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BITYITE 2M, FROM ERÄJÄRVI COMPARED WITH RELATED Li
BRITTLE MICAS
SEPPO I. LAHTI and RISTO SAIKKONEN
LAHTI, SEPPO I. and SAIKKONEN, RISTO, 1985: Bityite 2M, from Eräjärvi
compared with related Li—Be brittle micas. Bull. Geol. Soc. Finland 57, Part
1—2, 207—215.
Bityite was encountered in thee lithium pegmatite dykes in the Eräjärvi area in
Orivesi, southern Finland. The mineral is closely associated with beryl occurring
in pseudomorphs after it or in cavities with bertrandite, fluorite and fluorapatite
as a fine-scaled white or yellowish mass with a pearly lustre. This is the first descrip-
tion of the mineral from Finland.
A bityite sample found in a small abandoned feldspar quarry called Maantien-
varsi was studied in detail. Wet chemical analysis shows (wt °/o): Si02 33.35,
AljO, 34.61, Fe2Oj tot. 1.37, MgO 1.84, CaO 14.07, BeO 7.21, NazO 0.10, K20
0.16, Li20 2.32, H20+ 5.33 and F 0.39. The formula computed on the basis of
24 anions is as follows (Z = 2):
CaK^KoojNaoo^Li, l9Al3.6gMg0 35Fe0.13)(All 53Be2.21Si4.26)Oi9.3o(OH)4 ,4Fo.16. The
mica is monoclinic and the space group is C2/c or Cc with a = 4.99 Å, b = 8.68 Å,
c= 19.04 Ä and
(3
= 95.17°. The refractive indices are
c*
= 1.650, ß= 1.658, 7= 1.660
and —2Vcalc. = 52.9°. The specific gravity is near 3.05 and the density is 3.12
g/cm3. The following infrared absorbtion bands were recorded for the mineral
(cm-1): 3620, 3451, 1634, 1400, 949, 707, 537 and 431. The lack of absorbtion
near 800 cm-1 caused by Al—O—AI stretching vibrations indicates ordering of
tetrahedral Si and Al + Be.
The analytical data collected from the literature show a series between tri-
octahedral bityite Ca2(Li2Al4)(Al2Be2Si4)O20(OH)4 and octahedral mica margarite
Ca2Al4(Si4Al4)O20(OH)4. Natural Li—Be brittle micas are always deficient in Li,
and there seems to be a miscibility gap between the end members. In contrast to
the ideal formula, at most 1—1.2 of the two possible Li atom sites are filled. The
valency balance is achieved by substituting AP+ for Li1+ + Be2+ and O2- for
(OH + F)1-. The replacement of cations may be more complex.
The Maantienvarsi Li—Be mica is typical bityite with the high content of Li
and Be and its properties agree well with those of similar micas. The sum of
octahedral cations, 5.35, is the highest known of the Li-Be micas reported in the
literature.
Key words: bityite, brittle micas, physical properties, chemical composition, unit
cell, infrared spectroscopy, Eräjärvi, Finland.
Seppo I. Lahti and Risto Saikkonen: Geological Survey, SF-02150 Espoo, Fin-
land.
208 Seppo /. Lahti and Risto Saikkonen
Introduction
White Li- and Be-rich brittle mica replacing
beryl was detected in the granitic pegmatite
»Maantienvarsi» dyke during pegmatite studies
carried out by one of the authors (SIL) in the
Eräjärvi pegmatite area (Lahti 1981, 1983).
Mineralogical studies confirmed that its chemi-
cal composition and physical properties are
comparable to those given by Strunz (1956) for
bityite. The mineral is relatively common in
pseudomorphs after beryl or in cavities in the
Maantienvarsi dyke. Bityite was later also en-
countered in two nearby dykes, »Suonlaita»
and »Keskimetsä»; their mineralogy has been
described earlier by Lahti (1981). Each of the
dykes is zoned and exhibits large sugar albite or
cleavelandite replacement bodies and fracture
fillings containing beryl, columbite—tantalite,
Li phosphates and silicates.
The mineral is not easy to recognize and may
be confused with other micas. About three kilo-
metres north of the Maantienvarsi pegmatite
there is the well-known Viitaniemi pegmatite
(Volborth 1954, Lahti 1981), in which fine-grained
greenish yellow muscovite (gilbertite) with be-
ryllium phosphates (väyrynenite and hydroxyl
herderite) in pseudomorphs after beryl is a com-
mon mineral. Spectrographic determination of
one muscovite sample indicates 570 ppm berylli-
um. In the Leikattu pegmatite, in the western
part of the area, the pseudomorphs are filled
with montmorillonite and bertrandite, whereas
in the Juurakko dyke black or silky light green
chlorite occurs with beryl and bertrandite. The
beryllium content of the Juurakko black, iron-
rich chlorite is as high as 1100 ppm (emission
spectrography).
After a careful search for altered beryl crystals,
small amounts of bityite were encountered in
one pegmatite dyke in the Kitee—Tohmajärvi
area, eastern Finland (see Kallio and Alviola
1975) and in the Kaatiala dyke, western Finland
(see Haapala 1966). The mineral may also occur
elsewhere in beryl-bearing dykes where hydro-
thermal solutions caused alteration at the end of
the crystallization of the dyke.
Bityite has been reported as a rare mineral
from only a few pegmatite deposits. Its proper-
ties and occurrence are not fully known. The
type bityite was described by Lacroix (1908)
from Mt Bity, Madagascar. The mineral bow-
leyite described by Rowledge and Hayton (1947)
from Londonderry, western Australia, is also
bityite (Strunz 1956). Bityite or intermediate mi-
cas between bityite and margarite have later
been reported from several localities in Africa
(Gallagher and Hawkes 1966), from the USSR
(Beus 1956, Kutukova 1959), from Germany
(Tennyson 1960) and from Italy (Lin and Gug-
genheim 1983).
A great variety of names have been given for
Li-Be micas in the literature. To avoid confu-
sion in definitions, the present authors use the
following classification (see Figure 3 showing
the atomic composition of the micas in Li vs. Be
diagram):
— bityite contains both Li and Be >
1
atom/
formula
— beryllium margarite contains Be >
1
atom/
formula and Li <
1
atom/formula
— lithian and/or beryllian margarite contains
both Li and Be<l atom/formula.
For this study bityite was separated from a
pseudomorph after beryl of the Maantienvarsi
pegmatite dyke (abandoned feldspar quarry).
One of the authors (SIL) is responsible for the
mineralogical studies and the other (RS) for the
chemical analysis.
Occurrence
Bityite always occurs in close association with
beryl, replacing it either in pseudomorphs after
beryl (Fig. 1) or in cavities associated with
altered beryl crystals in a dyke in the middle of
the Maantienvarsi pegmatite. The 2—4-m-wide
dyke occurs in an almost E—W direction in the
contact between a Svecokarelian late orogenic
Bityite 2M, from Eräjärvi compared with related Li-Be brittle micas 209
Fig. 1. Bityite-bearing pseudomorph (B) after beryl in a peg-
matite sample from the Maantienvarsi feldspar quarry.
Associated minerals: A = albite, Q = quartz and M =
muscovite.
microcline granite stock and surrounding mica
schist. The main minerals are perthitic micro-
cline, quartz, albitic plagioclase, muscovite and
black tourmaline. There is a small abandoned
feldspar quarry at the dyke.
The dyke is zoned. The border zone is com-
posed of fine-grained oligoclase pegmatite of
increasing grain size so much so that the inter-
mediate zone contains microcline crystals one
metre long. The rare minerals, including beryl,
bityite, spodumene, cassiterite, Fe-tantalite, le-
pidolite, zircon, lithiophilite and its alteration
products, are concentrated in albite-rich pegma-
tite parts between the big microcline crystals or
in small sugar albite or cleavelandite veins
crosscutting them.
The pseudomorphs filled with bityite are
always small, 0.5—3.0 cm in diameter, and also
contain varying amounts of small, purple fluo-
rite crystals and occasionally thin transparent
bertrandite crystal plates, light green fluorapa-
tite, quartz and corroded remnants of beryl.
The cavities rich in bityite are also small and
contain the same minerals, but the crystals of
fluorite, fluorapatite and bertrandite are often
euhedral. Bityite has crystallized as an altera-
tion product of beryl from fluorine-rich, prob-
ably acid hydrothermal solutions or fluids, al-
though Cerny (1968) has suggested that alumo-
beryllosilicates of alkalies and alkali earths are
indicative of alkaline parent solutions.
Physical properties
Bityite occurs as a fine-scaled (0<O.3 mm)
white or yellowish mass. The strong pearly
lustre of the mineral is one of its characteristic
properties. The indices of refraction measured
by the immersion method are as follows: a =
1.650 ±0.001, 0= 1.658 ±0.001, 7= 1.660
±0.001, 7
—
a = 0.010 and — 2Vcalc. = 52.9°.
The determinations were carried out in Na light.
The index of the refraction of the liquids was
tested on an Abbe refractometer. The optic axial
angle could not be measured in a universal stage
because the mica flakes are so thin.
The refractive indices of the Maantienvarsi
bityite are very close to those reported for the
Madagascar bityite (a =1.651, ß= 1.659 and
7=1.661; Strunz 1959) and Londonderry bit-
yite (a= 1.650, and 7=1.661; Rowledge and
Hayton 1947). Substitution of Al by Li + Be
causes an increase in the refractive indices, and
the corresponding values of the Be—Li marga-
rites intermediate in composition are generally
much lower.
The specific gravity of the Maantienvarsi bit-
yite, almost 3.05, was measured with heavy liq-
uids. The density of the liquids was tested with
a Westphal balance. Exact determination failed
owing to the fine particle size of the mineral,
but the value obtained is close to the calculated
density 3.12 g/cm3 and the specific gravity of
other bityites described in the literature.
X-ray studies
The mineral was studied with a precession
camera. O-, 1- and 2-level, a- and b-axis photo-
14
210 Seppo I. Lahti and Risto Saikkonen
graphs were taken using Zr-filtered Mo radia-
tion. The precession photographs show mono-
clinic symmetry for the mineral with systematic
absences of h + k^2n for general reflections
and l=?t2n for hOl reflections, indicating
possible space groups of C2/c or Cc. On the
basis of three mica flakes studied the mineral is
a two layer-type modification (polytype 2M,).
The unit cell dimensions measured from the
films are as follows: a = 4.99 Å, b = 8.68 Å,
c = 19.04 Å, 0 = 95.17° and V = 821.33 Å3. The
shrinkage of the films was corrected using
silicon reflections exposed to the films.
The unit cell data on the Maantienvarsi bit-
yite and the type bityite from Mt Bity Mada-
gascar, are almost identical. Professor Th. G.
Sahama kindly donated a small sheet of the type
bityite (from Madagascar) for our studies. He
had got the mica sample from Professor Hugo
Strunz, who used the same material in his
studies in 1956. The precession camera studies
show that the symmetry, unit cell dimensions
(a = 5.02 Å, b = 8.69 Å, c = 19.07 Å, ß = 95.08°
and V = 828.64 Å3) and intensities of the reflec-
tions of both bityites are nearly identical. X-ray
powder diffraction studies on the same material
with a Debye-Scherrer camera (114.6 mm in
diameter) confirmed the results and both of the
x-ray powder diffraction film data are com-
parable to those given for bityite in JCPDS-
card 11—400.
The unit cell of the Li- and Be-rich brittle
mica of intermediate composition from the
Mops pegmatite (the chemical composition
given in Table 1, analysis no. 10) and studied in
detail by Lin and Guggenheim (1983) is slightly
greater. Their results are, however, consistent
with our determinations, because the substitu-
tion of Al by Li + Be reduces the lateral dimen-
sions of both octahedral and tetrahedral sheets
in particular.
Chemical analysis
For the chemical analysis bityite was sep-
arated with heavy liquids (dijodmethan) and a
Franz isomagnetic separator. The density frac-
tion 3.05—3.15 was accepted for study. The
chemical composition of bityite was first anal-
ysed semiquantitively with an electron micro-
probe and optical emission spectrograph and
then with wet chemical methods.
The sample was dried at 105°C. One portion
(0.1 g) was fused with sodium carbonate. Silica
was determined gravimetrically by dehydration
with hydrochloric acid and the residual silica
retained in the filtrate was determined color-
imetrically. Aluminium, magnesium and cal-
cium in the filtrate of silica were analysed by
atomic absorption spectrophotometry.
One portion (0.1 g) was decomposed by treat-
ment with hydrofluoric-sulphuric acid, and
aliquots were taken to measure manganese and
total iron colorimetrically, and sodium, potas-
sium, beryllium and lithium by atomic absorp-
tion spectrophotometry. H20+ was deter-
mined gravimetrically using the Penfield method.
Fluorine was analysed with an ion-selective
electrode.
Table 1 shows the chemical analysis (no. 12)
of the mineral compared with eleven Li—Be
brittle mica analysis taken from the literature.
Computed on the basis of 24 (O, OH, F) the
formula of the mineral may be written as fol-
lows (Z = 2):
Ca, 19K0 03Na0 02(Li, 19Al3 68Mg0 35Fe013)5 35
(Al,
.53Be2
2,Si4 26)80 19 30(OH)4 54F0 ,6.
Infrared spectrum
The infrared spectrum of the Maantienvarsi
bityite is compared in Figure 2 with the spec-
trum of Li and Be poor margarite (Li < 500 ppm
and Be <50 ppm, spectrographic determina-
tions) from Enontekiö, northern Finland. The
spectra were recorded at the Technical Research
Centre of Finland with a Perkin-Elmer 983
spectrophotometer fitted with a Data Station
3600 computer. The conventional pressed disc-
Bityite 2M, from Eräjärvi compared with related Li-Be brittle micas 211
Table 1. The chemical composition of Maantienvarsi bityite (no. 12) compared with the composition of the related Li—Be
brittle micas reported in the literature.
1 2 3 4 5 6 7 8 9 10 11 12
Si02 32 29.84 30.16 30.88 31.95 30.44 33.1 36.1 33 31.26 33.37 33.35
Ti02 0.1 — 0.00 — — — 0.0 0.2 0.0 0.00 0.00
AI2O3 42 47.35 45.59 46.22 41.75 45.56 37.0 40.8 37 44.37 36.24 34.61
Fe203 0.0 0.81 0.13 0.37 — 0.38 0.02 0.02 0.02 0.17 0.19 1.37
MnO — — — — — — — — — 0.00 0.00
MgO — 1.22 1.06 0.87 0.13 0.67 0.1 0.1 0.1 0.04 1.84
CaO 14 11.70 13.18 13.90 14.30 13.48 14.5 12.7 14.6 13.64 14.42 14.07
BeO 2.5 1.18 1.88 2.67 2.27 3.26 7.2 3.8 7.2 4.1 7.30 7.21
BaO — — 0.00 — — 0.04 _ _
NazO 0.8 1.93 1.08 0.70 0.40 0.57 0.1 0.4 0.1 0.19 0.29 0.10
K2O 0.5 0.55 0.52 0.21 0.16 0.25 0.03 0.1 0.03 0.01 0.04 0.16
Li20 0.05 0.47 0.72 0.38 2.73 0.78 1.8 1.9 2.0 2.1 2.39 2.32
H2O + 6 4.48 3.80 3.40 6.50 3.98 6.1 4.7 6.0 5.1 5.72 5.33
F — 0.70 1.64 1.35 — 0.82 — — — 0.00 0.39
97.95 100.23 99.76 100.95 100.19 100.23 99.95 100.82 100.05 100.94 100.00 100.75
—0 = F — 0.29 0.69 0.57 — 0.35 — — — 0.16
97.95 99.94 99.07 100.38' 100.19 99.882 99.95 100.82 100.05 100.94 100.003 100.594
Atoms based on 24 (O, OH, F)
4.01 4.26 4.26
2.73 1.50 1.53
1.26 2.24 2.21
8.00 8.00 8.00
3.98 3.94 3.68
— 0.00 0.00
0.02 0.02 0.13
— — 0.00
— 0.01 0.35
— — 0.00
1.08 1.23 1.19
5.08 5.20 5.35
1.88 1.97 1.93
0.05 0.07 0.02
0.00 0.01 0.03
1.93 2.05 1.98
4.37 4.87 4.54
— — 0.16
4.37 4.87 4.70
19.63 19.13 19.30
All iron assumed ferric — not determined 3 ignition loss as H20 + 2 omitted wt% S 0.11, Cl 0.20 1 omitted wt%
Cr20, 0.40, CI 0.13 4 omitted (ppm) B 1500, Ga 160, Cu 14, Sn 88, Cr<20, Ti<200, emission spectrographic
determinations.
The locations of the samples and references:
1. Namhere Mine, Ankola, Uganda (Gallagher and Hawkes
1966).
2. USSR (Beus 1956).
3. Urals, USSR (Kutukova 1959).
4. Urals, USSR (Kutukova 1959).
5. Maharitra, Madagaskar (Strunz 1956).
6. Urals, USSR (Kutukova 1959).
7. No Beer pegmatite, Bikita district, Zimbabwe (Gallagher
and Hawkes 1966).
8. Mops pegmatite, Salisbury district, Zimbabwe (Gal-
lagher and Hawkes 1966).
Si 4.25 3.96 4.04 4.08 4.12 4.01 4.21 4.62 4.20
Al 2.95 3.66 3.36 3.07 3.18 2.96 1.59 2.21 1.60
Be 0.80 0.38 0.60 0.85 0.70 1.03 2.20 1.17 2.20
EZ 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00
Al 3.63 3.75 3.83 4.13 3.16 4.12 3.96 3.95 3.95
Ti 0.01 — — — — — 0.00 0.02 0.00
Fe3 + 0.00 0.08 0.01 0.04 — 0.04 0.00 0.00 0.00
Mn — — — — — —
Mg — 0.24 0.21 0.17 0.03 0.13 0.02 0.02 0.02
Ba — — — — — 0.00 —
Li 0.03 0.25 0.39 0.20 1.41 0.41 0.92 0.98 1.02
EY 3.67 4.32 4.44 4.54 4.60 4.70 4.90 4.97 4.99
Ca 1.99 1.66 1.89 1.97 1.97 1.90 1.98 1.74 1.99
Na 0.21 0.50 0.28 0.18 0.10 0.15 0.02 0.10 0.02
K 0.08 0.09 0.09 0.04 0.03 0.04 0.00 0.02 0.00
EX 2.28 2.25 2.26 2.19 2.10 2.09 2.00 1.86 2.01
OH 5.32 3.97 3.39 3.00 5.59 3.50 5.18 • 4.01 5.09
F — 0.29 0.69 0.56 — 0.34 —
(OH + F) 5.32 4.26 4.08 3.56 5.59 3.84 5.18 4.01 5.09
O 18.68 19.74 19.91 20.44 18.41 20.16 18.82 19.99 18.91
9. Benson 4 pegmatite, Mtoko District, Zimbabwe (Gal-
lagher and Hawkes 1966).
10. Mops pegmatite, Zimbabwe (Lin and Guggenheim 1983,
microprobe analysis of the sample no. 8 in this table).
11. Londonderry feldspar quarry, western Australia (Row-
ledge and Hayton 1947).
12. Maantienvarsi, Finland (This study).
212 Seppo I. Lahti and Risto Saikkonen
technique, in which 1 mg of the powdered
sample was ground with 300 mg KBr, was used.
The following absorption bands were re-
corded for Maantienvarsi bityite (cm-1): 3620,
3451, 1634, 1400, 949, 707, 537 and 431. The
spectrum is near to that given by Farmer and
Velde (1973) for two »beryllium margarite»
samples from Zimbabwe. They studied the
material of Gallagher and Hawkes (1966),
whose analyses are referred to in Table 1 (nos.
8, 10 and 9) in this study. In fact, the composi-
tion of the sample richest in Be (7.2 wt %) and
Li (2.0 wt %) is near the composition of the
Maantienvarsi Li- and Be-rich mica called bit-
yite by the present authors.
Farmer and Velde (1973) have discussed the
infrared spectrum of Li- and Be-bearing mar-
garites and related micas in detail. The sharp
absorption bands that characterize the spectrum
of both the bityite and the margarite studied by
the present authors distinguish them from those
reported by Farmer and Velde (1973). In their
interpretation, the diffuse absorptions of the
micas are due to the random substitution of
tetrahedral Al by Be compensated by additional
Li in octahedral layers.
The sharp infrared spectrum of the Maan-
tienvarsi bityite indicates the well-ordered struc-
ture of the mineral. Evidence also exists for the
absence of absorbtion near 800 cm-1, which
Fig. 2. The infrared spectrum of (a) bityite from the Maantienvarsi pegmatite compared with that of (b) margarite from
Enontekiö, northern Finland.
Bityite 2M, from Eräjärvi compared with related Li-Be brittle micas 213
should indicate Al—O—AI bonds in the struc-
ture (Farmer and Velde 1973). Silicon and alu-
minium + beryllium do not occupy alternate
sites in the tetrahedral layers and thus the Al—
O—AI stretching vibrations typical of disor-
dered micas are prevented.
The infrared spectrum or margarite and bit-
yite shows many common absorbtions. The
most prominent differences are below 700
cm-1, but the absorption bands between 3460
and 3650 cm^' caused by OH groups are also
different. As shown by Farmer and Velde (1973)
and confirmed by the present authors, bityite
has two strong OH absorptions near 3620 and
3450 cm-1 in contrast to the one strong and
one to two weak absorptions in the spectrum of
margarite.
Discussion
Bityite was described as a new mineral by
Lacroix (1908) and the unit cell data for the
mineral were given by Strunz (1956). Lin and
Guggenheim (1983) carried out a detailed crystal
structure analysis of a related Li- and Be-bear-
ing mica, and later Guggenheim (1984) exam-
ined closely the structure of the Li-Be and other
brittle micas. The structural determination by
Lin and Guggenheim (1983) was preceded by a
study by Farmer and Velde (1973), who used
infrared spectroscopy to investigate the or-
dering of tetrahedral cations of brittle micas.
The data available on the chemistry and proper-
ties of bityite and related micas are, however,
limited.
Strunz (1956) generalized the formula of bit-
yite as Ca2(Li2Al4)(Al2Be2Si4)O20(OH)4. Most of
the Li- and Be-bearing micas described in the
literature are intermediate in composition
between trioctahedral bityite and dioctahedral
margarite Ca2Al4(Al4Si4)O20(OH)4. Table 1 lists
the chemical analyses and atomic composition
of bityite and related micas given in the lit-
erature. Beryllium occurs in the tetrahedral coor-
dination replacing aluminium and silica. Lithium
and aluminium occupy octahedral cation sites
with minor magnesium and iron.
The valency balance between octahedral and
tetrahedral layers is achieved by substituting
Al5+ for Li1+ +Be2+. Replacement of 02~ by
(OH + F)1" also seems probable, because near-
ly all the micas analysed show excess (OH + F).
The fluorine concentration is not very high, al-
though the mineral often occurs with fluorite
or other fluorine-rich minerals. The substitu-
tions of various elements in the Li-Be brittle
micas have been examined in many papers (see
e.g. Strunz (1956), Ginsburg (1957), Schaller et
al. (1967) and Guggenheim (1984)).
Calcium is the main interlayer cation, but
potassium and especially sodium can replace it
in small amounts. The miscibility gap towards
ephesite Na2(Li2Al4)(Al4Si4)O20(OH)4 is very
large, as shown by Schaller et al. (1967). Beus
(1956), however, gives an analysis (see Table 1,
no. 2 in the present study) of a lithian beryllian
margarite, where 22 % of the interlayer cations
are occupied by sodium. However, no beryllium-
bearing ephesites are known.
Only limited data are available on the trace
element composition of brittle micas. In general
trace elements seem to show marked variation
between samples.
If we follow the formula proposed for the
mineral by Strunz (1956), no ideal bityite is
known. Figure 4 shows that the content of Li in
micas, and similarly also the sum of octahedral
cations, which is generally between 4.3 and 5.4,
increases concomitantly with the increasing
content of Be. Three bityite analyses given in
Table 1 (no. 10 from Zimbabwe, no. 11 from
Londonderry and no. 12 from Finland) contain
more than five cations in the octahedral sites,
when six are available.
The ratio of Si/Al + Be in tetrahedral sites in
the Li- and Be-bearing brittle micas is near
1:1, and the cations show the ordered struc-
ture indicated by the infrared studies by Farmer
and Velde (1973) and the present authors, and
214 Seppo I. Lahti and Risto Saikkonen
2-1
<
3
CU
o 1
o
t—
<
A
m 12,ii
.10 mS
7«
I I
.3 .6
•2 .4
1.
2.0
3
CC
O
• 1.0
o
h-
<
(b) .12
g. .10
0 12 3
Be ATOMS/ FORMULA
I LITHIAN ANO/OR BERYLLI AN
M
ARGARITE
H BERYLLIUM MARGARITE
IH BITYITE (A = IDEAL COMPOSITION)
Fig. 3. The Li versus Be diagram of the Li—Be brittle micas
given in Table 1. The figure shows the classification of
micas used in this study.
confirming the crystal structure determination
by Lin and Guggenheim (1984). The aluminium
content fluctuates, but decreases generally with
increasing Li + Be substitution. The Be/Li ratio
varies between 1.0 and 2.4, being generally
more than unity.
The Li vs. Be plot (Figure 3) shows that, com-
pared with the ideal formula of bityite, the Li
content is never ideal: at most
1
—1.2 of two
possible atom sites are filled. From a detailed
crystal structure analysis, Lin and Guggenheim
(1983) have concluded that the octahedral M(l)
site in the structure of the Li—Be brittle mica
from the Mops pegmatite, Zimbabwe (see Table
1, analysis no. 10) contains 0.55 Li and 0.45
vacancy, whereas the other octahedral sites
M(2) and M(3) are fully occupied by Al. Lithium
for vacancy substitution in the octahedral sheets
is possibly a common feature of all the Li- and
Be-bearing margarites and bityites and may
explain the intermediate structure of the micas.
Two separate groups — bityites and Li- and
Be-bearing margarites — can be distinguished
in the figures for the Li vs. Be (Figure 3), Li vs.
octahedral cation sum (Figure 4b) and Be vs.
3.0 4.0 5.0 6.0
SUM OF OCTAHEDRAL CATIONS/FORMULA
12.0-
tr
o
LL_
\
I/)
S
O
<
4)
CD
1.0
(a)
7..9 • .12
.10
3.0 4.0 5.0 6.0
SUM OF OCTAHEDRAL CATIONS/ FORMULA
Fig. 4. a) The Be versus octahedral cation sum diagram, b)
the Li versus octahedral cation sum diagram. The figures
were compiled from the chemical analyses given in Table 1.
octahedral cation sum (Figure 4a). This in-
dicates a miscibility gap between the end mem-
bers. According to the classification, only four
samples (nos. 9—12) plot in the bityite field and
two in the beryllium margarite field (nos. 6 and
7) being, however, very near the border lines;
the others are lithian and/or beryllian mar-
garites. Analysis no. 10 is a microprobe analysis
of sample no. 8, but includes Li and Be from
the wet chemical data. Lin and Guggenheim
(1983) have suggested that the difference in the
silica content of the mica may be due to quartz
contamination. The type bityite from Madagas-
Bityite 2M, from Eräjärvi compared with related Li-Be brittle micas 215
car (no. 5) can be plotted in the lithium mar-
garite field, but it has been omitted because the
old analysis of the mineral is erroneous (Flei-
scher 1950) and contains excessively low con-
centrations of beryllium.
The mica sample studied from the Maantien-
varsi pegmatite, Finland, is typical bityite in
composition and closely resembles bityite (bow-
leyite) from Londonderry with a nearly similar
content of Li and Be. The physical properties
also agree well with those reported for similar
micas. The mineral is rich in silica and poor in
aluminium, having the lowest Al/Si ratio 1.22
of all the micas given in Table 1. The contents
of magnesium and iron in the mineral are con-
siderable compared with those in other Li- and
Be-bearing micas. The sum of the octahedral
cations, 5.35, is the highest known in bityites.
Acknowledgements. The authors are indebted to Jukka
Keskinen, Jussi Kuusola and Hannu Mäkitie, who helped in
many ways during the studies. Special thanks are due to Rit-
va Forsman for drawing the figures, to Mirja Saarinen for
the X-ray powder diffraction runs, to Ragnar Törnroos for
the microprobe determinations and to Raimo Lahtinen and
Ari Puisto for the optical emission spectrographic analyses.
The IR spectrum of bityite and margarite were recorded by
Lasse Kalervo at the Technical Research Centre of Finland,
Espoo. Reijo Alviola and the late Professor Th. G. Sahama
kindly provided reference material for our studies.
Professor Atso Vorma and Dr. Kai Hytönen read the
manuscript and offered valuable criticism.
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