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The Mineral Identification Key |  |
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The Mineral Identification Key | |
Table IIIA-2: Nonmetallic Luster; Hardness Greater Than 5½ and Less Than 7; Cleavage not prominent. (Can not be scratched by a knife blade, but can be scratched by quartz) [Previous Table] [Next Table]
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 5 to 5½ | Colorless to 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. |
| 5 to 6 | White, Yellow, Red, Brown, Black | Glassy | OPAL SiO2.nH2O |
Amorphous | Massive, 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 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 color. |
| 5 to 5½ | Azure-blue to Sky-blue, more rarely Bluish-white to Bluish-green | Vitreous to sub-vitreous | LAZULITE/ SCORZALITE MgAl2(PO4)2 (OH)2 FeAl2(PO4)2(OH)2 |
Monoclinic | Usually as finely crystalline crusts or granular, crystals rare and usually millimeter size, acutely pyramidal, tabular | 3.08 to 3.38 (increases with Fe content) | White streak; Rare. |
| 5 to 6 | Sky-blue to Bluish-green to Apple-green, Green-grey | Vitreous to sub-vitreous | TURQUOISE CuAl6(PO4)4(OH)8.4H2O |
Triclinic | Usually massive in crusts and fracture fillings, dense to finely crystalline, crystals rare, short-prismatic | 2.6 to 2.8 | White to pale-green streak, may exhibit a perfect cleavage (pinacoidal), but rarely seen. Massive material may test softer due to granular/earthy texture |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 5 to 6 | White to Grey, Yellowish, Brownish, Orange, Purple | 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 apart without subtle chemical or optical tests. An intermediate member, wernerite, is probably the most common chemical form found – though it is not recognized as a species. It is probably best to label samples of these materials simply as scapolite, unless specific locality information dictates otherwise. | |
| 5½ | Light-green to Yellow-green, Yellow-brown to Reddish-brown, Colorless | Vitreous, to resinous | WILLEMITE (Phenakite Group) Zn2SiO4 |
Trigonal | Usually massively crystalline or granular, rarely in prismatic hexagonal crystals | 4.05 to 4.20 | Fluoresces bright green |
| 5½ to 6 | Greyish-white to White, Colorless | Vitreous to sub-vitreous, may be dull | LEUCITE K(AlSi2O6) |
Tetragonal and Isometric | Usually massive, granular, disseminated grains, crystals equant or blocky (soccer ball shaped), often multiply twinned | 2.45 to 2.50 | Restricted to mafic and ultramafic volcanic and hypabyssal rocks. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 5½ to 6 | White, often tinted yellowish or greenish, Grey, Reddish-brown | Vitreous to greasy | NEPHELINE (Na,K)AlSiO4 |
Hexagonal | Usually as crystalline grains or massively crystalline, crystals rare, hexagonal prisms with pinacoidal or pyramidal terminations | 2.55 to 2.67 | May exhibit a distinct prismatic cleavage in massively crystalline material, but rarely seen. |
| 5½ to 6 | Black to Light-brown | Resinous or pitchy, may appear sub-metallic | ALLANITE-(Ce) (Epidote Group) (Ce,Ca,Y)2(Al,Fe)3(SiO4)3(OH) |
Monoclinic | Usually massive, may be platy, metamict, crystals tabular to prismatic to acicular | 3.4 to 4.2 | May give a light-brown streak, Allanite-(La) and allanite-(Y) are closely related species, but very rare. |
| 5½ to 6 | Brown, Yellowish-brown, Reddish-brown; Dark-brown to Iron-black; | Metallic adamantine to submetallic | BROOKITE TiO2 |
Orthorhombic | Usually tabular, elongated and striated crystals, commonly pyramidal or pseudohexagonal | 4.08 to 4.18 | Found in alpine veins in gneiss and schist. Also found in contact metamorphic and hyrothermal veins |
| 5½ to 6½ | Colorless to White or Greyish-white, Pale-tan | Vitreous | EUCRYPTITE (Phenakite Group) LiAlSiO4 |
Trigonal | Usually coarsely crystalline, granular or compact, crystals equant | 2.66 | Soluble in acid producing silica gel, fluoresces bright pink to red. |
| 5 to 6½ | Green or Chartreuse, Yellow to Yellow-green to Yellow-brown to Brown, Pink to Red, Black, White, Purple, Blue | Vitreous to sub-vitreous | VESUVIANITE ("Idocrase") |
Tetragonal | Usually as stout prismatic crystals | 3.32 to 3.43 | Usually restricted to skarns, rodingites, and certain alkali syenites, and calc-silicate rocks. |
| 6 to 6½ | Pale-yellow, Brownish-red to Reddish- or Greenish-brown, Pale-orange | Vitreous to greasy | CHONDRODITE (Humite Group) (Mg,Fe)5(SiO4)2(F,OH)2 |
Monoclinic | Usually in equant crystalline grains, crystals equant, blocky | 3.1 to 3.23 | Soluble in acids producing silica gel, may fluoresce yellowish-white to yellow. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 6 to 6½ | Yellow to Dark-orange, Reddish-orange | Vitreous to sub-vitreous | HUMITE (Mg,Fe)7(SiO4)3(F,OH)2 |
Orthorhombic | Usually in equant crystalline grains, crystals extremely rare, also equant, blocky | 3.20 to 3.32 | Soluble in acids producing silica gel. Relatively rare. |
| 6 to 6½ | Pale- to Medium-green, Pale-yellow, Tan to Pinkish-tan, Grey to White | Vitreous to sub-vitreous, may be somewhat pearly on freshly broken exposures | PREHNITE Ca2Al2Si3O10(OH)2 |
Orthorhombic | Usually botryoidal to mammillary aggregates encrusting matrix, also stalactic and as radiating "bowtie" or "hourglass" aggregates, crystals extremely rare, short-prismatic to tabular | 2.90 to 2.95 | Slowly soluble in HCl producing silica gel. Often associated with Zeolites in traprock. |
| 6 to 6½ | Dark Reddish-brown to Black | Usually sub-adamantine to adamantine, but may also be metallic | RUTILE TiO2 |
Tetragonal | Usually as prismatic to acicular crystals, often reticulated, may be vertically striated | 4.18 to 5.25 | Often as an inclusion in quartz |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 6½ | Black | Metallic to submetallic | PYROLUSITE (Rutile Group) MnO2 |
Tetragonal | Usually in earthy masses with a much lower hardness (2), but actual crystals are 6½, rare, short-prismatic to equant, usually in druzes of small crystals | 5 | May exhibit one perfect cleavage, prismatic. Most dendrites are not pyrolusite. |
| 6 to 7 | Black to Dark-brown, may also be Yellowish-grey, more rarely Red, White or Colorless | Adamantine to metallic in crystals, greasy on fracture surfaces, may be earthy or submetallic in botryoids, concretions, and massive forms | CASSITERITE (Rutile Group) SnO2 |
Tetragonal | Usually massive as botryoidal crusts or concretions ("wood tin"), crystals usually short prismatic and complexly twinned producing unusual shapes ("knees," stubby five-pointed "stars," etc.) | 6.8 to 7.1 | |
| 6½ to 7 | Grey to Bluish-grey, Brown to Honey-brown or Yellow to Golden-brown, more rarely Green or Violet | Vitreous to sub-vitreous | FERRO-AXINITE/ MANGANAXINITE Ca2FeAl2BSi4O15(OH) Ca2MnAl2BSi4O15(OH) |
Triclinic | Usually as thin wedge-shaped "axhead" crystals, often arranged in rosettes | 3.23 to 3.32 (ferro-), 3.30 to 3.36 (mangan-) | End members difficult to distinguish, though low end and high end S.G. may do the trick. Magnesio-axinite and tinzenite are two rare related species. |
| Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| 6½ to 7 | Pale-yellow to Olive-green to Olive-brown, Black | Vitreous to sub-vitreous (forsterite) or submetallic to dull (fayalite) | FAYALITE/ FORSTERITE (Olivine Group) Fe2(SiO4) Mg2(SiO4) |
Orthorhombic | Usually as crystalline massive or granular, crystals short-prismatic | 4.39 (fayalite) to 3.24 (forsterite) | End members distinguished by S.G. and luster. "Peridot" is the varietal name for gem material in the fayalite-forsterite series, usually forsterite. |
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