Page 106 - Laboratory Manual in Physical Geology

DARK TO MEDIUM-COLORED NONMETALLIC (NM) MINERAL IDENTIFICATION

STEP 2: STEP 4:
STEP 1: STEP 3:
What is the What is Compare the mineral’s Find mineral name(s) and
mineral’s the physical properties to other check the mineral
hardness? mineral’s distinctive properties below. database for additional
cleavage? properties (Figure 3.21).
Translucent to opaque dark gray; blue-gray, or black; May have silvery Plagioclase feldspar
iridescence; 2 cleavages at nearly 90° and with striations; H 6
Translucent to opaque brown, gray, green, or red; 2 cleavages at Potassium feldspar (K-spar)
nearly right angles; Exsolution lamellae; H 6
Green to black; Vitreous luster; H 5.5–6.0; 2 cleavages at about 124° Actinolite (amphibole)
Cleavage and 56° plus uneven fracture; Usually forms long blades and masses
excellent of needle-like crystals
or good
Dark gray to black; Vitreous luster; H 5.5–6.0; 2 cleavages at about Hornblende (amphibole)
124° and 56° plus uneven fracture; Forms long crystals that break into
blade-like fragments
HARD Dark green to black; Dull to vitreous luster; H 5.5–6.0; two cleavages Augite (pyroxene)
(H > 5.5) at nearly right angles (93° and 87°) plus uneven fracture; Forms short
crystals with squarish cross sections; Breaks into blocky fragments
Scratches Transparent or translucent gray, brown, or purple; Greasy luster; Quartz
glass Massive or hexagonal prisms and pyramids; H 7 Smoky quartz (black/brown var.),
Amethyst (purple var.)
Not Gray, black, or colored (dark red, blue, brown) hexagonal prisms with Corundum
scratched flat striated ends; H 9 Emery (black impure var.),
by masonry Ruby (red var.) Sapphire (blue var.)
nail or knife
blade Opaque red-brown or brown; Luster waxy; Cryptocrystalline; H 7 Jasper (variety of quartz)
Transparent to translucent dark red to black; Equant (dodecahedron) Garnet
Cleavage crystal form or massive; H 7
absent, Opaque gray; Luster waxy; Cryptocrystalline; H 7 Chert (gray variety of quartz)
poor, or
not visible Opaque black; Luster waxy; Cryptocrystalline; H 7 Flint (black variety of quartz)
Black or dark green; Long striated prisms; H 7–7.5 Tourmaline
Olive green, Transparent or translucent; No cleavage; Usually has Olivine
many cracks and conchoidal to uneven fracture; Single crystals or
masses of tiny crystals resembling green granulated sugar or
aquarium gravel; The crystals have vitreous (glassy) luster
Opaque dark gray to black; Tarnishes gray to rusty yellow-brown; Magnetite
Cleavage absent; Strongly attracted to a magnet; May be
magnetized; H 6–6.5
Opaque green; Poor cleavage; H 6–7 Epidote
Opaque brown prisms and cross-shaped twins; H 7 Staurolite
Yellow-brown, brown, or black; vitreous to resinous luster (may also Sphalerite
be submetallic); Dodecahedral cleavage; H 3.5–4.0; Rotten egg smell
when scratched or powdered
Cleavage Purple cubes or octahedrons; Octahedral cleavage; H 4 Fluorite
excellent
or good Black short opaque prisms; Splits easily along 1 excellent Biotite (black mica)
cleavage into thin sheets; H 2.5–3
Green short opaque prisms; Splits easily along 1 excellent Chlorite
cleavage into thin sheets; H 2–3
SOFT
(H < 5.5) Opaque rusty brown or yellow-brown; Massive and amorphous; Limonite
Yellow-brown streak; H 1–5.5
Does not Rusty brown to red-brown, may have shades of tan or white; Earthy Bauxite
scratch glass and opaque; Contains pea-sized spheres that are laminated
internally; H 1–5; Pale brown streak
Scratched by Deep blue; Crusts, small crystals, or massive; Light blue streak; Azurite
masonry nail H 3.5–4
or knife Cleavage
blade absent, Opaque green or gray-green; Dull or silky masses or asbestos; Serpentine
poor, or White streak; H 2–5
not visible
Opaque green in laminated crusts or massive; Streak pale green; Malachite
Effervesces in dilute HCI; H 3.5–4
Translucent or opaque dark green; Can be scratched with your Talc
fingernail; Feels greasy or soapy; H 1
Transparent or translucent green, brown, blue, or purple; Brittle Apatite
hexagonal prisms; Conchoidal fracture; H 5
Opaque earthy brick red to dull red-gray, or gray; H 1.5–5; Hematite
Red-brown streak; Magnet may attract the gray forms

FIGURE 3.19 Identification chart for dark to medium-colored minerals with nonmetallic (NM) luster on freshly broken surfaces.

Mineral Properties, Identification, and Uses ■ 91