Troktolite
Category: Plutonic
Type Gabbroic rock.
Commons Troctolite is a mafic intrusive rock. It is a variety of gabbro composed predominantly of plagioclase and olivine. It may also contain small amounts of pyroxene. It is distinguished from typical gabbro by the absence of pyroxenes.
Name origin Troctolite is named after the Greek word for trout. It was first mentioned in 1875 by Arnold von Lasaulx, a German mineralogist and petrologist.
Locality Hualalai Volcano, Island of Hawaii (Hawaiian Islands), Kaupulehu lava flow from 1800 (sample donated by J. Lexa)
GPS: 19° 45' 50,22" N, 155° 58' 44,18" W
Major minerals Plagioclase rich in Ca and olivine.
Accessory minerals Pyroxene.
Classification
Colour White to light gray with green olivine crystals.
Structure Massive (compact).
Granularity
Texture Phaneritic, equigranular.
Alterations Rock is unaltered.
Petrographic characteristics A 7 cm troctolite xenolith is found within the alkalic basalt of the 1800 Kaupulehu lava flow, which erupted from the northwest rift (at an elevation of 1700 m above sea level) of the Hualalai volcano. Troctolite is distinctive in that it is composed solely of plagioclase and olivine. Although it belongs to the gabbro group, rocks that are typically dark, troctolite is very light in color. White plagioclase and green olivine grains can be distinguished macroscopically. The Kaupulehu flow is one of the youngest lava flows of the Hualalai volcano. At the time of the eruption, the basaltic lava flow exhibited very low viscosity and high velocity, exceeding 10 m/s (40 km/h) and causing extensive damage. The total length of the lava flow from the rift vent to the sea is 15 km (Guest et al., 1995). Xenoliths of troctolite and other mafic and ultramafic rocks, such as dunite, wehrlite, websterite, gabbro, and anorthosite (Bohrson & Clague, 1988; Chen et al., 1992; Schamberger & Hammer, 1996)—formed accumulations within the lava flow (Richter & Murata, 1961; Jackson et al., 1981; Guest et al., 1995).
Usage
Literature
Photomicrographs
Normative composition
Normative minerals
SiO2
TiO2
ZrO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
F
S
CO2
Total
Molar proportion of normative mineral
Molecular mass of normative mineral
Weight % of normative mineral
Oxide
(wt. %)
46.01
0.93
19.81
9.56
0.11
10.97
9.00
2.73
0.09
0.02
99.23
Molecular
weight
60.08
79.88
101.96
71.85
70.94
40.31
56.08
61.98
94.20
141.95
Molecular
proportion
0.7658
0.0116
0.1943
0.1346
0.0016
0.2721
0.1605
0.0440
0.0010
0.0001
ap
0.0005
0.0001
0.0001
328.68
0.05
il
0.0116
0.0116
0.0116
151.75
1.77
or
0.0057
0.0010
0.0010
0.0010
556.67
0.53
ab´
0.2643
0.0440
0.0440
0.0440
524.46
23.10
an
0.2986
0.1493
0.1493
0.1493
278.21
41.53
zvyšky
0.1230
0.2721
di
0.0215
0.0033
0.0074
0.0107
0.0107
226.38
2.43
hy´
0.3844
0.1196
0.2648
0.3844
110.21
42.36
ol
0.1922
0.1196
0.2648
0.1922
160.32
30.81
ne
0.0083
0.0021
0.0041
0.0041
284.11
1.17
ab
0.2397
0.0420
0.0399
0.0399
524.46
20.95
D: 0.2086
D1: 0.0164
D2: 0.0164
Mg/(Mg+Fe2+): 0.689
Total of normative wt. % 99.23
Comment
Chemical composition
-
SiO2
46.01TiO2
0.93Al2O3
19.81FeO
9.56MnO
0.11MgO
10.97CaO
9.00Na2O
2.73K2O
0.09P2O5
0.02Total
99.23Mg(Mg/Fe2+)
0.67
-
SiO2
40.45TiO2
5.95Al2O3
16.44FeO
13.61MnO
0.12MgO
11.93CaO
7.66Na2O
2.35K2O
0.11P2O5
0.02LOI
0.33Total
99.55Mg(Mg/Fe2+)
0.61
-
SiO2
42.88TiO2
0.05Al2O3
20.73FeO
4.99MnO
0.07MgO
19.09CaO
11.41Na2O
0.23K2O
0.03P2O5
0.03Total
99.51Mg(Mg/Fe2+)
0.87