Rocky Star Geology

Properties of Selected Minerals *

Information on this page is from:
1. Hurlbut: Dana´s Manual of Mineralogy 18^th Edition
2. Mason and Berry: Elements of Mineralogy
3. U.S.G.S. Bulletin 770: The Data of Geochemistry.

These minerals are significant as they have been used in the laboratory and/or found on field trips.

1. To find a mineral, click on a letter.
2. To return to the alphabetical index, use your browser's Back button.

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Apatite
Class - Phosphates
Cleavage - poor, basal
Color- variable (green, bluish green, green, blue, violet, brown, colorless, red)
Crystal System - Hexagonal
Formula - Ca₅(PO₄)₃ (F, Cl, OH)
Fracture - conchoidal
Hardness - 5
Luster - non-metallic (vitreous)
Streak - white
Other - Sometimes called Collophane.

Aragonite - see Calcite.

Arsenopyrite
Class - Sulfide
Cleavage - distinct
Color - silver white to steel gray
Crystal System - Orthorhombic
Formula - FeAsS
Fracture - none
Hardness - 5.5 - 6
Luster - metallic
Streak - dark grayish black
Other - Most abundant arsenic mineral.

Augite
Class - Silicates
Cleavage - good, two planes at nearly right angles
Color - usually dark green to black
Crystal System - Monoclinic
Formula - Ca(Mg, Fe, Al)(Al, Si)₂O₆
Fracture -
Hardness - 6
Luster - non-metallic (vitreous)
Streak - white to gray
Other - Most important ferromagnesium mineral in dark igneous rocks. Augite is an example of the monoclinic members of the pyroxene group.

Azurite
Class - Carbonate
Cleavage - perfect
Color - azure blue to very dark blue
Crystal System - Monoclinic
Formula - Cu₃(CO₃)₂(OH)₂
Fracture - none
Hardness - 3.5 - 4
Luster - non-metallic (vitreous)
Streak - light blue
Other - Forms in upper oxidized zone in reaction between carbonated water and copper minerals.

Barite
Class - Sulfate
Cleavage - perfect
Color - colorless to white, also yellow, brown, reddish, gray, greenish, or blue
Crystal System - Orthorhombic
Formula - BaSO₄
Fracture - None
Hardness - 3 - 3.5
Luster - non-metallic (vitreous)
Streak - white
Other - Often found in crystals as a "Desert Rose".

Biotite - Black Mica
Class - Silicates
Cleavage perfect basal
Color - black, dark green or brown
Crystal System - Monoclinic
Formula - K(Mg, Fe)₃(AlSi₃O₁₀)(OH)₂
Fracture -
Hardness - 2.5
Luster - non-metallic (vitreous or pearly)
Streak - white to gray
Other - Used as an insulator. Part of the Mica Group:

Biotite
Lepidolite
Muscovite
Paragonite
Phlogopite

Calcite
Class - Carbonates
Cleavage - perfect rhombohedral
Color - colorless and transparent when pure
Crystal System - Trigonal
Formula - CaCO₃
Fracture - none
Hardness - 3
Luster - non-metallic (vitreous - may be earthy in fine-grained massive forms)
Streak - white to grayish
Other - Effervesces freely in cold dilute hydrochloric acid (HCl). Polymorph of Aragonite.

Chalcedony (cryptocrystalline quartz)
Class - Silicates
Cleavage - none
Color - various
Crystal System - Trigonal
Formula - SiO₂
Fracture - conchoidal
Hardness - 7
Luster - non-metallic (waxy or dull)
Streak - white
Other - Chalcedony is a group name for a variety of forms of extremely fine grained forms of quartz.
The group includes:

agate - banded forms;
carnelian or sard - red to brown;
chert and flint - white, pale yellow, gray, or black;
jasper - generally red or brown;
silicified wood - reddish or brown;
tiger-eye - yellow, brown, or blue color

Chalcopyrite
Class - Sulfides
Cleavage - none
Color - brass yellow, tarnished to iridescent
Crystal System - Tetragonal
Formula - CuFeS₂
Fracture - uneven
Hardness - 3.5 - 4
Luster - metallic
Streak - greenish black
Other - Important source of copper.

Chert - see Chalcedony.

Chlorite
Class - Silicates
Cleavage - perfect basal
Color - green
Crystal System - Monoclinic
Formula - (Mg, Fe, Al)₆(Al, Si)₄O₁₀(OH)₈
Fracture -
Hardness - 2.5
Luster - non-metallic (vitreous to earthy)
Streak - white, pale green
Other - Alteration product of biotite and other ferromagnesian minerals in igneous rocks.

Cinnabar
Class - Sulfide
Cleavage - perfect
Color - red, brownish red
Crystal System - Trigonal
Formula - HgS
Fracture - none
Hardness - 2 - 2.5
Luster - non-metallic (Adamantine)
Streak - scarlet
Other - Important ore of mercury.

Corundum
Class - Oxide
Cleavage - none
Color - various: blue (sapphire), pink to blood red (ruby), also yellow (oriental topaz), yellow brown, green (oriental emerald), purple (oriental amethyst) to violet, pure corundum is white
Crystal System - Trigonal
Formula - Al₂O₃
Fracture -
Hardness - 9
Luster - non-metallic (adamantine to vitreous)
Streak - white
Other - Abrasive (emery) see color listings.

Diamond
Class - Native Element
Cleavage - perfect
Color - pale yellow, brown, white to blue-white, orange, pink, blue, green, red, black
Crystal System - Isometric
Formula - C
Fracture - conchoidal
Hardness - 10
Luster - non-metallic (Adamantine)
Streak - white
Other - Popular gemstone.

Dolomite
Class - Carbonate
Cleavage - rhombohedral
Color - colorless and transparent, or white, gray or greenish, yellowish brown, also pink or rose
Crystal System - Trigonal
Formula - CaMg(CO₃)₂
Fracture -
Hardness - 3.5 4
Luster - non-metallic (vitreous to pearly)
Streak - white
Other - Effervesces slightly in dilute cold hydrochloric acid (HCl) in contrast to Calcite.

Epidote
Class - Silicate
Cleavage - perfect
Color - Yellowish to brownish green ("pistachio green") to black
Crystal System - Monoclinic
Formula - Ca₂(Al, Fe)₃Si₃O₁₂(OH)
Fracture - none
Hardness - 7
Luster - non-metallic (vitreous)
Streak - white or grayish white
Other - In low- to medium-grade metamorphic rocks.

Flint/Jasper - see Chalcedony

Fluorite
Class - Halides
Cleavage - perfect octahedral
Color - pure form is colorless and transparent, commonly wine yellow, green, greenish blue, violet blue, also white, gray, sky blue, purple, bluish black, or brown
Crystal System - Isometric
Formula - CaF₂
Fracture -
Hardness - 4
Luster - non-metallic (vitreous)
Streak - white
Other - Fluorescent under ultra-violet light.

Galena
Class - Sulfide
Cleavage - cubic
Color - lead gray
Crystal System - Isometric
Formula - PbS
Fracture -
Hardness - 2.5
Luster - metallic
Streak - lead gray
Other - Important source of lead.

Garnet
Class - Silicate
Cleavage - none
Color - variable but dark red and reddish brown are common
Specifically:

Almandite is red or reddish brown,
Andradite is yellow, greenish yellow, greenish brown or black,
Grossular is white when pure but yellow, pink, green or brown if not pure,
Pyrope is red to nearly black,
Spessartine is orange to dark red or brown,
Uvarovite is emerald green

Crystal System - Isometric
Formula - A group of isomorphous minerals with the general formula:
(Ca, Mg, Fe⁺²,Mn⁺²)(Al, Fe⁺³Cr⁺³)(SiO₄)₃
Fracture -
Hardness - 7 - 7.5
Luster - non-metallic (vitreous to resinous)
Streak - white or pale shade of the color of the specimen
Other - Garnet "sand" is used to manufacture sand paper.

Glaucophane
Class - Silicate
Cleavage - Perfect
Color - Pale blue, lavender blue, dark blue to black
Crystal System - Monoclinic
Formula - Na₂(Mg, Fe, Al)₅Si₈O₂₂(OH)₂
Fracture - None
Hardness - 6
Luster - non-metallic (vitreous)
Streak - white to blue gray
Other - Found in low and medium grade metamorphic rocks as an essential mineral in the Glaucophane schists.

Goethite
Class - Hydroxide
Cleavage - perfect, parallel to side pinacoid
Color - blackish brown to yellow or reddish brown to brownish yellow in earthy forms
Crystal System - Orthorhombic
Formula - FeO(OH)
Fracture - uneven
Hardness - 5 - 5.5
Luster - metallic to dull; fibrous material silky; crystals adamantine
Streak - brownish yellow, orange yellow
Other - Usually forms from weathering of iron minerals, Glauconite, Magnetite, Pyrite, or Siderite.

Graphite
Class - Native Element
Cleavage - perfect basal
Color - black, dark gray
Crystal System - Hexagonal
Formula - C
Fracture - none
Hardness - 1 - 2
Luster - non-metallic (dull)
Streak - black
Other - Common mineral of regionally metamorphosed rock.

Gypsum
Class - Sulfate
Cleavage - good in one direction
Color - colorless and transparent, also white, gray, yellowish when massive
Crystal System - Monoclinic
Formula - CaSO₄ 2H₂O
Fracture - conchoidal
Hardness - 2
Luster - non-metallic (vitreous, pearly)
Streak - white
Other - Also known as: aggregates with parallel fibers is Satin Spar; fine-grained material is Alabaster; coarse, colorless and transparent is Selenite.

Halite
Class - Halide
Cleavage - perfect cubic
Color - colorless, white, gray, yellow, red, sometimes blue or purple
Crystal System - Isometric
Formula - NaCl
Fracture - none
Hardness - 2.5
Luster - non-metallic (vitreous)
Streak - white
Other - Most common water-soluble mineral.

Hematite
Class - Oxide
Cleavage - none
Color - gray (crystals). sometimes iridescent, earthy material dull to bright red
Crystal System - Trigonal
Formula - Fe₂O₃
Fracture - uneven with basal parting
Hardness - 5 - 6
Luster - metallic to submetallic in earthy varieties
Streak - red or red brown
Other - Primary ore of iron. Magnetite has a higher iron content but hematite is more common.

Hornblende (Amphibole)
Class - Silicate
Cleavage - perfect with two planes meeting at 56 and 124
Color - dark green, dark brown, black
Crystal System - Monoclinic
Formula - NaCa₂(Mg, Fe, Al)₅(Si, Al)₈O₂₂(OH)₂
Fracture - none
Hardness - 6
Luster - non-metallic (vitreous)
Streak - white to gray
Other - Common in medium grade metamorphic rocks and plutonic igneous rocks.
Part of Amphibole Group:

Alkali Amphibole,
Cummingtonite,
Hornblende,
Tremolite-Actinolite.

No entries at this time.

Kaolinite
Class - Triclinic
Cleavage - perfect basal but rare due to small grain size of clay
Color - white, often stained brown or gray by impurities
Crystal System - Silicate
Formula - Al₄Si₄O₁₀(OH)₈
Fracture - none
Hardness - 2
Luster - non-metallic (pearly if coarsely crystalline, usually dull and earthy
Streak - white
Other - Formed by decomposition of aluminosilicates, especially feldspars, either by weathering or hydrothermal activity.

Limonite (See Goethite).

Magnetite
Class - Oxide
Cleavage - none
Color - black
Crystal System - Isometric
Formula - Fe₃O₄
Fracture - none
Hardness - 5.5 - 6.5
Luster - metallic
Streak - black
Other - Strong magnetic nature.
Part of the Spinel Group:

Chromate,
Franklinite,
Magnetite,
Spinel

Malachite
Class - Carbonate
Cleavage - perfect
Color - bright green
Crystal System - Monoclinic
Formula - Cu₂(CO₃)(OH)₂
Fracture - uneven
Hardness - 3.5 - 4
Luster - non-metallic (silky, velvety, dull, translucent)
Streak - pale green
Other - Used as a gemstone.

Microcline (see Orthoclase Feldspar).

Muscovite (White Mica)
Class - Silicate
Cleavage - perfect basal
Color - white (hence the name Muscovy, White Russian)
Crystal System - Monoclinic
Formula - KAl₂(AlSi₃O₁₀)(OH)₂
Fracture - none
Hardness - 2.5 (on cleavage), 4 (across cleavage)
Luster - non-metallic (Vitreous, pearly)
Streak - white
Other - Used as an insulator.
Part of the Mica Group:

Biotite,
Lepidolite
Muscovite,
Paragonite,
Phlogopite.

No entries at this time.

Olivine
Class - Silicate
Cleavage - indistinct
Color - usually olive-green (hence the name), also white (Forsterite) and brown to black (Fayalite)
Crystal System - Orthorhombic
Formula - (Mg, Fe)₂SiO₄
Fracture - none
Hardness - 6.5
Luster - non-metallic (Vitreous)
Streak - white or gray
Other - Hydrothermal alteration produces serpentine; oxidation of the iron and removal of the magnesium and silica produces goethite or hematite.

Opal
Class - Silicate
Cleavage - none
Color - colorless (Hyalite) or white, also gray, brown or red (due to fine-grained impurities); also rich iridescence and play of colors (var. precious opal)
Crystal System - none, amorphous
Formula - SiO₂ n · H₂O
Fracture - conchoidal
Hardness - 5.5 - 6.5
Luster - non-metallic (Vitreous or waxy, colored versions are resinous)
Streak - white
Other - Solidified colloidal gel.

Orpiment
Class - Sulfide
Cleavage - perfect
Color - lemon-yellow to brownish yellow
Crystal System - Monoclinic
Formula - As₂S₃
Fracture - none
Hardness - 1.5 - 2
Luster - non-metallic (pearly, resinous)
Streak - pale yellow
Other - Arsenic mineral.

Orthoclase (K-spar, potassium feldspar, potash feldspar)
Class - Silicate
Cleavage - two planes at nearly right angles
Color - usually white or pink
Crystal System - Monoclinic
Formula - KAlSi₃O₈
Fracture - none
Hardness - 6
Luster - non-metallic (vitreous, sometimes pearly)
Streak - white
Other - Part of the Feldspar Group:

Potash Feldspar includes:
- Sanidine,
- Microcline,
- Orthoclase;
Soda-Lime Feldspar is the Plagioclase Series
Orthoclase is the Potash Feldspar of most plutonic rocks

Plagioclase
Class - Silicate
Cleavage - two planes at nearly right angles
Color - white or gray, sometimes reddish to reddish brown
Crystal System - Triclinic
Formula - (Ca, Na)(Al, Si)AlSi₂O₈
Fracture - none
Hardness - 6
Luster - non-metallic (vitreous, pearly)
Streak - white
Other - The Plagioclase Series is divided into six species. Using Albite as a reference they are:

Albite is 100 - 90% (or NaAlSi₃O₈)
Oligoclase is 90 - 70%
Andesite is 70 - 50%
Labradorite is 50 - 30%
Bytownite is 30 - 10%
Anorthite is 10 - 0% (or CaAl₂Si₂O₈)

Pyrite
Class - Sulfide
Cleavage - none
Color - pale brass yellow
Crystal System - Isometric
Formula - FeS₂
Fracture - conchoidal
Hardness - 6 - 6.5
Luster - metallic
Streak - greenish black or brownish black
Other - Also known as "fool´s gold."

Pyrolusite
Class - Oxide
Cleavage - perfect
Color - light to dark steel gray
Crystal System - Tetragonal
Formula - MnO₂
Fracture - uneven
Hardness - 6 - 6.5 (crystals), 2 - 6 (massive material)
Luster - metallic
Streak - black
Other - Occurs as dendritic pattern.

Quartz
Class - Trigonal
Cleavage - none
Color - crystals usually transparent (See Chalcedony)
Crystal System - Silicate
Formula - SiO₂
Fracture - conchoidal
Hardness - 7
Luster - non-metallic (vitreous)
Streak - white
Other - Piezoelectric (see Chalcedony).

Realgar
Class - Sulfide
Cleavage - good
Color - aurora-red to orange-yellow
Crystal System - Monoclinic
Formula - AsS
Fracture - none
Hardness - 1.5 - 2
Luster - non-metallic (resinous to greasy)
Streak - orange-red
Other - Arsenic mineral.

Rutile
Class - Oxide
Cleavage - distinct
Color - reddish brown, pure TiO₂ is white
Crystal System - Tetragonal
Formula - TiO₃
Fracture - conchoidal to uneven
Hardness - 6 - 6.5
Luster - non-metallic (Adamantine to metallic)
Streak - pale brown or grayish black; TiO₂ is white
Other - Titanium ore.

Serpentine
Class - Silicate
Cleavage - none observable
Color - usually green, also yellow, brown, reddish brown, and gray
Crystal System - Monoclinic
Formula - Mg₆Si₄O₁₀(OH)₈
Fracture - none
Hardness - 4 - 6 (variable)
Luster - non-metallic (waxy or greasy, silky)
Streak - white
Other - Coloration often resembles a serpent's skin hence the name.

Sodalite
Class - Silicate
Cleavage - poor
Color - often azure blue, may be white or pink
Crystal System - Isometric
Formula - Na₈(AlSiO₄)₆Cl₂
Fracture - none
Hardness - 6
Luster - non-metallic (vitreous)
Streak - white
Other - Associated with volcanic and plutonic rocks of the Nepheline/Syenite family.

Sphalerite
Class - Sulfide
Cleavage - perfect dodecahedral
Color - brown to yellow
Crystal System - Isometric
Formula - (Zn, Fe)S
Fracture - None
Hardness - 3.5 - 4
Luster - non-metallic (resinous to nearly metallic)
Streak - brown to light yellow
Other - Important metal for zinc. Sphalerite occurs in hydrothermal veins and replacement bodies.

Staurolite
Class - Silicate
Cleavage - distinct
Color - brown
Crystal System - Monoclinic (pseudo-orthorhombic)
Formula - Fe₂Al₉O₆(SiO₄)₄(O, OH)₂
Fracture - none
Hardness - 7
Luster - non-metallic (Vitreous or resinous)
Streak - gray
Other - Twins often in the shape of a star hence the name.

Sulfur
Class - Native Element
Cleavage - none
Color - yellow to yellowish brown, may be white around volcanic areas
Crystal System - Orthorhombic
Formula - S
Fracture - conchoidal to uneven
Hardness - 1.5 - 2.5
Luster - non-metallic (Resinous to greasy)
Streak - white
Other - Often associated with the "bad smell" from volcanic areas (incomplete oxidation of hydrogen sulfide, H₂S).

Talc
Class - Silicate
Cleavage - perfect basal
Color - usually pale green, also white or gray, may be stained reddish or brown by iron oxides
Crystal System - Monoclinic
Formula - Mg₃Si₄O₁₀(OH)₂
Fracture - none
Hardness - 1
Luster - non-metallic (pearly, greasy or dull)
Streak - white
Other - Mineral of low- to medium-grade metamorphic rocks rich in magnesium. It may be a major rock-forming mineral, such rocks are soapstone or steatite.

Topaz
Class - Silicate
Cleavage - perfect
Color - colorless or white, also transparent pale blue, yellow, yellowish brown
Crystal System - Orthorhombic
Formula - Al₂SiO₄(OH, F)₂
Fracture - none
Hardness - 8
Luster - non-metallic (vitreous)
Streak - white
Other - Transparent topaz is used as a gemstone.

Tourmaline
Class - Silicate
Cleavage - poor
Color - usually black, also brown, dark blue, colorless (iron-free varieties); with lithium, pink, green, and blue; transparent form used as gemstone
Crystal System - Trigonal
Formula - Na(Mg, Fe)₂Al₆(BO₃)₃(Si₆O₁₈)(OH)₄
Fracture - none
Hardness - 7.5
Luster - non-metallic (Vitreous)
Streak - white
Other - Occurs in granite pegmatites as rather large (2 - 4 inch) crystals; piezoelectric property used in construction of pressure gauges.

Turquoise
Class - Hydrated Phosphate
Cleavage - perfect
Color - sky blue, bluish green to apple green
Crystal System - Triclinic
Formula - CuAl₆(PO₄)₄(OH)₈ 4H₂O
Fracture - conchoidal to smooth in massive material
Hardness - 5 - 6
Luster - non-metallic (waxy, vitreous)
Streak - white or greenish to pale green
Other - Fine cryptocrystalline form is used as an ornamental or semiprecious stone.

No entries at this time.

Vermiculite
Class - Silicate
Cleavage - perfect
Color - yellow to brown
Crystal System - Monoclinic
Formula - Mg₃Si₄O₁₀(OH)₂ n · H₂O
Fracture - none
Hardness - 1.5
Luster - non-metallic (Pearly, sometimes submetallic)
Streak - white
Other - Hydrothermal alteration of Phlogopite and Biotite.

Wollastonite
Class - Silicate
Cleavage - perfect to good
Color - usually white, sometimes grayish
Crystal System - Triclinic
Formula - CaSiO₃
Fracture - splintery
Hardness - 5
Luster - non-metallic (Vitreous, somewhat silky)
Streak - white
Other - Forms from metamorphism of siliceous limestones, occurs in igneous contact zones and high-grade regionally metamorphosed zones.

No entries at this time.

* This List was updated on July 21, 2003

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Rocky Star Geology

Properties of Selected Minerals*

Properties of Selected Minerals *