Datolite
Introduction
The English  word was derived from the Greek word dateisthai (?to divide?).It  occurs in cavities and veins in hypabyssal and volcanic basic igneous rocks. It  is almost always a secondary mineral and it may be associated with calcite,  prehnite, and zeolites.
Physical  properties
The physical  and optical properties are outlined in Table 1.Datolite generally is colorless  but it can show other varieties in color such as pale-yellow, tinted pink, and  pale-green. The luster is vitreous and the cleavage is absent so it exhibits  conchoidal fracture. The hardness and specific gravity ranges between 5.0-5.5  and 2.8-3.0 g/cm3, respectively. Optically, datolite is biaxial and  negative.It does not display any luminescence.
Table 1. Physical properties of Datolite
Physical Property Characteristic
Chemical formula CaBSiO4(OH)
Mineral group Gadolinite group
Forms [110] [011]      [112] [100] [111]
Habits Crystals commonly short prismatic often with a variety of forms. Also granular or as      porcelain-like compact masses
Color White, colorless, yellowish, reddish, gray, brown by impurities, light green
Color (thin section)
colorless
Luster Vitreous to greasy
Transparency transparent  to translucent, rarely opaque
Cleavage absent
Fracture Conchoidal to uneven
Hardness 5.0-5.5---knife blade
Specific gravity 2.8-3.0
Streak     Uncolored/white
Luminescence None
Optically Biaxial      negative
α=1.660
β=1.684
γ=1.690
Tenacity brittle
III.Chemistry  analyses
The majority  of datolite chemical composition is SiO2. Typical chemical  analyses are displayed in Tables 2, 3, and 4.
Table 2. Typical chemical analysis of datolite from various locations
Chemical formula 1 2 3 4 5 6 7
SiO2 38.00 36.16 36.94 35.92 37.43 37.08 37.52
B2O3 19.38 22.60 21.15 20.61 21.87 21.90 21.53
Al2O3 0.65 0.54 0.31 0.31 0.57 0.52 0.38
Fe2O3 0.28 Trace 0.09 0.18 0.57 0.17 --------
FeO --------- --------- --------- --------- --------- ---------- 0.31
MnO 0.39 ---------- 0.02 ---------- --------- ---------- ----------
MgO 0.09 ---------- 0.00 ---------- --------- Trace 0.65
CaO 35.39 35.30 35.17 35.92 34.67 34.61 33.44
Na2O ---------- ---------- Trace --------- -------- --------- 0.13
K2O ---------- ----------- Trace --------- --------- --------- 0.12
H2O+ 5.71 5.40 6.06 5.38 5.67 5.78 5.70
H2O- 0.26 ---------- 0.10 -------- 5.67 -------- 0.20
Total 100.15 100.00 99.84 100.21 100.23 100.06 99.98
α 1.623 1.625 1.626 1.6260 1.626 1.6241 1.626
β 1.652 1.648 ------- 1.6539 1.653 1.6525 1.654
γ 1.669 1.665 1.686 1.6699 1.670 1.6690 1.670
2V -------- ---------- 72 deg 74 11?, 74 16? 74 16? 74 11? ---------
D ---------- 2.989 -------- 2.993 3.001 ---------- 2.996
1 Datolite in calcite veins in diabase sill,  Silver Harbor, Lake Superior, Ontario
2 Datolite from Northern Italy
3 Large datolite crystals, Tiflis, Georgia,  Russia
4 Datolite, in fissures in diabase with zeolites,  albite, and calcite?Radotin, Bohemia
5 Datolite, from epidiorite dyke in hornblende  schist
6 Colorless and white datolite, with white  prehnite in cavities in diabase?Italy
7 Datolite, copper-pyrrhotite contact?Japan
(Deer et al. 1963)
Table 3.Chemical composition (wt. %) of Datolite  Specimens
Component Chemical      composition (wt. %)
SiO2 37.42
TiO2 0.002
Al2O3 0.18
Fe2O3 0.07
FeO 0.16
MnO 0.005
CaO 34.72
MgO 0.001
Na2O 0.09
K2O 0.04
B2O3 21.11
H2O+ 5.92
H2O- Not observed
∑ 99.72
Table 4. My chemical analysis
    oxygens/ 5
Si 1.03
Ti 0
Al 0.02
    Fe3+ 0
    Fe2+ 0
Mn 0.01
Ca 1.02
Mg 0.02
Na 0
K 0
B 0.9
H 0.95
    Total 8.95
Formula weight 275.2692
weight % oxides
SiO2 22.48293
B2O3
Al2O3 0.740802
Fe2O3 0.0000
MnO 0.257675
MgO 0.292877
CaO 20.73577
H2O 6.219003
    Crystal System     Monoclinic
a 9.66
b 7.64
c 4.83
α 90.15
β 90.15
γ 90.15
    cell volume 356.4644
Z 4
    density 5.12848
(1 1 0) d 5.992374
2 theta 14.77028
(0 1 1) d 4.082568
2  theta 21.75012
(1 1 2) d 2.239937
2 theta 40.22582
(1  0 0) d 9.66
2  theta 9.146785
(1 1 1) d 3.76052
2  theta 23.63856
IV.Structure
The structure of datolite was first determined by using film data (Foit et al. 1973).  The space group and unit cell dimensions of datolite were first described in  1929. It is an orthosilicate containing separate SiO4 tetrahedra.   Its structure may be described as consisting of superimposed complex sheets of  linked oxygen and O,OH tetrahedra around silicon and boron atoms respectively. In the sheet SiO4 and B(O,OH)4 tetrahedra alternate  forming rings of four and eight tetrahedra (Danas 1993).  The sheets bonded  together by calcium atoms (Foit et al. 1973).In the sheets, the  bridging oxygens are bonded to one Si, one B, and one or two Ca atoms; whereas  the non-bridging oxygens are bonded either to one Silicon atom and two Calcium  atoms or to one B, one Hydrogen, and two Calciums (Foit et al. 1973).
The  crystallographic data for datolite is illustrated in Table 5. The structure of  datolite is displayed in Figure 1.The various structure plans are displayed in  Figure 5.
Table 5. Crystallographic Information
Crystallographic info Characteristics
Crystal  System Monoclinic
Point group 2/m
Space group P21/c
Unit Cell  Parameters a = 9.62, b        = 7.6, c = 4.84
Z 4
Density  (literature value) 2.98
Density (calculated value) 5.13
β = 90.15
Beta
a:b:c =  1.2643:1:0.6321
Axial ratios
V. X-ray Diffraction
An X-ray  diffraction pattern was obtained for a sample of datolite and is shown in Figure  2. The background and alpha2-radiation were subtracted so that the peaks can  be located using the program package DSMNT (Scintag, Inc., 1998).The processed  pattern can be seen in Figure 3. A powder pattern was calculated and presented  in Figure 4. The observed and calculated diffraction peaks are given in Table  6.
VI. Occurrences
There are several occurrences of datolite.  The most common of  them is in cavities and veins in hydrothermal and volcanic basic igneous rocks. In most cases, datolite is a secondary mineral and may be associated with  calcite, prehnite, and zeolites.Datolite can occur in deposits in limestone  granite contacts and occasionally in granite. Particularly datolite-rich rocks  can occur from contact metamorphosed sediments, where is associated with garnet,  calcite, and fluorite (Danas 1993).
VII. References
Deer, W.A., R.A. Howie, and J.  Zussman (1963) Rock Forming Minerals Vol. 1
Longmans, London.  171-175pp.
Foit, F.F., Jr., M.W. Phillips,  and G.V. Gibbbs. (1973) A refinement of the crystal
Structure of Datolite,  CaBSiO4(OH).American Mineralogist 58, 909-914.
Gaines, R.V., H.C.W. Skinner,  E.E.Foord, B. Mason, A. Rosenzweig, V.T. King and
E. Dowty (1997). Dana's; ew  Mineralogy, Eighth Edition, New York, John Wiley & Sons, Inc.
Figure 5.Crystal Plans of Datolite
Figure 5a
Plan of a part of a crystal of datolite showing the forms c(001), a(100), M(110), o(120), t(013), g(012), m(011), T(702), x(101), H?(308), y(104), n(111), L(112), W(114), b(121), U(123), and D(133). Parker mine
Figure 5b
Plan of a crystal of datolite showing the c(001), a(100), M(110), t(013), g(012), m(011), x(101),

 
Figure 5c
Doubly  terminated crystal of datolite showing the forms a(100), M(110), m(011), n(111), L(112), e(112), g(111),  and D(133). Parker mine.
Figure 5d
Crystal  of datolite showing the forms c(001), a(100), M(110), m(011), n(111), and D(133).
Parker  Mine.
Figure 5e
Plan of a crystal of datolite showing the c(001), M(110), g(012), m(011), n(111), e(112), g(111), Q(122), b(121), D(133), and k(4.7.10). Parker mine
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