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Scapolite Solid solutions of marialite,

Hand Specimen identification: The scapolites are characterized by prismatic crystals with square cross sections and 45° cleavage. Massive samples often have a distinct woody or fibrous ap-pearance. Scapolite may be confused with feldspar.

Chemical Composition : Na4(AlSi3O8)3Cl, and meionite, Ca4(Al2Si2O8)3(CO3,SO4)

Hardness : 5 to 6

Specific Gravity2.55

Cleavage/Fracture : good {100}, poor {110}/

conchoidal

Luster/Transparency : vitreous/transparent to

translucent

Color : colorless, variable

Streak : white

Optical Properties : Scapolite may appear similar to feldspars but is uni-axial and has different cleavage. Uniaxial 1- 2, v = 1.540, e = 1.536, d = 0.004.

Crystallography : Tetragonal, a = 12.11, c = 7.56, Z = 2; space group I4/m; point group 4/m.

Habit : Crystals are typically tetragonal prisms, often woody looking. Massive varieties are common.

Structure and composition : The structure, closely related to that of nepheline, con-sists of AlO4 and SiO4 joined in a three-dimensional framework. The structure contains two types of holes for anions and anionic groups: One holds Na and Ca; the other Cl or CO3. CaAl substitutes for NaSi freely; SO4 and F may substitute for Cl and CO3.1The two2 im-

portant end members are marialite, Na AlSi O Cl, 1 2 1 4 2 3 8 3

and meionite, Ca4 Al2Si2O8 3 CO3,SO4 . Natural scapolites have intermediate compositions. Chemically, scapolites are equivalent to plagioclase feldspar plus CaCO3, NaCl, or CaSO4. Minor K, OH, and F may be present.

Occurrence and associations : Most scapolite occurrences are in calcic metamorphic rocks: marbles, marls or mafic gneisses, and amphibo-lites. Rare occurrences are reported from igneous rocks. Associated minerals include plagioclase, clinopy-roxene, hornblende, apatite, garnet, and sphene.

Varieties :

Related Minerals :

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