Bytownite

Na0:3¡0:1 Ca0:7¡0:9 Al1:7¡1:9 Si2:3¡2:1 O8c 2001 Mineral Data Publishing, version 1.2°

Crystal Data: Triclinic. Point Group: 1: Crystals, °attened along [010], are rare, to 3 cm;
commonly as cleavable masses or anhedral grains in massive aggregates. Twinning: Common
after the Albite, Carlsbad, and Pericline laws, developed in simple to complex combinations.
Physical Properties:
Cleavage: Perfect on f001g, good on f010g, imperfect on f110g.
Fracture: Uneven to conchoidal. Tenacity: Brittle. Hardness = 6{6.5 D(meas.) = 2.72{2.74D(calc.) = 2.713{2.726
Optical Properties: Transparent to translucent. Color: Colorless, white, gray.
Streak: White. Luster: Vitreous, pearly on cleavages.
Optical Class: Biaxial ({). Dispersion: r < v; weak. ® = 1.565{1.573 ¯ = 1.569{1.580° = 1.578{1.585 2V(meas.) = 87± to ¡78± (low); 81± to ¡83± (high).
Cell Data: Space Group: P 1 or I1 for certain compositions.
c = 14.187 ® = 93:5± ¯ = 115:9± ° = 90:63± Z = 8a = 8.178

b = 12.870

X-ray Powder Pattern: Rustenburg mine, Transvaal, South Africa.
3.20 (100), 4.03 (80), 3.75 (80), 3.17 (80), 3.62 (70), 2.94 (70b), 2.52 (70)
Chemistry:
SiO2
Al2 O3Fe2 O3MgO
CaO

(1)
49.06
32.14
0.27
0.20
15.38

(2)
50.54
31.70

(3)
45.62
35.02

14.36

18.24

Na2 O
K2 O
H2 O+
H2 O¡

(1)
2.57
0.17
0.13
0.03

(2)
3.40

(3)
1.12

Total

99.95

100.00100.00(1) Rustenburg mine, Transvaal, South Africa. (2) Na0:30 Ca0:70 Al1:70 Si2:30 O8 :(3) Na0:10 Ca0:90 Al1:90 Si2:10 O8 :Polymorphism & Series:
Mineral Group:

Low- and high-temperature structural modi¯cations are recognized.

Feldspar group, plagioclase series.

Occurrence: A rock-forming mineral typical of ma¯c intrusive igneous rocks, as gabbros andanorthosites; as phenocrysts in basalts; rare in metamorphic rocks.
Association:

Pyroxenes, olivine.

Distribution: One of the less frequently identi¯ed feldspars, although still very common.
\Type" material from Ottawa, Ontario, Canada was later shown to be a mixture. Elsewhere inCanada, from the Shawmere anorthosite, Foleyet Township, Ontario, and on Yamaska Mountain,
near Abbotsford, Quebec. On Rhum Island, Scotland. At Eycott Hill, near Keswick, Cumberland,England. From NÄarÄo dal, Norway. In Greenland, from Fisken½sset and Storo. In the Bushveldcomplex, Transvaal, South Africa. In the USA, in the Stillwater complex, Montana; fromCornwall, Lebanon Co., and Phoenixville, Chester Co., Pennsylvania; near Lakeview, Lake Co.,
Oregon; in the Lucky Cuss mine, Tombstone, Cochise Co., Arizona; and from the Grants district,
McKinley Co., New Mexico. From Isa Valley, Western Australia.
Name:For the \type" locality, in Ottawa (formerly Bytown), Ontario, Canada.

References: (1) Dana, E.S. (1892) Dana's system of mineralogy, (6th edition), 325{327,334{335. (2) Deer, W.A., R.A. Howie, and J. Zussman (1963) Rock-forming minerals, v. 4,framework silicates, 94{165. (3) Phillips, W.R. and D.T. Gri®en (1981) Optical mineralogy,352{360. (4) Goodyear, J. and W.J. Du±n (1954) The identi¯cation and determination ofplagioclase feldspars by the X-ray powder method. Mineral. Mag., 30, 306{326. (5) Fleet,S.G., S. Chandrasekhar, and H.D. Megaw (1966) The structure of bytownite (`body-centeredanorthite'). Acta Cryst., 21, 782{801.
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