Mineral ID

Common Rock-Forming Mineral Identification Key

This key is a much abbreviated version of the original by Alan Plante and Donald Peck, which may be found at rockhounds.com, and is used for educational purposes in accordance with the original copyright statement. This statement may be found at the bottom of the document, along with a link to rockhounds.com.

Step 1: Is the Luster Metallic or Submetallic?
Yes- [Go to Section I: Metallic or Submetallic Luster Key, Step 2]
No- [Go to Section II: Nometallic Luster Key (Soft), Step 4]

Section I: Minerals with a Metallic or Submetallic Luster

Step 2: Will the mineral leave a mark on paper? (Hardness less than 2½?)
Yes- [Go to Table IA]
No- [Go to Step 3]

Section I: Minerals with a Metallic or Submetallic Luster & Hardness Greater than 2½

Step 3: Can the mineral be scratched by a knife? (Hardness less than 5½?)
Yes- [Go to Table IB]
No- [Go to Table IC]

Section II: Minerals with a Non-Metallic Luster

Step 4: Does the mineral have a definitely colored streak? (Leaves a colored powder streak on unglazed porcelain?)
Yes[Go to Table IIA]
No[Go to Step 5]

Section II: Minerals with a Non-Metallic Luster

Step 5: Can the mineral be scratched by a fingernail? (Hardness less than 2½?)
Yes[Go to Step 6]
No- [Go to Step 7]

Section II: Minerals with a Non-Metallic Luster & Hardness Less than 2½

Step 6: Does the mineral have prominent cleavage?
Yes- [Go to Table IIB-1]
No- [Go to Table IIB-2]

Section II: Minerals with a Non-Metallic Luster & Hardness Greater than 2½

Step 7: Can the mineral be scratched by a copper penny? (Hardness less than 3?)
Yes- [Go to Step 8]
No- [Go to Step 10]

Section II: Minerals with a Non-Metallic Luster & Hardness Greater than 3

Step 8: Does the mineral have a prominent cleavage?
Yes- [Go to Table IIC-1]
No- [Go to Step. 9]

Section II: Minerals with a Non-Metallic Luster, Hardness Greater than 2½ & No Prominent Cleavage

Step 9: Will a thin splinter fuse in a candle flame? (Fusibility of 1?)
Yes- [Go to Table IIC-2a]
No- [Go to Table IIC-2b]

Section II: Minerals with a Non-Metallic Luster & Hardness Greater than 3

Step 10: Can the mineral be scratched by a knife? (Hardness less than 5½?)
Yes- [Go to Step 11]
No- [Go to Step 12]

Section II: Minerals with a Non-Metallic Luster & Hardness Less than 7

Step 11: Does the mineral have a prominent cleavage?
Yes- [Go to Table IID-1]
No- [Go to Table IID-2]

Section III: Minerals with a Non Metallic Luster & Hardness Greater than 5½

Step 12 Can the mineral be scratched by a sharp quartz point? (Hardness less than 7?)
Yes- [Go to Nonmetallic Luster Key (Hard): Step 13]
No- [Go to Nonmetallic Luster Key (Hard): Step 14]

Section III: Minerals with a Non Metallic Luster & Hardness Less than 7

Step 13 Does the mineral have a prominent cleavage?
Yes- [Go to Table IIIA-1]
No- [Go to Table IIIA-2]

Section III: Minerals with a Non Metallic Luster & Hardness Greater than 7

Step 14 Does the mineral have a prominent cleavage?
Yes- [Go to Table IIIB-1]
No- [Go to Table IIIB-2]

Table IA: Minerals with Metallic or Submetallic Luster & Hardness of less than 2½: (Will readily leave a mark on paper.)

Hardness	Color	Streak	Cleavage	Name	System	Habit	SG	Notes
1+	Dark-red to Vermilion	Rust-red		HEMATITE Fe₂O₃	Trigonal	ocherous masses, granular, often as a pigment – "rust" – in rocks	5.2	exhibits a wide range of H. depending on form; crystalline hematite is harder.
1 to 1½	Steel-grey to Iron-black	Black	one perfect cleavage	GRAPHITE C	Trigonal	may be in small hexagonal plates	2.23	has a greasy feel.
2½	Grey-black to Black	Bluish-black to Lead-grey	perfect cleavage in three directions at 90^o to each other	GALENA PbS	Isometric	usually in cubic crystals or masses exhibiting cubic cleavage, also in granular masses.	7.6	Sometimes won’t mark paper

Table IB: Minerals with Metallic or Submetallic Luster & Hardness greater than 2½, but less than 5½: (Will not easily mark paper, but can be scratched with a pocket knife.)

Hardness	Color	Streak	Cleavage	Name	System	Habit	SG	Notes
2½	Bluish-black to Lead-grey	Grey-Black to Black	perfect cleavage in three directions at 90^o to each other	GALENA PbS	Isometric	usually in cubic crystals or masses exhibiting cubic cleavage, also in granular masses	7.6	Will usually mark paper, sometimes not.
5 to 5½	Dark brown to black	Yellow-brown or Yellow-ocher		GOETHITE (pronounced "Ger-ta-ite.") FeO(OH)	Orthorhombic	usually in radiating botryoidal aggregates, mammillary, or stalactic	4.37	Includes "Limonite"
5½ to 6½	Dark-brown to Steel-grey to black	Rust-red or Indian-red		HEMATITE Fe₂O₃	Trigonal	usually massive in radiating, reniform, or micaceous aggregates	4.8 to 5.3	usually harder than a knife, but some forms can be softer. (See under Sections IA & IC.)

Table IC: Metallic or Submetallic Luster and Hardness greater than 5½: (Can not be scratched by a knife.)

Hardness	Color	Streak	Cleavage	Name	System	Habit	SG	Notes
5 to 5½	Dark-brown to black	Yellow-brown or Yellow-ocher		GOETHITE (pronounced "Ger-ta-ite.") FeO(OH)	Orthorhombic	usually in radiating botryoidal aggregates, mammillary, or stalactic	4.37	Includes "Limonite"
5½ to 6	Dark-brown to Steel-grey to black	Rust-red or Indian-red		HEMATITE Fe₂O₃	Trigonal	usually massive in radiating, reniform, or micaceous aggregates, crystals rhombohedral	4.8 to 5.3	usually black and usually harder than a knife. (See under Sections IA & IB.)
6	Black	Black		MAGNETITE Fe₃O₄	Isometric	massive or in octahedral crystals	5.18	strongly magnetic
6 to 6½	Brass-yellow	Black (may be greenish-black)		PYRITE FeS₂	Isometric	massive or in striated cubic or pyritohedral crystals, may be twinned	5.0	most common brass-yellow metallic mineral

Table IIA: Minerals with a Nonmetallic Luster, Definitely Colored Streak, and Hardness 1 to 6:

Streak	Hardness	Color	Cleavage	Name	System	Habit	SG	Notes
Rust-red to Indian-red	1+ to 6½	Dark-brown to Steel-grey to black		HEMATITE Fe₂O₃	Trigonal		4.8 to 5.3	hardness and S.G. lower in earthy massive materials, harder and denser in crystals and crystalline materials, crystals usually 5½ to 6½ with a metallic to sub-metallic luster

Table IIB-1: Streak not colored, Cleavage Prominent, Hardness less than 2½: (Can be scratched with a fingernail, Streak: white or none.)

Cleavage	Hardness	Luster	Color	Name	System	Habit	SG	Notes
Perfect in one direction, good in two directions (prismatic)	2	sub-vitreous to pearly (on cleavage faces) to silky or dull	Colorless to White or Grey (may be stained other colors by impurities)	GYPSUM CaSO₄^.2H₂O	Monoclinic	may be in compact masses without cleavage ("alabaster"), fibrous masses ("satin spar") or prismatic crystals, often twinned ("selenite")	2.32
Perfect in three directions (cubic)	2	vitreous to sub-vitreous	Colorless to White (may be stained by impurities)	HALITE (Common Salt) NaCl	Isometric	usually as granular crystalline masses or small cubic crystals	2.17	tastes somewhat "salty." Halite and sylvite can be difficult to distinguish, but halite tastes noticeably "saltier".
Perfect in one direction	2 to 2½	pearly on cleavage surfaces	White, may be Pale- green or Pale-yellow	MUSCOVITE (Mica Group) Kal₂(Si₃Al)O₁₀ (OH,F)₂	Monoclinic, pseudo- hexagonal	usually in "books" of thin sheets or as aggregates of foliated thin scales, crystals usually elongated stacks of sheets with a hexagonal or "diamond" cross section	2.76 to 2.88	sheets or flakes elastic
Perfect in one direction	2 to 2½	pearly on cleavage surfaces	White to Greenish- white or Yellowish-brown Dark Brown to Black (Biotite)	BIOTITE-PHLOGOPITE (Mica Group): KMg₃(Si₃Al)O₁₀(F,OH)	Monoclinic, pseudo- hexagonal	usually as aggregates of foliated thin scales or "books" of sheets, crystals rare, usually same as for muscovite	2.86	Sheets or flakes elastic. Light varieties difficult to distinguish from muscovite, which is much more common.
Perfect in one direction (rarely seen)	2 to 2½	earthy	White to Tan, may be Greyish	KAOLINITE Al₂Si₂O₅(OH)₄	Triclinic	usually as compact earthy masses	2.6	has an earth odor when moistened, sticks to a dry tongue

Table IIB-2: Streak Not Colored, Cleavage Not Prominent, Hardness less than 2½: (Can be scratched with a fingernail; Streak: white or none; Cleavage absent or not obvious.)

Hardness	Color	Luster	Cleavage	Name	System	Habit	SG	Notes
2 to 2½	White to Tan, may be Greyish	earthy luster	Perfect in one direction, may not be seen	KAOLINITE Al₂Si₂O₅(OH)₄	Triclinic	usually as compact earthy masses	2.6	earth odor when moistened, sticks to a dry tongue

Note: There are very few common to rare non-metallic species less than 2½ in hardness which do not have at least one good cleavage. Any sample that keys out to this point and is not one of the above listed minerals needs to be re-examined. It probably either has a prominent cleavage or is harder than 2½. Or it is a very rare to extremely rare species not covered here.

Table IIC-1: Streak not colored, Cleavage prominent, Hardness 2½ to 3: (Can not be scratched by a fingernail, can be scratched by a copper penny, streak white or one.)

Cleavage	Hardness	Color	Luster	Name	System	Habit	SG	Notes
Perfect in three directions (cubic)	2	Colorless to White (may be stained by impurities)	vitreous to sub-vitreous	HALITE (Common Salt) NaCl	Isometric	usually as granular crystalline masses or small cubic crystals	2.17	tastes somewhat "salty."
Perfect in three directions, corners 120^o or 60^o (rhombic)	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 cleavable masses producing rhombs, granular masses (limestone and marble),scalenohedral ("dogtooth") crystals, rhombohedral crystals, flattened rhombohedral ("nailhead") crystals, or a wide variety of related shapes; The many varied habits of calcite crystals make this one of the species that can be tough to identify by crystal form alone. Few people, if any, are familiar with all the forms it takes. (there are over 800 crystals forms known for this species to date…)	2.71	White streak; clear rhombs show a doubled image of print viewed through them, effervesces in cold, dilute acid – even in vinegar or Coke to a small degree.
Perfect in two directions, imperfect to good in a third (rhombic)	3 to 3½	Colorless to White or Greyish-white, may be tinted other colors	vitreous and/or pearly	ANHYDRITE CaSO₄	Orthorhombic	usually in coarsely crystalline masses exhibiting a pseudo-cubic cleavage, or in granular masses with no cleavage apparent, crystals rare and usually equant or thick tabular	2.9 to 2.98

Table IIC-2: Nonmetallic Luster; Streak Not Colored; Hardness 2½ to 3; Cleavage Not Prominent; Infusible in a Candle Flame.

Color	Hardness	Luster	Name	System	Habit	SG	Notes
White to Tan, may be Greyish	2 to 2½	earthy	KAOLINITE Al₂Si₂O₅(OH)₄	Triclinic	usually as compact earthy masses	2.6	has an earth odor when moistened, sticks to a dry tongue
Colorless to White or Greyish-white, may be tinted other colors	3 to 3½	vitreous and/or pearly	ANHYDRITE CaSO₄	Orthorhombic	usually in coarsely crystalline masses exhibiting a pseudo-cubic cleavage, or in granular masses with no cleavage apparent, crystals rare and usually equant or thick tabular	2.9 to 2.98

Table IID-1: Nonmetallic Luster, Hardness Greater Than 3 but Less Than 5½, and Prominent Cleavage: (Can not be scratched by a copper penny, can be scratched by a knife, streak white or none.)

Cleavage	Hardness	Color	Luster	Name	System	Habit	SG	Notes
Distinct in one direction (prismatic)	3½ to 4	Colorless to White or Grey, may be tinted other colors	vitreous to sub-vitreous or resinous	ARAGONITE (Aragonite Group) CaCO₃	Orthorhombic	usually in prismatic crystals, sometimes flattened, often acicular, usually twinned producing a pseudo-hexagonal cross section, may also be 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, higher S.G. and different crystal forms and habits – though some crystals may mimic the forms found in calcite.
Perfect in one direction ( may have a good cleavage in another direction)	4½ to 5 parallel to the length of the crystal, 7 to 7½ across the crystal	Blue most common, also Green, may be Grey, even Black, due to inclusions	vitreous to subvitreous	KYANITE Al₂SiO₅	Triclinic	most often as bladed long-prismatic crystals, usually poorly formed and rarely terminated, may be in parallel groupings or randomly oriented in the matrix	3.56 to 3.76	only common mineral with significant hardness difference in two directions.
Two Directions at ~56^o and ~124^o , prismatic – Amphibole Group; good to perfect in both directions	5 to 6	Grey to Lavender-blue or Pale-blue White to Light-Green to Dark-green	vitreous, silky in asbestiform varieties sub-metallic in the "bronzite" and "hypersthene" varieties	_{AMPHIBOLE (includes mixtures such as Hornblende)} (Ca,Na)(Fe, Mg)Si₂O₈	Monoclinic	usually in lath-like crystal aggregates, striated along their length, and in fibrous ("asbestiform") aggregates ("crocidolite"), or in bladed crystal aggregates, may also be somewhat acicular, tremolite may be asbestiform ("byssolite") or in fibrous mats ("mountain leather,"mountain cork")	2.8 to 3.4 (increasing with Fe content)	pale-grey to bluish-grey streak; glaucophane usually light-grey to lavender-blue, ferroglaucophane usually darker. The glaucophanes and riebeckites are impossible to tell apart without subtle chemical or optical tests. tremolite usually white to greyish-white or pale greenish-white, actinolite medium- to dark-green.
Two directions at nearly 90^o, prismatic – Pyroxene Group. good to perfect in both directions	5 to 6	Pale-to Medium-green, Pale-yellow to light Yellowish-brown, Pale-bronze may be White or Greenish-white to Greyish-white	vitreous to sub-vitreous, may be sub-metallic in the "bronzite" and "hypersthene" varieties	_PYROXENE (Ca,Na)(Fe, Mg)Si₂O₆	Orthorhombic & Monoclinic	usually granular in rocks, rarely as short-prismatic crystals	3.21 to 3.60	Enstatite-ferrosilite and clinoenstatite-clinoferrosilite form solid solution series. They are very difficult to tell apart. "Bronzite" and "hypersthene" are varietal names for intermediate members of the series.
Cleavage	Hardness	Color	Luster	Name	System	Habit	SG	Notes
Three cleavage directions, rhombohedral – Trigonal Carbonates: perfect in all three directions	3, 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 luster, may be waxy or dull on weathered crystals, pearly on cleavage surfaces	CALCITE CaCO₃	Trigonal	may be in cleavable masses producing rhombs, granular masses (limestone and marble), scalenohedral ("dogtooth") crystals, rhombohedral crystals, flattened rhombohedral ("nailhead") crystals, or a wide variety of related shapes (there are over 800 crystals forms known for this species to date…)	2.71	clear rhombs show a doubled image of print viewed through them, , effervesces in cold, dilute acid – even in vinegar or Coke to a small degree. The many varied habits of calcite crystals make this one of the species that can be tough to identify by crystal form alone. Few people, if any, are familiar with all the forms it takes.
Three cleavage directions, rhombohedral – Trigonal Carbonates: perfect in all three directions	3½ to 4	White to Tan or Pink, Grey, Greenish, tends towards Brown with increasing Fe, Red with Mn	usually vitreous to sub-vitreous, but may be pearly	DOLOMITE CaMg(CO₃)₂	Trigonal	usually as massive, granular, or in curved rhombohedral crystal clusters "fingernail" shape, druzes may have a characteristic "saddleback" arrangement	2.85	streak same color as sample, but pale; effervesces in cold acid, though not as vigorously as calcite, may be fluorescent. The Dolomite Group minerals are best told apart – and from calcite – by their S.G. when color and habit coincides.
Three cleavage directions, rhombohedral – Trigonal Carbonates: perfect in all three directions	3½ to 4	Dark-brown to Tan to Cream, may be Blackish-brown due to weathering	vitreous to sub-vitreous, may be oily to resinous on weathered, oxidized surfaces	SIDERITE (Calcite Group) FeCO₃	Trigonal	usually in cleavable coarsely crystalline masses, though not cleavable into rhombs, crystals rhombohedral, more rarely tabular, very rarely prismatic	3.8 to 4.0	white streak; somewhat soluble in cold acids, but with little or no effervescence
Cleavage	Hardness	Color	Luster	Name	System	Habit	SG	Notes
Three cleavage directions – one cleavage perfect, one imperfect, both prismatic, one good, basal	3 to 3½	Colorless to White, usually tinted other colors	vitreous to sub-vitreous, may be pearly on cleavage surfaces	ANHYDRITE CaSO₄	Orthorhombic	usually in coarsely crystalline masses showing pseudo-cubic cleavage, or granular or fibrous, crystals uncommon and usually equant or thick tabular	2.98	white streak; dissolves in cold acids without effervescence
Three cleavage directions – Perfect in two directions, basal and prismatic, imperfect in a third, prismatic (rhombic overall)	3 to 3½	Colorless to White or Greyish-white, may be tinted other colors, streak white	vitreous, pearly on basal cleavage surfaces	BARITE BaSO₄	Orthorhombic	usually in clusters or aggregates of platy to tabular crystals	4.5	heavy for its size. The three members of the Barite Group are most easily told apart by their S.G.s. Anglesite is noticeably heavier than the other two, barite may feel heavier than celestite.
Four cleavage directions - perfect in all four directions, octahedral	4	Colorless, Green, Purple, Blue, Yellow, Pink	vitreous	FLUORITE CaF₂	Isometric	usually in cubic crystals or coarsely crystalline masses exhibiting octahedral cleavage, crystals may also be octahedral	3.18	streak white; often fluorescent. The term fluorescent is derived from this mineral's name.

Notes: Amphiboles are a large and difficult group to tell apart with hand specimens. The best bet is site-specific knowledge.

Pyroxenes are also difficult to distinguish from one another. Locality information is usually the best bet for determining what you have.

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.)

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 (certain massive forms) 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
3½ to 4	Colorless to White or Grey, may be tinted other colors	vitreous to sub-vitreous or resinous	ARAGONITE (certain massive forms) 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.
5	White to Colorless, Greens, Blues, Lavender, Yellows, Purples	vitreous to sub-vitreous, may be dull in massive material	APATITE 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

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.)

Cleavage	Hardness	Color	Luster	Name	System	Habit	SG	Notes
Perfect in one direction (prismatic)	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 (prismatic)	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 (prismatic)	6½ to 7	Colorless to White, Grey, Pale-brown to Pale-yellow, more rarely Pale-blue to Pale-green	vitreous to subvitreous, 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
perfect in two directions (prismatic), sometimes 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	PYROXENE (Fe, Ca, Mg, etc.)₂Si₂O₆	Monoclinic	usually in compact massive material, granular or short fibrous, with cleavage not apparent	3.0 to 3.43
Cleavage	Hardness	Color	Luster	Name	System	Habit	SG	Notes
Perfect in one direction, imperfect in the second (prismatic)	6 to 6½	White to colorless, Cream to Tan to Pale-yellow, Pink to Brownish-red; green in some Microcline	vitreous to sub-vitreous, may be dull in granular masses, may exhibit pale blue to white opalescence ("moonstone")	ORTHOCLASE and MICROCLINE (Potassium Feldspar Group) KalSi₃O₈	Monoclinic	usually massive, coarsely crystalline to granular, crystals usually short prismatic, blocky, often twinned	2.54 to 2.63	PEARLY LUSTER Microcline and orthoclase can be difficult to tell apart, particularly when they occur together in granites and pegmatites. The best bet in such cases is locality specific information.
Perfect in one direction, good in the second (prismatic)	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"	PLAGIOCLASE Feldspar NaAlSi₃O₈ – CaAl₂Si₂O₈ ALBITE – ANORTHITE	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, crystals rare in intermediate members	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)	TWINNING looks like it has ridges 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.

Note: Two cleavage directions at ~56^o and ~124^o = 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.

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)

Hardness	Color	Luster	Name	System	Habit	SG	Notes
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.
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.
6½ to 7	Pale-yellow to Olive-green to Olive-brown, Black	vitreous to sub-vitreous (forsterite) or submetallic to dull (fayalite)	OLIVINE Fe₂(SiO₄) – Mg₂(SiO₄) FAYALITE – FORSTERITE	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 name for gem material in the fayalite-forsterite series, usually forsterite. Names for intermediate members of the series, such as "chrysolite" and "hortonolite" are based on chemical composition and should not be used unless the composition is known.

Table of Contents Return to Step 13

Table IIIB-1: Nonmetallic Luster; Hardness 7 to 10; Has a Prominent Cleavage. (Can not be scratched by quartz.)

Cleavage	Hardness	Color	Luster	Name	System	Habit	SG	Notes
One cleavage direction: perfect (prismatic)	6½ to 7	Colorless to White, Grey, Pale-brown to Pale-yellow, more rarely Pale-blue to Pale-green	vitreous to subvitreous, silky in fibrous material, may be dull on weathered surfaces	SILLIMANITE Al₂SiO₅	Orthorhombic	usually as crude prismatic crystals with a nearly square cross-section,surfaces often rough and altering to muscovite, may be in columnar to fibrous aggregates ("fibrolite")	3.23 to 3.27	restricted to high-temperature and pressure metamorphic environments.

Table IIIB-2: Nonmetallic Luster; Hardness 7 to 10; Cleavage not prominent.. (Can not be scratched by quartz.)

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	OLIVINE Fe₂(SiO₄) – Mg₂(SiO₄) FAYALITE – FORSTERITE	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 name for gem material in the fayalite-forsterite series, usually forsterite. Names for intermediate members of the series, such as "chrysolite" and "hortonolite" are based on chemical composition and should not be used unless the composition is known.
7	Colorless, Milky, Smoky-grey to Black, Amethyst, Rose, Yellow to Brownish-yellow, may be tinted other shades by inclusions	vitreous	QUARTZ (Crystalline) SiO₂	Trigonal	usually in pyramidaly terminated prismatic hexagonal crystals or glassy massive, "rock crystal," "smoky" and "amethyst" varieties may be very large, "citrine" usually small to medium in size, "rose" usually rather small to micro-scale	2.65	Abundant! in milky and massive forms. Common in rock crystal, smoky and amethyst forms, citrine rare, rose crystals very rare. (See also immediately below for crypto- and non-crystalline forms.)
7 to7½	All Dark Colors except Blue	vitreous	GARNET Ca₃Cr₂(SiO₄)₃	Isometric	usually as small to tiny dodecahedral crystals	3.40 to 4.2
7 to 7½	Medium- to Dark-brown, Reddish-brown	vitreous to sub-vitreous, may also be dull	STAUROLITE (Fe,Mg,Zn)₂Al₉(Si,Al)₄O₂₂(OH)₂	Monoclinic pseudo. Orthorhombic	usually as either short- to long-prismatic crystals, frequently twinned ("fairy crosses"), with twining at either 90^o or 60^o	3.74 to 3.83	usually in intermediate grade pelitic metamorphic rocks
7½	Brown, White, Yellow, Orange, Green, Blue	adamantine, oily on fracture surfaces	ZIRCON ZrSiO₄	Tetragonal	usually as small tetragonal prisms with pyramidal terminations, may also be bipyramidal without the prism	4.6 to 4.71	fluoresces orange-yellow, yellow and orange. This mineral may also be metamict (structurally disrupted by radiation) and then exhibits a lower H. (6 to 7) and S.G 3.9 to 4.6 range.

Copyright

This "Mineral Identification Key" is copyrighted by Alan Plante and Donald Peck, 2000; all rights reserved. Permission to copy all or parts of the Key is granted for noncommercial use by individuals and mineral clubs, study groups, or educational institutions. Publication in any medium for profit, including print and electronic, is expressly prohibited, except by written permission from the authors. Permission is required before posting a copy of the Key to another web site.

Alan Plante, 8 Hamlin Avenue, Gorham, NH 03581
Donald Peck, 6 Indian Rock Road, Warren, NJ 07059

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