Mineral Identification Key

The Mineral Identification Key

Copper, Michigan, Seaman Museum specimen

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

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 hardness depending on form; crystalline hematite is harder.

1 to 1½ Steel-grey to Iron-black Black One perfect direction (basal) GRAPHITE
C Trigonal May be in small hexagonal plates 2.23 Has a greasy feel.

1 to 1½ Bluish-black Black to Greenish-black One perfect direction (basal) MOLYBDENITE MoS₂ Trigonal Usually in somewhat foliated appearing masses, often leaf-like hexagonal plate 4.7 Has a greasy feel; heavier than graphite

Hardness Color Streak Cleavage Name System Habit SG Notes

1 to 2 Iron-black Black One perfect direction PYROLUSITE
MnO₂ Tetragonal May be splintery or in radiating fibrous masses 4.7 Sometimes won’t mark paper. Most dendrites are not pyrolusite.

1½ Lead-grey Lead-grey One perfect direction NAGYÁGITE
Pb₅Au(Te,Sb)₄S_5-8 Orthorhombic pseudo-
tetragonal Usually platy masses, may be granular 7.4 to 7.5 Flakes flexible. Rare.

1½ Steel-grey Steel-grey One perfect direction TETRADYMITE
Bi₂Te₂S Trigonal Usually in foliated to bladed masses, acute rhombohedral crystals rare 7.1 to 7.4 Flexible. Relatively rare.

1½ to 2 Silvery-white Grey SYLVANITE
(Au,Ag)₂Te₄ Monoclinic Usually granular or in bladed aggregates, often appears as skeletal forms on rocks, resembling writing (cuneiform). 8 to 8.2 May not mark paper. Rare.

Hardness Color Streak Cleavage Name System Habit SG Notes

2 Grey-black to Lead-grey Black to Grey-black One perfect direction, two imperfect BISMUTHINITE Bi₂S₃ Orthorhombic In bladed crystals showing cross striations 6.78 Similar to stibnite but heavier. Rare

2 Grey-black Bluish-black to Silvery-black One perfect direction (prismatic), two imperfect STIBNITE
Sb₂S₃ Orthorhombic Usually as thick bladed crystals with striations both parallel to and across the long axis; crystals often bent or "kinked" 4.5 Fuses in a candle flame, sometimes won’t mark paper.

2 to 2½ Grey-black to Lead-grey Black ACANTHITE
Ag₂S Isometric Pseudo-cubic, usually massive 7.3 Bright-steel-grey on fresh surfaces but darkens upon exposure, easily cut with a knife (sectile). Sometimes won’t mark paper

2 to 2½ Iron-black Iron-black STEPHANITE
Ag₅SbS₄ Orthorhombic Usually massive, more rarely as short prismatic to tabular crystals 6.2 to 6.5 Rare

Hardness Color Streak Cleavage Name System Habit SG Notes

2 to 2½ Bright-red Bright-red to Deep-red One perfect direction CINNABAR
HgS Trigonal Usually in granular masses 8.1 Luster actually adamantine, sometimes appearing sub-metallic to metallic. May not mark paper.

2 to 2½ Brownish-red to Scarlet or Vermilion Deep Ruby-red to Bright Ruby-red One distinct direction PYRARGYRITE/ PROUSTITE
Ag₃(Sb,As)S₃Ag₃(As,Sb)S₃ Trigonal Prismatic, pyramidal, rhombohedral, and scalenohedral crystals , also massive, usually as complex intergrown crystal aggregates 5.57 to 5.58 Isostructural species difficult to distinguish, though pyrargyrite is usually darker in color and more common than proustite. Rare.

2 to 2½ Silvery-white Silvery-white with decidedly reddish tones One perfect direction (basal), one good BISMUTH
Bi Trigonal Usually in laminated granular masses, may be arborescent or reticulated, artificial crystals in pseudo-cubic "hopper" groups 9.8 Heavy. Rare. May not mark paper.

Hardness Color Streak Cleavage Name System Habit SG Notes

2½ Brass-yellow to Silvery-white Yellowish- to Greenish-grey CALAVERITE
AuTe₂ Monoclinic Usually granular, rarely in distinct elongated crystals 9.35 Very heavy, easily fusible in candle flame (leaving globules of gold). (May not mark paper.) Rare.

2½ Grey-black to Black Bluish-black to Lead-grey Perfect 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

2 to 3 Black Black One imperfect direction POLYBASITE
(Ag,Cu)₁₆Sb₂S₁₁ Monoclinic, pseudo-
hexagonal Usually massive or in crude pseudo-hexagonal plates 6.1 to 6.2 Rare

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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 hardness depending on form; crystalline hematite is harder.
1 to 1½	Steel-grey to Iron-black	Black	One perfect direction (basal)	GRAPHITE C	Trigonal	May be in small hexagonal plates	2.23	Has a greasy feel.
1 to 1½	Bluish-black	Black to Greenish-black	One perfect direction (basal)	MOLYBDENITE MoS₂	Trigonal	Usually in somewhat foliated appearing masses, often leaf-like hexagonal plate	4.7	Has a greasy feel; heavier than graphite
Hardness	Color	Streak	Cleavage	Name	System	Habit	SG	Notes
1 to 2	Iron-black	Black	One perfect direction	PYROLUSITE MnO₂	Tetragonal	May be splintery or in radiating fibrous masses	4.7	Sometimes won’t mark paper. Most dendrites are not pyrolusite.
1½	Lead-grey	Lead-grey	One perfect direction	NAGYÁGITE Pb₅Au(Te,Sb)₄S_5-8	Orthorhombic pseudo- tetragonal	Usually platy masses, may be granular	7.4 to 7.5	Flakes flexible. Rare.
1½	Steel-grey	Steel-grey	One perfect direction	TETRADYMITE Bi₂Te₂S	Trigonal	Usually in foliated to bladed masses, acute rhombohedral crystals rare	7.1 to 7.4	Flexible. Relatively rare.
1½ to 2	Silvery-white	Grey		SYLVANITE (Au,Ag)₂Te₄	Monoclinic	Usually granular or in bladed aggregates, often appears as skeletal forms on rocks, resembling writing (cuneiform).	8 to 8.2	May not mark paper. Rare.
Hardness	Color	Streak	Cleavage	Name	System	Habit	SG	Notes
2	Grey-black to Lead-grey	Black to Grey-black	One perfect direction, two imperfect	BISMUTHINITE Bi₂S₃	Orthorhombic	In bladed crystals showing cross striations	6.78	Similar to stibnite but heavier. Rare
2	Grey-black	Bluish-black to Silvery-black	One perfect direction (prismatic), two imperfect	STIBNITE Sb₂S₃	Orthorhombic	Usually as thick bladed crystals with striations both parallel to and across the long axis; crystals often bent or "kinked"	4.5	Fuses in a candle flame, sometimes won’t mark paper.
2 to 2½	Grey-black to Lead-grey	Black		ACANTHITE Ag₂S	Isometric	Pseudo-cubic, usually massive	7.3	Bright-steel-grey on fresh surfaces but darkens upon exposure, easily cut with a knife (sectile). Sometimes won’t mark paper
2 to 2½	Iron-black	Iron-black		STEPHANITE Ag₅SbS₄	Orthorhombic	Usually massive, more rarely as short prismatic to tabular crystals	6.2 to 6.5	Rare
Hardness	Color	Streak	Cleavage	Name	System	Habit	SG	Notes
2 to 2½	Bright-red	Bright-red to Deep-red	One perfect direction	CINNABAR HgS	Trigonal	Usually in granular masses	8.1	Luster actually adamantine, sometimes appearing sub-metallic to metallic. May not mark paper.
2 to 2½	Brownish-red to Scarlet or Vermilion	Deep Ruby-red to Bright Ruby-red	One distinct direction	PYRARGYRITE/ PROUSTITE Ag₃(Sb,As)S₃Ag₃(As,Sb)S₃	Trigonal	Prismatic, pyramidal, rhombohedral, and scalenohedral crystals , also massive, usually as complex intergrown crystal aggregates	5.57 to 5.58	Isostructural species difficult to distinguish, though pyrargyrite is usually darker in color and more common than proustite. Rare.
2 to 2½	Silvery-white	Silvery-white with decidedly reddish tones	One perfect direction (basal), one good	BISMUTH Bi	Trigonal	Usually in laminated granular masses, may be arborescent or reticulated, artificial crystals in pseudo-cubic "hopper" groups	9.8	Heavy. Rare. May not mark paper.
Hardness	Color	Streak	Cleavage	Name	System	Habit	SG	Notes
2½	Brass-yellow to Silvery-white	Yellowish- to Greenish-grey		CALAVERITE AuTe₂	Monoclinic	Usually granular, rarely in distinct elongated crystals	9.35	Very heavy, easily fusible in candle flame (leaving globules of gold). (May not mark paper.) Rare.
2½	Grey-black to Black	Bluish-black to Lead-grey	Perfect 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
2 to 3	Black	Black	One imperfect direction	POLYBASITE (Ag,Cu)₁₆Sb₂S₁₁	Monoclinic, pseudo- hexagonal	Usually massive or in crude pseudo-hexagonal plates	6.1 to 6.2	Rare