Only a few rare metagabbros contain igneous clinopyroxene and orthopyroxene
partially replaced by hornblende.
Various factors, including magma composition, control whether orthopyroxene
or pigeonite occurs as the low-Ca pyroxene, but high temperatures yield orthopyroxene
and low-pressure crystallization favours pigeonite.
Galileo's near-infrared mapping spectrometer hasn't examined the mineral composition of Ida's entire surface, but those regions it has looked at consist mainly of the mineral olivine, with traces of orthopyroxene
. In contrast, the moon's surface appears to have roughly equal mixtures of these two minerals and one other, clinopyroxene.
The Model composition of gabbroic rcoks includesplagioclase (pl) 45 - 70 Percent , clinopyroxene (cpx) 15 - 30 Percent , orthopyroxene
(opx) 10 - 20 Percent, magnetite and ilmenite.
Coughlan (1986) recognized five units in the Staples Mountain Gabbro, designated I to V from inferred bottom to top: I--gabbro, anorthositic gabbro, and olivine gabbro; II--subophitic augite gabbro; III--interlayered norite and anortbosite; IV--unlayered augite gabbro and minor olivine gabbro; V --unlayered gabbronorite, with increased orthopyroxene
and no olivine, in contrast to unit IV.
This residual nature is confirmed by the higher Cr # in spinel and Mg # in orthopyroxene
and olivine from the foliated peridotite suggesting their derivation by higher degree of partial melting from a depleted mantle source.
Rock types sampled in the Duck Lake Pluton include gabbro, orthopyroxene
gabbro, gabbronorite, olivine gabbronorite, anorthosite, dunite, and wehrlite (Grammatikopoulos 1992; White 1996).
The main primary minerals include plagioclase, clinopyroxene, orthopyroxene
, amphibole and minor quartzdescribed as under: Plagioclase: Plagioclase crystals are euhedral to subhedral, lathlike and columnar in shape, and exhibit polysynthetic twinning (according to the albite and occasionally to the combined albite and Carlsbad laws) and oscillatory zoning.
6) Silica-rich glass around partly dissolved orthopyroxene
crystals in peridotites.
They essentially consist of plagioclase, orthopyroxene
and clinopyroxene with accessory amounts of epidote and quartz.
The serpentinite-Ellsworth contact is sheared, and the serpentinite has a harzburgite protolith with elongate orthopyroxene
is more abundant than clinopyroxene and occurs as subhedral phenocrysts, some largely replaced by amphibole.