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PETROLOGY


Figure 3. Top: 4x, displays a groundmass dominated by larnite microlaths


Figure 3. Bottom:20x, merwinite mircolaths surrounded by spinel and oxides.

Right: 10x, hypidiomorphic texture of sadanagaite.

Two calcium silicate phases are found within the Granada material, being larnite and merwinite (fig. 3). The most abundant phase is larnite, which occurs as microlaths less than 1.0 mm in size, composing approximately 45% of the groundmass. The microlaths typically lack a prominent orientation and display quenched cooling patterns, with a non-trachytic texture relative to the groundmass. However, some localized microlaths are trachytic around microphenocrysts of amphibole grains in a distinct radial orientation. Polysynthetic twinning is localized around a few individual grains, typically occurring around crystal rims.

Merwinite occurs as microlaths that are less than 1.0 mm in size. The microlaths are anhedral and make up an estimated modal composition of 5%. Larnite microlaths are more anhedral and display lower order colors in polarized light whereas merwinite displays higher order colors. Merwinite is nearly pure in composition with some iron enrichment.


Figure 4. Top: 20x, trachyitic texture of larnite around sadanagaite;


Figure 4. Bottom: 10x, hypidiomorphic texture of sadanagaite.

Sadanagaite, an amphibole group mineral, is the second most abundant phase in the Granada material (fig. 4). It comprises approximately 15% of the groundmass, as subhedral to euhedral microphenocrysts that are less than 1.0 mm in size. Sadanagaite lacks pleochroism and displays prominent cleavage faces, which resemble pyroxene group minerals; particularly augite. The crystals occur as clusters, displaying a hypidiomorphic texture. The overall texture is slightly ophitic, where the amphibole crystals exceed the size of the larnite crystals (fig. 4). Compositionally, sadanagaite is an intermediate amphibole, corresponding to a hypothetical composition of 48% anthophyllite, 21% grunerite, and a 30% calcium amphibole end member composition (Table 1).


Table 1. Electron microprobe analysis (EMP) results of Granada material


Figure 5a: 40x, magnesiowustite cluster on larnite


Figure 5b: 20x, flower petal magnesiowustite structure on larnite;


Figure 5c: 20x, magnesiowustite grape-like clusters;


Figure 5d: 4x, iron metal crystal and quartz crystals.

Magnesiowustite occurs as spherules, and makes up approximately 6% of the groundmass (fig. 5). The spherules occur in clusters that are only a few dozen microns in diameter, with some displaying unusual grape-like clusters and flower petal structures (fig. 5). The spherules are orange to opaque in plane light. Compositionally, magnesiowustite grains have an unusual high concentration of manganese. Moreover, Frondel (1940) found that the manganese substitution is related to orientated inclusions of manganosite, implying a very low oxygen fugacity. Some spherules have darker rims, which may reflect an iron-enrichment. Magnesiowustite occurs chiefly with ilmenite and is modally concentrated as clusters on larnite microphenocrysts.

Ilmenite grains occur as anhedral to subhedral elongated microphenocrysts that are less than 1.0 mm in size; comprising approximately 4% of the groundmass. They typically occur with magnesiowustite spherules as localized clusters; however some individual microphenocrysts were found in association with amphibole phenocrysts. Compositionally, ilmenite has a significant pyrophanite component (MnTiO3), corresponding to a composition of 9% pyrophanite, 2% geikielite, and 90% ilmenite.

Spinel comprises approximately 4% of the material, with a crystalline habit of irregular decimated patches that are 1.0-2.0 mm in size. The patches are massive and granular, appearing opaque to green in plane light, with an anomalous green reflectance in polarized light. Moreover, the patches display conchoidal fracture and an interstitial texture relative to the microphenocrysts of sadanagaite. The crystals that were analyzed by EMP show an intermediate composition between galaxite and hercynite. Spinel corresponds to an end member composition of 25% galaxite, 20% hercynite, and 55% spinel.

The most abundant opaque phase is magnetite, composing 16% of the groundmass. The grains occur as 1.0-2.0mm anhedral to subhedral crystals, and are ophitic in reference to the other opaque phases.

The largest phase is iron metal, with grains being variable in size from 1.0-4.0 mm, and comprising approximately 4% of the specimen. Grains occur as poorly shaped anhedral phenocrysts (fig. 5). Compositionally they are relatively pure, with no kamacite component (Fe,Ni). Some iron phenocrysts show alteration halos of oxidation.

EMP identified various accessory minerals (less than 1%), within the Granada material including: quartz, plagioclase, orthoclase, albite, and calcite. Quartz was the only physically verified phase, occurring as localized crudely anhedral hexagonal zoned clustered microphenocrysts (fig. 5).

Plagioclase is intermediate in composition (andesine), whereas albite and orthoclase are pure.

Calcite was identified on the outer edge of the section as localized massive decimated patches; determined to be secondary in origin. Moreover, EMP analysis of some calcite suggests an intergrowth with quartz.

 

 

 

 

 

 

Photo Abbreviations: Larnite=l; merwinite=mw; sadanagaite=sa; opaque=op (magnetite); ilmenite=il; spinel=sp; magnesiowustite=wu; iron=Fe; quartz=qtz; oxidation=oxi; all blue areas in photomicrographs are epoxy-filled vesicles; all photomicrographs in plane light