Crystal Serpentine

Chemical Formula

The Serpentine group is composed of several related minerals. A generic formula that includes all members is:
X_2-3Si₂O₅(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.
 

Composition

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.)

Color

White, yellow, green, gray, brown, black, purple; sometimes multicolored, especially green and yellow.

Streak

White

Hardness

2 - 5

Crystal System

Monoclinic

Crystal Forms 
and Aggregates

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.

Transparency

Translucent to opaque. Rarely transparent.

Specific Gravity

2.5 - 3.2

Luster

Greasy, waxy, or silky

Cleavage

Usually not discernible because of crystal development.
Chrysotile may exhibit basal cleavage.

Fracture

Conchoidal, splintery

Tenacity

Brittle. Fibrous Serpentine is flexible and elastic.

Other ID Marks

1) Has a greasy feel.
2) Yellow Serpentine often fluoresces a cream-white color in shortwave ultraviolet light.

In Group

Silicates; Phyllosilicates

Striking Features

Greasy feel, soft compared to similar minerals, and may flexible and elastic.

Environment

Serpentine is fairly common in many environments, and is an important rock forming mineral in many metamorphic environments.

Rock Type

Metamorphic

Popularity (1-4)

2

Prevalence (1-3)

1

Demand (1-3)

1