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

2. Information on hydrous pyroxenoids
Pectolite Serandite
Composition NaCa2Si3O8(OH) NaMn2Si3O8(OH)
Colors White, colorless Pink, brown
Hardness
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)
b (Å)
c (Å)
V (Å3) a b g
r (g/cm3)
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 Å3
M2 octahedra Å3
Na polyhedra Å3
Jacobsen (1998)

8. Cation ordering in serandite-pectolite
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