- #71
- 7,536
- 3,353
This sample is also a melteigite:
This sample consists of subhedral elongated grains of aegirine-augite, diopside, and apatite in a groundmass of (altered) nepheline, calcite, and titanite:
Nepheline altered to chlorite and sericite/muscovite (below, surrounded by apatite grains) so this sample is probably F2 fenitized to some degree.
Altered nepheline (center, below) contains small crystals of (possibly) epidote, if true then presence implies hydrothermal alteration.
A higher-magnification XP view of the crystalline inclusions- one in the upper left has the characteristic habit:
Some calcite in veins. Opaques: grains and a few “skeletal” arrangements of needlelike grains (ilmenite? magnetite? (60 degree symmetry? 90 degree symmetry? reflecting the primary mineral?) filled with pyroxene, calcite, and cryptocrystalline (probably)TiO2. These features are really striking and can best be seen with epi-darkfield imaging:
Biotite replaced with chlorite (anomalous blue birefringence), here with (F2?) titanite (very high relief) and pyroxene (high relief), a grain of carbonatite on the lower right:
Sample lacks melanite (see below).
From a relevant paper: Ijolites and melteigites (mela-ijolites)consist of euhedral prismatic crystals of clinopyroxene and apatite set in a matrix of nepheline and minor calcite. Sphene and strongly zoned (5-12% TiO2) melanite are common accessories. Pyroxenes are pleochroic in shades of light yellow-green to apple- green. Colorless to light brown cores of Al-diopside can be found, but the bulk of the pyroxenes are low Ti, low Al, Na-rich diopsides. The compositional zoning is one of Na and Fe enrichment along an aegirine-hedenbergite trend similar to that determined for the urtite pyroxenes, e.g.Di70Hd20Aeg10 to Di40Hd40Aeg20. Pyroxenes from ijolites which contain melanite are richer in Na and Fe on average than pyroxenes from rocks which lack melanite.
This sample consists of subhedral elongated grains of aegirine-augite, diopside, and apatite in a groundmass of (altered) nepheline, calcite, and titanite:
Nepheline altered to chlorite and sericite/muscovite (below, surrounded by apatite grains) so this sample is probably F2 fenitized to some degree.
A higher-magnification XP view of the crystalline inclusions- one in the upper left has the characteristic habit:
Some calcite in veins. Opaques: grains and a few “skeletal” arrangements of needlelike grains (ilmenite? magnetite? (60 degree symmetry? 90 degree symmetry? reflecting the primary mineral?) filled with pyroxene, calcite, and cryptocrystalline (probably)TiO2. These features are really striking and can best be seen with epi-darkfield imaging:
Biotite replaced with chlorite (anomalous blue birefringence), here with (F2?) titanite (very high relief) and pyroxene (high relief), a grain of carbonatite on the lower right:
Sample lacks melanite (see below).
From a relevant paper: Ijolites and melteigites (mela-ijolites)consist of euhedral prismatic crystals of clinopyroxene and apatite set in a matrix of nepheline and minor calcite. Sphene and strongly zoned (5-12% TiO2) melanite are common accessories. Pyroxenes are pleochroic in shades of light yellow-green to apple- green. Colorless to light brown cores of Al-diopside can be found, but the bulk of the pyroxenes are low Ti, low Al, Na-rich diopsides. The compositional zoning is one of Na and Fe enrichment along an aegirine-hedenbergite trend similar to that determined for the urtite pyroxenes, e.g.Di70Hd20Aeg10 to Di40Hd40Aeg20. Pyroxenes from ijolites which contain melanite are richer in Na and Fe on average than pyroxenes from rocks which lack melanite.