Chemical Formula
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The Serpentine group is composed of several related minerals. A generic formula that includes all members is: X2-3Si2O5(OH)4 Where X = Mg, Fe2+, Fe3+, Ni , Al, Zn, or Mn. (One of the two Si atoms may also be replaced by an Al or Fe atom in a few rare members.) This leads to a complete formula of (Mg,Fe,Ni,Al,Zn,Mn)2-3(Si,Al,Fe)2O5(OH)4. (The subscripted 2-3 after the first set of elements denotes that there are two or three of the selected elements, depending on the charge of the element. Some very rare forms of Serpentine contain water [H2O] in place of the hydroxyl [OH]. Since the charge of water is different from hydroxyl, the formula of these rare forms is slightly altered.) The two most common members of the Serpentine Group are: Antigorite - (Mg,Fe)3Si2O5(OH)4 Chrysotile - Mg3Si2O5(OH)4 Chrysotile is not a single mineral, but a group of polymorphous minerals with the same chemical composition but different crystal lattice. Chrysotile polytypes are: Clinochrysotile (monoclinic) Orthochrysotile (orthorhombic) Parachrysotile (orthorhombic) Lizardite (hexagonal) Other members of the Serpentine group are: Amesite - Mg2Al(SiAl)O5(OH)4 Cronstedtite - Fe2+2Fe3+(SiFe3+)O5(OH)4 Fraipontite - (Zn,Al)3(Si,Al)2O5(OH)4 Nepouite - Ni3Si2O5(OH)4 There are several additional members of the Serpentine group not discussed here due to there extreme rarity.
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Composition
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Basic silicate of magnesium, iron, aluminum, nickel, zinc, and manganese. (Some rare varieties contain water in place of the hydroxyl, and substitute a different metal in exchange of the charge difference.)
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Color
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White, yellow, green, gray, brown, black, purple; sometimes multicolored, especially green and yellow.
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Streak
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White
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Hardness
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2 - 5
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Crystal System
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Monoclinic
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Crystal Forms and Aggregates
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Antigorite, Clinochrysotile, Cronstedtite, Fraipontite, and Nepouite all crystallize in the monoclinic crystal system. Orthochrysotile and Parachrysotile crystallize in the orthorhombic crystal system. Lizardite crystallizes in the hexagonal crystal system. Amesite crystallizes in the triclinic crystal system. Serpentine aggregates include massive, platy, fibrous (forming tough, flexible and elastic fibers),botryoidal, columnar, earthy, platy, micaceous, in pyramidal groupings, in parallel bladed groups, and as fibrous veins. Fibrous veins may be straight, but are more often in curved, angled veins. Some fibrous forms are extremely soft and flexible and resemble wool. Serpentine also pseudomorphs after many minerals, where it form in the same crystal shape as the pseudomorphed mineral.
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Transparency
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Translucent to opaque. Rarely transparent.
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Specific Gravity
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2.5 - 3.2
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Luster
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Greasy, waxy, or silky
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Cleavage
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Usually not discernible because of crystal development. Chrysotile may exhibit basal cleavage.
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Fracture
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Conchoidal, splintery
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Tenacity
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Brittle. Fibrous Serpentine is flexible and elastic.
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Other ID Marks
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1) Has a greasy feel. 2) Yellow Serpentine often fluoresces a cream-white color in shortwave ultraviolet light.
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In Group
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Silicates; Phyllosilicates
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Striking Features
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Greasy feel, soft compared to similar minerals, and may flexible and elastic.
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Environment
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Serpentine is fairly common in many environments, and is an important rock forming mineral in many metamorphic environments.
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Rock Type
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Metamorphic
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Popularity (1-4)
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2
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Prevalence (1-3)
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1
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Demand (1-3)
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1
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