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The Mineral Identification Key |  |
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The Mineral Identification Key | |
Table IIIA-1: Hardness greater than 5½ but less than 7; Cleavage prominent (Can not be scratched by a knife, can be scratched by quartz.) [Previous Table] [Next Table]
| Cleavage | Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| Perfect in one direction | 4 to 5 parallel to cleavage and crystal faces, 6 to 7 across crystal and cleavage faces | Blue, Green, Grey, White, very rarely Black | Vitreous to sub-vitreous | KYANITE Al2SiO5 |
Triclinic | Usually in bladed aggregates, parallel or radiating, may also be single flattened prismatic crystals | 3.56 to 3.67 | Only common mineral in which the hardness is so notably different along its length vs. across its width. |
| Perfect in one direction | 5 to 6½ | Light-brown to Black | Resinous or pitchy/ submetallic | ALLANITE-(Ce) (Epidote Group) |
Monoclinic | Usually in coarsely crystalline massive form, crystals tabular, prismatic to acicular (may be metamict), may also be finer grained with no apparent cleavage | 3.4 to 4.2 | May give a light brown streak; Allanite-(La) and Allanite-(Y) are closely related species, but extremely rare. Most of the Epidote group minerals may exhibit a second, poor, cleavage, but it is usually not seen. See also clinozoisite below. |
| Perfect in one direction | 6½ to 7 | Green to Yellow-green, Yellow, Grey, Brownish-green, Greenish-black, Black | Vitreous to sub-vitreous , dull in weathered crystals and massive materials | EPIDOTE Ca2(Fe,Al)3(SiO4)3(OH) |
Monoclinic | Usually in short to long prismatic crystals, may also be thick tabular or acicular; also massive, coarse to fine granular, rarely fibrous | 3.38 to 3.49 | Over 200 different forms are known; Commonest of the Epidote Group species. Most of the Epidote group minerals may exhibit a second, poor, cleavage, but it is usually not seen. See also clinozoisite below. |
| Cleavage | Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| Perfect in one direction | 6 to 7 | Grey, Green to Yellowish-green, Pinkish, Brown, Blue, Purple, Colorless | Vitreous, may be pearly on cleavage surfaces | ZOISITE (Epidote Group) Ca2Al3(SiO4)3(OH) |
Orthorhombic | Usually in aggregates of crude parallel crystals with vertical striations on the faces, more rarely as well-formed prismatic crystals in clusters or singly | 3.15 to 3.36 | The blue gem variety "tanzanite" and pink gem variety "thulite" are rare. Most of the Epidote group minerals may exhibit a second, poor, cleavage, but it is usually not seen. See also clinozoisite below. |
| Perfect in one direction | 6½ to 7 | Colorless to White, Grey, Pale-brown to Pale-yellow, more rarely Pale-blue to Pale-green | Vitreous to sub-vitreous, silky in fibrous material, may be dull on weathered surfaces | SILLIMANITE Al2SiO5 |
Orthorhombic | Usually as crude prismatic crystals with a nearly square cross-section, may be in columnar to fibrous aggregates ("fibrolite") | 3.23 to 3.27 | Surfaces often rough and altering to muscovite, restricted to high-temperature and pressure metamorphic environments |
| Cleavage | Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| Two directions at or nearly 90o. Perfect in one direction distinct in other (prismatic) | 6½ to 7 | White to Greyish-white, Colorless, Pinkish-white to Pale-lavender, more rarely Greenish to Yellowish, Brown, Rose-red to Dark-red | Vitreous to sub-adamantine, "brilliant" | DIASPORE AlO(OH) |
Orthorhombic | Usually in thin platy crystalline aggregates, crystals thin, platy, may be prismatic or acicular, rarely tabular | 3.35 to 3.45 | Very brittle. Often mistaken for milky to greyish quartz. |
| Perfect in two directions (prismatic) | 6 | Brown to Greenish-brown, Colorless, Black | Vitreous | PIGEONITE (Pyroxene Group) (Mg,Fe,Ca)(Mg,Fe)Si2O6 |
Monoclinic | Usually as matrix-bound crystalline grains in cooled lavas | 3.17 to 3.46 | Often inverts to augite after cooling. Relatively rare compared to other Pyroxenes |
| Perfect in two directions (prismatic) | 6½ to 7 | Grey to White, Colorless, Tan, Yellow, Pale-green to Bright-green ("hiddenite"), Pink to Lilac to Violet ("kunzite") | Vitreous to sub-vitreous | SPODUMENE (Pyroxene Group) LiAlSi2O6 |
Monoclinic | Crystals are usually prismatic with a diamond shaped cross-section; but matrix-frozen crystals - the most commonly seen - tend to show only two adjacent faces, the other two not developed; then they look like a triangular prism with two developed faces and a rough base to the 'triangle' - only half of the complete prism developed. | 3.03 to 3.23 | Crystals often very large, to 14 meters or more, crystals faces usually have a wood-grain look to them, gem quality material often with acid-etched crystal faces, heavily vertically striated. |
| Perfect in two directions (prismatic), often not apparent |
6 to 7 | Dark-green to Medium-green to (more rarely) Pale-green, White to Grey or Bluish-grey, Lavender to Violet | Vitreous to sub-vitreous, may be dull in granular material | JADEITE (Pyroxene Group) Na(Al,Fe)Si2O6 |
Monoclinic | Usually in compact massive material, granular or short fibrous, with cleavage not apparent | 3.24 to 3.43 | Massive material often a mix of jadeite and diopside, crystals extremely rare. [See also the softer Pyroxene Group minerals, IID, two cleavage directions, The hardness of many of them spans the 5 to 6 range, so they may key out to this point.] |
| Cleavage | Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| Perfect in one direction, distinct in the second | 6 | White to Colorless, Cream to Tan | Vitreous to sub-vitreous, may be pearly on cleavage | SANIDINE (Feldspar Group) K(Al,Si)4O8 |
Monoclinic | Usually as tabular crystals (phenocrysts) in rhyolites and other felsic extrusive volcanic rocks and as spherulites in obsidian | 2.56 to 2.62 | Association with rhyolites and other extrusive igneous rocks is distinctive. Carlsbad twins - mirrored inter-penetration twins exhibiting two terminations at each end - are common. |
| Perfect in one direction, good in the second | 6 to 6½ | White to Colorless, Cream to Tan and Pale-yellow, Salmon-pink to Red, Green to Blue-green ("Amazonite") | Vitreous to sub-vitreous, may exhibit opalescence ("adularia") | MICROCLINE (Feldspar Group) KAlSi3O8 |
Triclinic | Usually as coarsely crystalline rock-forming masses or macro-crystals in the 1 meter to 10s of meters range, smaller crystals usually short-prismatic to blocky, often twinned | 2.54 to 2.57 | Microcline and orthoclase are actually polytypes of a single species, and are extremely difficult to tell apart without subtle crystallographic observations. The best bet for these is site-specific knowledge about what is found there. |
| Perfect in one direction, imperfect in the second | 6 to 6½ | White to Colorless, Cream to Tan to Pale-yellow, Pink to Brownish-red | Vitreous to sub-vitreous, may be dull in granular masses, may exhibit pale blue to white opalescence ("moonstone") | ORTHOCLASE (Feldspar Group) KAlSi3O8 |
Monoclinic | Usually massive, coarsely crystalline to granular, crystals usually short prismatic, blocky, often twinned | 2.55 to 2.63 | Microcline and orthoclase are actually polytypes of a single species, and are extremely difficult to tell apart without subtle crystallographic observations. The best bet for these is site-specific knowledge about what is found there. |
| Cleavage | Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| Perfect in one direction, good in the second | 6 to 6½ | White to Light-grey and Colorless, in albite, may be tinged Blue or Green, darkening to Dark-grey to Black in anorthite, intermediate members Medium to Darker greys (rarer than end members) | Vitreous, labradorite exhibits opalescence/ "labradorescence" | ALBITE/ ANORTHITE (Plagioclase Series) NaAlSi3O8 CaAl2Si2O8 |
Triclinic | Usually massive, coarsely crystalline, platy lamellar habit in albite ("cleavlandite"), more randomly oriented and smaller grains in anorthite end of the series, including labradorite, crystals usually tabular and usually twinned in albite, usually short-prismatic and twinned in anorthite, euhedral crystals rare except for albite | 2.60 to 2.65 (albite) 2.63 to 2.66 (oligoclase) 2.66 to 2.68 (andesine) 2.68 to 2.74 (labradorite) 2.72 to 2.75 (bytownite) 2.74 to 2.76 (anorthite) |
S.G. increases with replacement of Na by Ca and addition of Al, members to the albite end of the series most easily distinguished from other feldspars by fine, closely-spaced striations on the {001} cleavage plane, members towards the anorthite end of the series by their dark color and randomly oriented grains in massive materials. Intermediate members usually difficult to distinguish from one another. |
| Perfect in one direction (prismatic), good in the other (pinacoidal) | 5½ to 6½ | Pink to Rose-red to Brownish-red, may also be Orangish-red in some weathered material | Vitreous to sub-vitreous, may be dull in massive material | RHODONITE (Mn,Fe,Mg,Ca) SiO3 |
Triclinic | Usually massive, coarse to fine granular, crystals tabular to equant, rare | 3.55 to 3.76 | Often coated and/or veined with black Mn-oxides due to chemical weathering ("spider-web" veining). |
| Cleavage | Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| Perfect in one direction (prismatic), imperfect in the other (pinacoidal) | 5½ to 6 | Black to Dark greenish-black | Vitreous to splendant | BABINGTONITE (Rhodonite Group) Ca2(Fe,Mn)FeSi5O14(OH) |
Triclinic | Usually as equant to short-prismatic crystals | 3.34 to 3.48 | May give a greenish to brownish-grey streak; usually associated with zeolites in basalts ("traprock") more rarely in granitic rocks. Rare. |
| Perfect in one direction, parting in the second direction (both prismatic) | 5 to 5½ | Yellowish-brown to Greenish-brown | Vitreous to sub-vitreous, may be somewhat resinous or oily. | TITANITE ("SPHENE") CaTiSiO5 |
Monoclinic | Usually in crude blocky to stout prismatic crystals, wedge-shaped | 3.48 to 3.60 | Streak white to pale brown; |
| Perfect in two directions (prismatic and pinacoidal) | 6½ | Pale yellowish-grey to Yellowish-green, Colorless, Pale-rose to Red | Vitreous to sub-vitreous | CLINOZOISITE (Epidote Group) Ca2Al3(SiO4)3(OH) |
Monoclinic | Usually as massive, granular to fibrous, crystals prismatic, often striated | 3.21 to 3.38 | Distinguished from other common Epidote group minerals by its second perfect cleavage. |
| Cleavage | Hardness | Color | Luster | Name | System | Habit | SG | Notes |
| Perfect in one direction (pinacoidal), imperfect in another (pinacoidal), distinct in a third (prismatic), and may have a fourth, good, direction (prismatic) | 5½ to 6 | White to Milky- or Cream-white, Grey, Yellowish to Tan or Beige, Salmon-pink, may also be Greenish or Bluish, Colorless | Vitreous to greasy | AMBLYGONITE/ MONTEBRASITE LiAl(PO4)F LiAl(PO4)(OH) |
Triclinic | Usually as crude equant to short-prismatic crystals, may be extremely large (meter scale), and as massive crystalline, coarse to fine granular | 3.11 | It is impossible to tell the two end members apart without subtle optical tests. Locality info may not help; eg. "amblygonite" specimens from Maine pegmatites have been shown to actually be montebrasite. It is probably best to label specimens as shown under Name unless there is proof positive of which end member you have |
Note: Two cleavage directions at ~56o and ~124o = Amphibole Group. A number of the Amphiboles have hardnesses in the 5 to 6 range, spanning previous sections and this one, and may key out to this point. See IID-1.
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