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The Mineral Identification Key |  |
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The Mineral Identification Key | |
Table IID-2: Nonmetallic Luster; Hardness greater than 3 but less than 5½, Cleavage not prominent: (Can not be scratched by a copper penny, can be scratched by a knife, streak white or none.) [Previous Table] [Next Table]
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 2½ to 3 | Bright-yellow, Orange-yellow, Orange, Red, Grey, Green, White | Sub-adamantine to vitreous, may be greasy | WULFENITE PbMoO4 |
Tetragonal | Usually as thin platy to thin tabular crystals, rarely pyramidal, more rarely prismatic | 6.7 to 7.0 | |
| 2½ to 3 | Orange-red to Ruby-red, Brownish-red to Brownish-yellow or Pale Straw-yellow | Sub-vitreous to sub-resinous | VANADINITE (Apatite Group) Pb5(VO4)3Cl |
Hexagonal | Usually in barrel-shaped hexagonal crystals, long or short prismatic, may be acicular in clusters or mats ("endlichite"), and as hollow prisms – "hopper" crystals | 6.88 | Streak white, may be yellowish |
| 2½ to 4, rarely 5 to 6 in dense massive material | Apple-green to Yellow-green, Dark-green to Dark Grey-green, Greenish-black, White, often mottled | Sub-vitreous to greasy | ANTIGORITE/LIZARDITE/ AMESITE (Serpentine Group) (Fe,Mg)3(Si,Al)2O5(OH)4 Mg3(Si,Al)2O5(OH)4 Mg2Al(Si,Al)2O5(OH)4 |
Monoclinic, Orthorhombic and Triclinic | Usually as crystalline masses, often platy or columnar | 2.4 to 2.79 | White streak; feels greasy. These Serpentine minerals are almost impossible to tell apart, particularly in massive forms. They may coexist at the same locality. Specific locality information may be the best bet for the ID. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 3, but may be 2 across the top surface of prismatic crystals with a flat termination | Usually Clear or White to Tan or Grey, but may be tinted many colors | Vitreous to sub-vitreous | CALCITE CaCO3 |
Trigonal | May be in banded masses or stalactic ("flowstone"), granular masses (limestone and marble), or fibrous | 2.71 | White streak; effervesces in cold, dilute acid – even in vinegar or Coke to a small degree. Cleavage is difficult to observe in some massive forms. |
| 3 to 3½ | Colorless to White or Greyish-white, may be tinted other colors | Vitreous to sub-vitreous | WITHERITE (Aragonite Group) BaCO3 |
Orthorhombic, pseudo-hexagonal | In coarse fibrous aggregates, may be radiating, and granular, more rarely as globular to botryoidal clusters | 4.29 | Often hazed, will effervesce in cold acid. Cleavage is difficult to observe in some massive forms. |
| 3½ | Bright-green to Yellow-green, Yellow to Orange-yellow, Brown, White, colorless | Sub-vitreous to resinous | PYROMORPHITE (Apatite Group) Pb5(PO4)3Cl |
Hexagonal | Usually in barrel-shaped hexagonal crystals, may also be spindle-shaped, hollow – "hopper" crystals – and rarely tabular or pyramidal, crystals may exhibit concentric color or structural zones due to zoned variations in composition | 7.04 | White streak; difficult to distinguish from mimetite without tests for PO4 vs. AsO4. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 3½ to 4 | Pale-yellow to Yellow-brown, Orange-yellow to Orange-red, White or Colorless | Sub-vitreous to resinous | MIMETITE (Apatite Group) Pb5(AsO4)3Cl |
Hexagonal | Usually in simple barrel-shaped crystals, rarely tabular to acicular, may be botryoidal or globular | 7.28 | Streak white; Difficult to distinguish from pyromorphite without tests for AsO4 vs. PO4. Named for the fact that it closely mimics pyromorphite. |
| 3½ to 4 | Colorless to White or Grey, may be tinted other colors | Vitreous to sub-vitreous or resinous | ARAGONITE CaCO3 |
Orthorhombic | In columnar, radiating or stellate aggregates, also stalactic | 2.95 | Often fluorescent, pale rose, yellow or bluish-white, effervesces in cold acids. Distinguished from calcite by its greater hardness and higher S.G. Cleavage is difficult to observe in some massive forms. |
| 3½ to 4 | Colorless to White or Grey, may be tinted other colors | Vitreous to sub-vitreous, silky when fibrous | STRONTIANITE (Aragonite Group) SrCO3 |
Orthorhombic | In granular or fibrous aggregates, may be columnar, crystals short prismatic to acicular, often pseudo-hexagonal in cross-section (Ca-rich) | 3.76 | Effervesces mildly in cold acids |
| 3½ to 4½ | White to Grey, may be tinted Yellowish to Brownish | Vitreous to sub-vitreous, may be pearly on cleavage surfaces | MAGNESITE (Calcite Group) MgCO3 |
Trigonal | Usually massive, granular, crystals may be rhombohedral or prismatic but are rare | 3.0 | White streak; only slightly soluble in cold acids. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 3½ to 4 | White to Greyish-white, Reddish to Reddish-brown, Yellowish | Dull to sub-vitreous | ALUNITE KAl3(SO4)2(OH)6 |
Trigonal | Usually massive, granular to dense, associated in rock with quartz, kaolinite, etc | 2.6 to 2.9 | Large deposits formed by action of sulfate bearing hydrothermal fluids. Small deposits formed by sulfates formed by the weathering of pyrite. |
| 3½ to 4 | Yellowish-green, Green to Yellow, Yellow-brown, Brown, more rarely Blue, White, Colorless | Vitreous to pearly or resinous | WAVELLITE Al3(PO4)2(OH,F)3.5H2O |
Orthorhombic | Usually stellate or hemispherical radial fibers or acicular crystals | 2.36 | White streak; dissolves in cold acids with no effervescence. |
| 4 to 4½ | Greyish-white to Dark-grey, Greenish- or Brownish-white, may be Green to Apple-green, Blue to Blue-green, Yellow, Pink or Brown | Vitreous to sub-vitreous, may look somewhat porcelain-like | SMITHSONITE (Calcite Group) ZnCO3 |
Trigonal | Usually in botryoidal, reniform, and stalactic masses, rhombohedral crystals rare and usually crude with somewhat curved and rough surfaces, druzy | 4.2 (pure) | White streak; soluble in cold acids, though not with the vigorous effervescence of calcite |
| 4½ to 5 | Colorless to White, may be Pale-yellow, Pale-green, Sky-blue, Pale-brown | Vitreous, pearly on some cleavage surfaces | HEMIMORPHITE Zn4Si2O7(OH)2.H2O |
Orthorhombic | Usually in radiating clusters of acicular or somewhat flattened long prismatic crystals, may also be in thick botryoidal crusts of thick to almost fibrous radiating crystals | 3.4 to 3.5 | May resemble prehnite in its botryoidal form, but S.G. is higher. |
| 4½ to 5 | White to Colorless, Pale-yellow, Pale-green, Pale-orange | Vitreous to adamantine | SCHEELITE CaWO4 |
Tetragonal | Usually as grains or flakes, crystals rare and usually either platy to thin tabular or short-prismatic bipyramids | 6.1 | White streak; fluoresces intense blue-white, more rarely yellowish white. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 4½ to 5 | White, Pale-pink, Pale-tan, Pale-blue | Vitreous to silky in fibrous material | PECTOLITE (Wollastonite Group) NaCa2Si3O8(OH) |
Triclinic | Usually in acicular sprays or radial fibrous aggregates | 2.84 to 2.90 | Partly decomposed in acid forming a gel |
| 5 to 5½ | Colorless to White, Pale-pink, may be Grey or other colors | Usually vitreous to silky in finer acicular aggregates, but may be dull or even greasy | NATROLITE (Zeolite Group) Na2[Al2Si3O10].2H2O |
Orthorhombic | Usually as long slender needles in stellate clusters or radial aggregates, may also be in jackstraw clusters | 2.20 to 2.27 | White streak; dissolves in cold acids |
| 5 to 5½ | Colorless to White, may be Yellow, Pink, or other colors | Vitreous | ANALCIME (Zeolite Group) Na[AlSi2O6].H2O |
Polymorphous (Iso., Tet., Orth., Mon., Tric., Trig.) | Usually in cubic or trapezohedral crystals or fine-grained masses, more rarely in other crystal forms | 2.22 to 2.63 | Dissolves in cold acids |
| 5 to5½ | White, usually tinted Pale-green, may be Pale-blue | Vitreous to greasy | DATOLITE (Gadolinite Group) Ca2B2Si2O8(OH)2 |
Monoclinic | May be either platy to short prismatic or blocky crystals, more rarely as spherical aggregates or massive, granular to compact | 2.9 to 3.0 | White streak; may show an imperfect basal cleavage, may fluoresce |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 5 to 6 | White, Yellow, Red, Brown, Black | Vitreous | OPAL SiO2.nH2O |
Amorphous | Glassy massive material, as fracture fillings, coatings, "nodules," etc. | 1.9 to 2.1 | Distinguished from massive quartz by lower hardness and S.G. Precious opal has an intense internal play of colors – the fire comes from the natural diffraction grating of ordered spheres of a diameter approximately the size of a wavelength of light; in fire opal the flashes are predominantly reds, yellows and oranges against a black background. Common opal is "opalescent," but without the intense flashes of colors. |
| 5 to 6 | White to Grey, Yellowish, Brownish, Orange, Purple | Vitreous to subvitreous, may be dull | MARIALITE/ MEIONITE (Scapolite series) 3NaAlSi3O8. NaCl 3CaAl2Si2O8.CaCO3 |
Tetragonal | Usually massive, either columnar or fibrous, (columnar masses may exhibit prismatic cleavage surfaces), also as short to medium prismatic crystals with bipyramidal terminations, usually somewhat crude | 2.55 to 2.72 | May fluoresce yellow. It is almost impossible to tell the end members of this series apart without subtle chemical or optical tests. Intermediate member, wernerite, is probably the most common , though it is not recognized as a species. It is best to label samples of these materials simply as scapolite, unless specific locality information dictates otherwise. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 5 | White to Colorless, Greens, Blues, Lavender, Yellows, Purples | Vitreous to sub-vitreous, may be dull in massive material | FLUORAPATITE (Apatite Group) Ca5(PO4)3F |
Hexagonal | Variable, may be massive, compact or granular, and may be in short- to long-prismatic crystals with a wide variety of habits, the hexagonal prism with pyramidal termination(s) being the most common, but may also be short-prismatic and even bipyramidal without the prism in between, also acicular | 3.1 to 3.2 | White streak; Closely related members of the Group include chlorapatite, hydroxyl-apatite, carbonate-fluorapatite and carbonate-hydroxylapatite, which can be difficult to distinguish from fluorapatite – but they are rare |
| 5 to 5½ | Yellowish- or Reddish-brown to Brown, Pale-green to nearly White | Resinous to waxy | MONAZITE (Ce,La,Y,Th)PO4 |
Monoclinic | Usually in crude blocky to tabular crystals, may be large, also as rounded grains | 4.6 to 5.3, increasing with Th content | Streak very pale brown; may be radioactive. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 5 to 5½ | Light-green to Yellow-green, Yellow-brown to Reddish-brown, Colorless | Vitreous, occasionally resinous | WILLEMITE Zn2SiO4 |
Trigonal | Usually massively crystalline or granular, rarely in prismatic hexagonal crystals | 4.05 to 4.20 | Fluoresces bright green |
| 5½ to 6 | Medium-blue to Violet-blue, Greyish-white to White, more rarely Greenish- or Yellowish-white | Vitreous to subvitreous | SODALITE Na8Al6Si6O24Cl2 |
Isometric | Usually as granular masses or grains in matrix, sometimes crudely cleavable, crystals rare, usually dodecahedral to cubo-dodecahedral, rarely octahedral | 2.14 to 2.30 | White to very pale blue streak; may be somewhat dull in massive material, usually associated with nepheline – never with quartz. |
| 5 to 5½ | Deep-blue to Medium-blue or Violet-blue, may be Greenish-blue, Colorless | LAZURITE (Sodalite Group) |
Isometric, Triclinic, and Monoclinic polytypes | Usually as compact massive material or grains, crystals rare, usually well-formed dodecahedral to cubo-dodecahedral | 2.38 to 2.45 | Luster dull to vitreous, usually found with pyrite |
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