Which minerals are magnetic
Of 176 Handbook of Mineralogy sheets, 5 record a magnetic property: magnetite, pyrrhotite, greenalite, pyrite and borax. Hematite, 69.94% iron, records none.
Which minerals are magnetic? Of the 176 Handbook of Mineralogy sheets in this site's reference set, 5 record a magnetic property and 3 use the plain word magnetic: magnetite, strongly, hardness 5.5 to 6.5; pyrrhotite; and greenalite. Pyrite is recorded as paramagnetic, borax as diamagnetic. Iron content does not predict it: hematite, 69.94% iron, records nothing.
In short
- Five of the 176 sheets in this site's reference set record a magnetic property. Magnetite reads strongly magnetic, greenalite moderately magnetic, pyrrhotite magnetic, varying in intensity inversely with iron content, pyrite paramagnetic and borax diamagnetic. The other 171 are silent on magnetism.
- A mention of 'magnet' is not a magnetic property. The string appears on 21 sheets. Sixteen use it only for magnetite as an associate (10 sheets), Magnet Cove in Arkansas (4) or the Magnet mine in Tasmania (2).
- Iron content does not predict the record. Worked from the formulas: magnetite is 72.36% iron and strongly magnetic; hematite is 69.94% iron and has no magnetic entry; siderite, 48.20%, has none; pyrite, 46.55%, is only paramagnetic. Pyrrhotite runs the other way, more magnetic with less iron.
- Is galena magnetic? Is stibnite, sphalerite, chalcopyrite, cassiterite, rhodochrosite or epidote? All seven have sheets in the set, and none records a magnetic property. A response from one of those specimens is information about something else in it, which is the reasoning worked through on the tetrahedrite page.
- Is hematite magnetic? Its sheet says nothing either way, which is absence from the source rather than a measurement. A physics answer from the University of Illinois describes hematite as weakly magnetic. Streak separates it from magnetite without a magnet: cherry-red against black.
| Species | Formula | Iron by mass, calculated | Magnetic property as recorded | Hardness | Streak |
|---|---|---|---|---|---|
| Magnetite | Fe3O4 (IMA: Fe2+Fe3+2O4) | 72.36% | Strongly magnetic | 5.5 to 6.5 | Black |
| Hematite | Fe2O3 | 69.94% | None recorded | 5 to 6 | Cherry-red or reddish brown |
| Pyrrhotite | Fe1-xS, x = 0 to 0.17 (IMA: Fe7S8) | 59.11 to 63.53% (Fe7S8: 60.38%) | Magnetic, varying in intensity inversely with iron content | 3.5 to 4.5 | Dark greyish black |
| Goethite | FeO(OH) | 62.85% | None recorded | 5 to 5.5 | Brownish yellow, yellow-orange, ochre-yellow |
| Fayalite | Fe2SiO4 | 54.81% | None recorded | 6.5 to 7 | White |
| Siderite | FeCO3 | 48.20% | None recorded | 3.75 to 4.25 | White |
| Pyrite | FeS2 | 46.55% | Paramagnetic; a semiconductor | 6 to 6.5 | Greenish black to brownish black |
| Chalcopyrite | CuFeS2 | 30.43% | None recorded | 3.5 to 4 | Greenish black |
| Greenalite | (Fe2+,Fe3+)2-3Si2O5(OH)4 | Variable, not calculated | Moderately magnetic | Not determined | Not recorded |
| Borax | Na2B4O5(OH)4·8H2O | 0% | Diamagnetic | 2 to 2.5 | White |
Which minerals are magnetic? What 176 reference sheets record, and how we counted
Method. Every one of the 176 Handbook of Mineralogy sheets in this site's reference set was converted with pdftotext -layout and searched, case-insensitively, for the string magnet, which catches magnetic, paramagnetic, diamagnetic, magnetite and place-names alike. It occurs on 21 sheets. Restricting the search to the Physical Properties field, where the Handbook records behaviour, leaves 5. The search needs no ligature handling, because magnet contains no fi, fl or ff.
The five, in the sheets' own words. Magnetite: Strongly magnetic. Greenalite: Moderately magnetic. Pyrrhotite: Magnetic, varying in intensity inversely with iron content. Pyrite: Paramagnetic; a semiconductor. Borax: diamagnetic, at the end of a line about solubility. Only the first three use the plain word magnetic, and greenalite is a fine-grained mineral of banded iron formations, recorded as grains to 1 mm, that few collectors will ever test. In practice, the two magnetic species in this set that a collector is likely to test are magnetite and pyrrhotite.
The other sixteen. Ten sheets name magnetite as an associate: amesite, antigorite, cobaltite, dolomite, fayalite, forsterite, hematite, labradorite, phlogopite and sanidine. Four cite Magnet Cove, Hot Spring County, Arkansas, in their distribution: benitoite, microcline, monticellite and vermiculite. Two, cerussite and crocoite, cite the Magnet mine in Tasmania. Counting the string would have given 21; counting the property gives 5. The set is the site's reference set, not every mineral species, so read the 5 as a count of what these 176 sheets record.
Is hematite magnetic? Iron content is not the answer
Hematite is 69.94% iron by mass and its Handbook sheet records no magnetic property; magnetite is 72.36% iron and strongly magnetic. The arithmetic uses standard atomic weights, iron 55.845 and oxygen 15.999: Fe2O3 has a formula mass of 159.69, of which iron is 111.69; Fe3O4 has 231.53, of which iron is 167.54. A gap of 2.42 points of iron separates a mineral the sheet calls strongly magnetic from one on which it is silent.
Pyrrhotite makes the same point from the other side. Its sheet gives the formula as Fe1-xS with x from 0 to 0.17, which is 63.53% iron at one end and 59.11% at the other, and the IMA list gives Fe7S8, 60.38%. The sheet records that its magnetism varies inversely with iron content: the pyrrhotite with less iron is the more magnetic one. Siderite, 48.20% iron, records nothing; pyrite, 46.55%, records only paramagnetism. Whatever decides magnetism, it is not the percentage of iron, which is why a magnet test is a test of species and not of iron.
How to separate hematite from magnetite without a magnet: streak. Hematite's sheet gives cherry-red or reddish brown, magnetite's gives black, and the method is on how to use a streak plate. Hardness does not help, at 5 to 6 against 5.5 to 6.5. Botryoidal kidney-ore hematite and its streak are on the hematite species page; pyrrhotite's magnetism and its tendency to decay are on the pyrrhotite page.
Is galena magnetic? The species people ask about, one line each
None of these seven sheets records a magnetic property, so the answer to each question below is no, on the record. Each has its own sheet in the set, and the hardness is given so you can tell the species from what else might be in the specimen.
- Is galena magnetic? No magnetic property recorded; PbS, hardness 2.5 to 2.75, perfect cubic cleavage. See galena, and whether galena is toxic.
- Is stibnite magnetic? None recorded; Sb2S3, hardness 2, perfect and easy cleavage on {010}.
- Is sphalerite magnetic? None recorded, although the sheet's formula is (Zn,Fe)S and it may carry iron.
- Is chalcopyrite magnetic? None recorded; CuFeS2, 30.43% iron, hardness 3.5 to 4. See chalcopyrite.
- Is cassiterite magnetic? None recorded; SnO2, hardness 6 to 7. See cassiterite.
- Is rhodochrosite magnetic? None recorded; MnCO3, hardness 3.5 to 4. See rhodochrosite.
- Is epidote magnetic? None recorded; hardness 6 to 7. See epidote.
If one of them pulls a magnet anyway, the magnet is reporting a second mineral, steel from a tool, or a misidentification. The test sequence for that, localise the pull first and then test the grain, is set out on is tetrahedrite magnetic and applies unchanged to every species in this list, so it is not repeated here. The same caution appears on how to tell if a rock is a meteorite, where a magnet sticking to a heavy black rock proves little and the streak plate does the work.
The rest of the identification material is indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.