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Pectolite and Serandite

Pectolite and Serandite. Pectolite NaCa 2 Si 3 O 8 (OH). Serandite NaMn 2 Si 3 O 8 (OH). Information on hydrous pyroxenoids. Pectolite Serandite Composition NaCa 2 Si 3 O 8 (OH) NaMn 2 Si 3 O 8 (OH) Colors White, colorless Pink, brown Hardness 5 5

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Pectolite and Serandite

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  1. Pectolite and Serandite Pectolite NaCa2Si3O8(OH) Serandite NaMn2Si3O8(OH)

  2. Information on hydrous pyroxenoids Pectolite Serandite Composition NaCa2Si3O8(OH) NaMn2Si3O8(OH) Colors White, colorless Pink, brown Hardness 5 5 Cleavage {001} {100} {001} {100} Environment: Secondary, alteration mineral in alkali-rich rocks, associated with Zeolites. Famous localities: Patterson, NJ (pectolite); Mont Saint-Hiliare, Quebec (serandite)

  3. Crystallographic data for serandite and pectolite Serandite Pectolite Space group P-1 P-1 a (Å) 7.7185 7.9910 b (Å) 6.9064 7.0278 c (Å) 6.7624 7.0236 V (Å3) 350.56 383.29 a 90.492 90.524 b 94.085 95.181 g 102.775 102.524 r (g/cm3) 3.431 2.880 from Jacobsen (1998)

  4. Hydrous Pyroxenoid structure (Pectolite) Yellow/orange - Ca (Mn) Green - Na Light blue - Si Dark blue - H

  5. Hydrous pyroxenoid structure: T-O layers Yellow/orange - Ca (Mn) Green - Na Light blue - Si Dark blue - H

  6. Si-OH bonding in pectolite-serandite Takéuchi and Kudoh (1977) proposed a split-mode bonding environment for H in pectolite, so that H would be bonded sometimes to O3 and sometimes to O4. However, this proposal could not be confirmed by later studies of serandite structure (Jacobsen 1998).

  7. Polyhedral volumes in serandite M1 octahedra 14.806 Å3 M2 octahedra 14.356 Å3 Na polyhedra 25.418 Å3 Jacobsen (1998)

  8. Cation ordering in serandite-pectolite Ca Site occupancy M1 ideal M2 ideal Mn NaCa2Si3O8(OH) NaMn2Si3O8(OH) Bulk composition Na and Ca have preferred sites in the structure. Na will always occupy an M1 site if one is available; Ca will occupy an M2 site. Ohashi & Finger (1978)

  9. Comparison of pectolite-serandite with other 3-repeat pyroxenoids • Underbonding of oxygens allows hydrogen in the structure • Apical oxygens are linked to the octahedra differently, causing the octahedra to distort differently with tetrahedral tilting Pectolite Wollastonite

  10. Reasons to study hydrous pyroxenoids Insight into: • Mechanisms of storing water in mantle rocks • How H in minerals affects the mineral’s properties • The unusual Si-OH bonding environment

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