounding material. The crystals appear to be somewhat worn or weathered, judging from the patterns on their faces.
no photo
mag-4 ($ 85.00)
Republic Mine, Hanover, Grant Co., New Mexico, U.S.A.
MAGNETITE specimen mag-5
$ 44.00
Dims: 3.8" x 2.8" x 1.5"(9.7 x 7.1 x 3.8 cm)
Wt: 10.18 oz.(288.6 g)
Minas Gerais, Brazil
At least 8 octahedral Magnetite crystals can be seen sticking out of the green schist matrix of this specimen. Their size ranges from 0.1 - 0.5"(0.3 - 1.3 cm) along an edge, and they have excellent form, with clean faces and edges and only one small crystal showing any damage. Their color is black, though some of the larger crystals show iridescence on their faces. They have a pearly to waxy luster and are, of course, opaque. The green schist matrix rock is massive, and likely contains more of these lovely crystals. I may have to beg Steve and Doris to let me buy this specimen so that I can go to work on it!
no photo
mag-5 ($ 44.00)
Minas Gerais, Brazil
MAGNETITE specimen mag-6
$ 50.00
Dims: 4.9" x 3.6" x 1.5"(12.4 x 9.1 x 3.8 cm)
Wt: 1 lb., 3.5 oz.(554 g)
Minas Gerais, Brazil
This specimen consists of a large green schist matrix rock in which are 3 visible Magnetite crystals. Two of them do not exceed 0.3"(8 mm) along an edge, but the largest( and most exposed) crystal is huge in comparison, measuring from 0.6 - 0.8"(1.5 - 2.0 cm) along various edges. All display excellent octahedral form, and the only damage to be found is on the largest crystal; there is a small notch in one edge, and some minor damage on one face that appear to have occurred before the specimen was isolated from its place of origin. There are likely more of these excellent crystals in the green schist matrix that are waiting to be exposed.
no photo
mag-6 ($ 50.00)
Minas Gerais, Brazil
MAGNETITE specimen mag-7
$ 31.00
Dims: 1.5" x 0.7" x 0.6" (3.8 x 1.8 x 1.5 cm)
Wt: 19.1 g
Serro, Minas Gerais, Brazil
This small thumbnail consists of a narrow, shapeless column of green-gray mica-schist host rock that holds a single octahedral Magnetite crystal at one end. This crystal is in good condition, though it is missing one point close to the host rock. It has excellent form with well-defined edges and clean faces that show a metallic luster. It has a dark gray coloration and there are small patches of rust scattered on a few of the visible faces. The piece stands upright, and would look very good in a domed thimble box, which I have included.
no photo
mag-7 ($ 31.00)
Serro, Minas Gerais, Brazil
MAGNETITE specimen mag-9
$ 64.00
Dims: 2.6" x 2.3" x 1.4" (6.6 x 5.8 x 3.6 cm)
Wt: 3.86 oz. (109.5 g)
Serro, Minas Gerais, Brazil
The finely-grained, green micaceous host rock of this specimen holds at least 2 Magnetite crystals, one of which is highly visible. It has the dark gray coloration and metallic luster that is common for this mineral, and shows an excellent though somewhat odd octahedral form. It has dimensions of 0.6 x 0.5 x 0.4" (1.5 x 1.3 x 1.0 cm) and looks as if it has been "stretched" so that it is longer along one axis than the others. Thus, what would normally have been 2 points on opposing sides of the octahedron are actually 2 edges! Nonetheless, it is in perfect condition, showing no visible damage whatsoever. About 1" (2.5 cm) from this crystal, a tiny metallic flash can be seen where a very small portion of another Magnetite crystal has been exposed. Some careful work with a steel dental probe or pick will expose this other crystal.
no photo
mag-9 ($ 64.00)
Serro, Minas Gerais, Brazil
MAGNETITE specimen mag-10
$ 70.00
Dims: 3.3 x 2.3 x 1.7" (8.4 x 5.8 x 4.3 cm)
Wt: 8.06 oz. (228.7 g)
Republic Mine, Hanover, Grant County, New Mexico, U.S.A.
This hand specimen is made up almost entirely of Magnetite, and consists of at least 40 octahedral crystals that are partially embedded in a massive host. The crystals range in diameter from 1 mm to 0.4" (1.0 cm) and are in excellent condition, showing very little damage. Though several of them are noticeably weathered, most have excellent form, with well-defined edges and clean faces that show a dull, submetallic luster. All have a black color and are completely opaque, and portions of the massive base show substantial rust-staining.
no photo
mag-10 ($ 70.00)
Republic Mine, Hanover, Grant County, New Mexico, U.S.A.
MAGNETITE specimen mag-11
$ 45.00
Dims: 2.1 x 1.9 x 1.4" (5.3 x 4.8 x 3.6 cm)
Wt: 4.70 oz. (132.8 g)
Republic Mine, Hanover, Grant County, New Mexico, U.S.A.
This small hand specimen consists entirely of a cluster of intergrown Magnetite octahedrons that have grown out of a more massive Magnetite base. These crystals do not exceed 0.4" (1.0 cm) along any axis and are generally in very good condition, as damage is limited to the edges of the cluster. All have good form, with well-defined edges and clean faces that possess a rather dull waxy luster. They have the standard black coloration for this type of material and are, of course, completely opaque. There is no host rock present on the specimen, but small areas show definite weathering and decomposition in the form of patches of limonite.
no photo
mag-11 ($ 45.00)
Republic Mine, Hanover, Grant County, New Mexico, U.S.A.
MAGNETITE specimen mag-12
$ 28.00
Dims: 2.1 x 1.6 x 1.5" (5.3 x 4.1 x 3.8 cm)
Wt: 5.5 oz. (155.2 g)
Republic Mine, Hanover, Grant County, New Mexico, U.S.A.
This small hand specimen is made up almost entirely of Magnetite. It is basically an almost formless chunk of Magnetite that is made up of heavily broken, intergrown crystals. There is a depression in it, however, that has protected the crystals in and around it. The largest of these has dimensions of 0.4 x 0.3 x 0.3" (1.0 x 0.8 x0.8 cm) and have good octahedral form, with well-defined edges and relatively clean faces that possess a dull, submetallic luster. All of the magnetite is black in coloration, but there are a few patches of a dull, pale brown, powdery material coating part of the specimen; it is very thin, though.
no photo
mag-12 ($ 28.00)
Republic Mine, Hanover, Grant County, New Mexico, U.S.A.
MAGNETITE specimen mag-13
$ 25.00
Dims: 0.7 x 0.6 x 0.6" (1.8 x 1.5 x 1.5 cm)
Wt: 19.9 g w/ specimen box
Illogo, Central Afican Republic
A single Magnetite octahedron makes up this thumbnail specimen. It is in very good condition, showing no fresh damage, and has good octahedral form, though a few corners are rounded. Its black color is standard for its species, and it has an unusual, silky sheen on its surfaces. A few tiny bits of a rust-stained crust rest on it, but there is no host rock present. It is affixed inside a plastic specimen box with an adhesive putty.
no photo
mag-13 ($ 25.00)
Illogo, Central Afican Republic
MAGNETITE specimen mag-14
$ 26.00
Dims: 0.9 x 0.7 x 0.7" (2.3 x 1.8 x 1.8 cm)
Wt: 24.9 g w/ specimen box
Illogo, Central Afican Republic
This Magnetite octahedron is in good condition, showing a small spot of what appears to be fresh damage on one tip, and a spot of healed damage on the opposing tip. It has excellent octahedral form and shows the standard black color of its species. Its silky luster is unusual, and makes me think that each surface is patterned at a microscopic level. There is no host rock present, and the piece is affixed inside a plastic specimen box.
no photo
mag-14 ($ 26.00)
Illogo, Central Afican Republic
MAGNETITE specimen mag-15
$ 125.00
Dims: 1.6 x 1.5 x 0.8" (4.1 x 3.8 x 2.0 cm)
Wt: 2.1 oz. (59 g)
Dashkesan, Azerbaijan
Several Magnetite crystals rest together on the dull, brown base of this specimen. These crystals are in excellent condition, showing little fresh damage, and measure up to nearly 0.9" (2.3 cm) in diameter. Their dodecahedral form is extremely unusual- I have never seen it before in Magnetite- and it is very good. The crystals have the standard dark silvery-gray color and dull metallic luster of their specie and are accompanied by several tiny quartz crystals.
no photo
mag-15 ($125.00)
Dashkesan, Azerbaijan
MAGNETITE specimen mag-16
$ 48.00
Dims: 1.4 x 0.6 x 0.5" (3.6 x 1.5 x 1.4 cm)
Wt: 17 g w/ specimen box
Diamantina, Minas Gerais, Brazil
A single Magnetite octahedron is partly embedded in the mica schist matrix of this thumbnail specimen. The crystal has dimensions of 0.4 x 0.4 x 0.4" (1.1 x 1.1 x 0.9 cm) and is in excellent condition, showing no damage. Though its octahedral form is not perfectly symmetrical, it is still excellent. It has the standard dark gray color and dull, submetallic luster. The piece is hot-glued into a plastic thumbnail box.
no photo
mag-16 ($ 48.00)
Diamantina, Minas Gerais, Brazil
MAGNETITE specimen mag-17
$ 29.00
Dims: 1.1 x 1.0 x 0.7" (2.8 x 2.5 x 1.8 cm)
Wt: 26 g
Serro, Minas Gerais, Brazil
This thumbnail specimen consists of a single Magnetite crystal. It is in good condition- most damage is old and partly healed- and has excellent, though rather oblong, octahedral form. It has the standard dark gray color and dull metallic luster, and has only a tiny amount of green mica schist attached to it.
no photo
mag-17 ($ 29.00)
Serro, Minas Gerais, Brazil
MAGNETITE specimen mag-19
$ 43.00
Dims: 1.8 x 1.4 x 1.1" (4.6 x 3.4 x 2.9 cm)
Wt: 1.6 oz. (45 g)
Hanover Mine, Grant County, New Mexico, U.S.A.
The crystals in this small Magnetite cluster reach up to 0.4" (1.1 cm) in diameter and are in excellent condition, showing almost no fresh damage. All have excellent octahedral form and show the standard black color and dull, submetallic luster. There is no host rock present.
no photo
mag-19 ($ 43.00)
Hanover Mine, Grant County, New Mexico, U.S.A.
MAGNETITE specimen mag-18
$ 25.00
Dims: 1.0 x 0.9 x 0.9" (2.5 x 2.4 x 2.4 cm)
Wt: 0.6 oz. (18 g)
unknown (likely Brazil)
A single Magnetite crystal comprises this thumbnail specimen. It is in moderately good condition, showing light, fresh damage on almost all corners, and has excellent octahedral form, though its faces are slightly pitted. These pits contain some dull, powdery rust, but the crystal has the standard dark gray color and dull metallic luster. There is no host rock present.
no photo
mag-18 ($ 25.00)
unknown (likely Brazil)
MAGNETITE specimen mag-20
$ 60.00
Dims: 2.6 x 2.2 x 1.7" (6.6 x 5.6 x 4.3 cm)
Wt: 11.3 oz. (321 g)
Kara Mine, Tasmania, Australia
A druse of intergrown Magnetite crystals rests on the more massive Magnetite base of this cabinet specimen. There are scores of these crystals; all are generally in good condition and reach diameters of nearly 0.3" (7 mm). All have excellent dodecahedral form, with well-defined edges and clean faces- a few appear to be elongated along one axis. Their black color and submetallic luster are standard for the specie. I am told that this particular form is somewhat rare for Magnetite.
no photo
mag-20 ($ 60.00)
Kara Mine, Tasmania, Australia
MAGNETITE specimen mag-22
$ 67.00
Dims:2.4x2.1x1.7" (6.1x5.3x4.3 cm)
Wt: 7.1oz. (202g)
Cedar City, Iron cty., Utah
This is an intergrown cluster of octahedral magnetite crystals with fluorapatite. The magnetite crystals reach 1.1" (2.8cm) on edge. There are two fluorapatite crystals growing up through the magnetite. One of these is terminated; the other one has been broken off. One of the larger magnetite crystals has a moderately damaged face. The remainder of the specimen only shows damage on the edges. This piece is firmly glued to a clear acrylic base.
no photo
mag-22 ($ 67.00)
Cedar City, Iron cty., Utah
MAGNETITE specimen mag-23
$ 75.00
Dims:3.2x1.5x0.8" (8.1x3.8x2.0 cm)
Wt: 3.3oz. (93g)
Chester, Vermont
Several octahedral magnetite crystals are embedded in a soft biotite mica schist in this specimen. These crystals reach 0.2" (0.5cm) on edge. At least four of these crystals have damaged tips, probably due to careless excavation.
no photo
mag-23 ($ 75.00)
Chester, Vermont
MAGNETITE specimen mag-24
$ 40.00
Dims:2.9x2.0x1.5" (7.4x5.1x3.8 cm)
Wt: 9.3oz. (263g)
Korodv, Russia
This specimen consists of a cavity in a semi-crystalline matrix of magnetite. Large octahedral magnetite crystals grew in this cavity, which was then filled with white massive calcite. The exposed magnetite crystals range in size up to 0.8" (2.0cm) on edge. There is some damage to this specimen, primarily along the edges, where it was broken away from the host rock.
no photo
mag-24 ($ 40.00)
Korodv, Russia
MAGNETITE specimen mag-25
$ 30.00
Dims:4.1x2.3x1.0" (10.4x5.8x2.5 cm)
Wt: 8.5oz. (240g)
Kcac Mine, San Benito cty., California
Hundreds of tiny to small (0.1", 0.3cm) crystals of magnetite dot the top surface of this specimen. These crystals are primarily octahedral, although I found two that display cubic faces. There is no damage to this specimen.
no photo
mag-25 ($ 30.00)
Kcac Mine, San Benito cty., California
MAGNETITE specimen mag-26
$ 25.00
Dims: 1.7 x 1.3 x 0.5" (4.3 x 3.4 x 1.3 cm)
Wt: 0.8 oz. (21 g)
Serro, Minas Gerais, Brazil
The finely-grained, green mica schist host rock of this specimen holds a single well formed magnetite crystal. It has the dark gray coloration and metallic luster that is common for this mineral, and shows an excellent though somewhat odd octahedral form, nearly stretched into a tabby form. The faces of the magnetite have round green crystalline patches which look like wavelite, but which I believe are just more green mica. There is also a nice colorful irridescence on all of the magnetite surfaces.
no photo
mag-26 ($ 25.00)
Serro, Minas Gerais, Brazil
MAGNETITE specimen mag-27
$ 36.00
Dims: 4.52x2.32x1.65" (11.49x5.90x4.19cm)
Wt: 15.55oz (440g)
Potosi, Bolivia
This specimen displays perhaps a hundred magnetite crystals scattered over one surface of a pale host rock. The magnetite crystals are black and vitreous, as octahedrons. Oddly enough, many (perhaps most of the larger crystals) are cracked - I can think of nothing that might cause this other than a rapid temperature change. The host rock is a conglomerate consisting of a cream colored granular mineral, tiny magnetite crystals, and rounded distorted colorless crystals that I believe are quartz - one of them displays a conchoidal fracture. They may be a glass, of course, as this is likely a volcanic rock.
no photo
mag-27 ($ 36.00)
Potosi, Bolivia
MAGNETITE specimen mag-28
$ 72.00
Dims: 4.77x2.85x1.87" (12.13x7.23x4.76cm)
Wt: 12.95oz (367g)
Bolivia
One surface of this specimen is covered with excellent crystals of magnetite, all as nearly perfect octahedrons. They have a steel gray color and vitreous luster. Many of the larger crystals do have some damage, although the specimen is still a good one. The host rock is volcanic with little blobs of glassy material scattered throughout. It has a crack which has filled with more magnetite, cementing the two pieces together.
no photo
mag-28 ($ 72.00)
Bolivia
 

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