Gabbro: R1831

Title: Gabbro: R1831
Description: Hand Sample: This is a overall black rock with relatively high density. This rock has a holocrystalline texture. The crystals it contains are mostly black,white and colorless. 60% of them are black hornblende, 30% of them are colorless olivine, and 5-10% of them are white vitreous (probably) feldspar.

Thin Section: From the thin section we can see a better holocrystalline texture with only some vesicles (black spots shown in PPL view). Many of the minerals display sieve (or embayed) texture (a).In (b) we can see the crystal consists of many needle-like microlites. In (1) we can see polysynthetic twins, so this is a feldspar (about 35% of all crystals). (2) displays undulatory extinction, which suggests that it is quartz (30%). (3) has a high birefringence color and parallel extinction, so we can say it’s olivine (10%). From its color in PPL (also shows pleochroism) and XPL we can say (4) is hornblende (10%).

Life History: This rock didn’t stay for a long time underground before it was blown out by the eruption. The general environment has relatively low pressure and temperature. It maybe experienced compositional change due to magma ejection or experienced the escape of gas.

Location:Salem, Massachusetts
Creator: 8585
Type: Extrusive Igneous Rock
Mediator: Tamara Carley

Media: https://exhibits.lafayette.edu/geology/files/original/3d6e97c5def88daee5c0785536362e6c.JPG; https://exhibits.lafayette.edu/geology/files/original/230d43ab2dccb36f296108502037d141.JPG; https://exhibits.lafayette.edu/geology/files/original/c30517502674dae34bf94e8340453d42.JPG; https://exhibits.lafayette.edu/geology/files/original/8b490f0782a45000e1835429836ec825.JPG; https://exhibits.lafayette.edu/geology/files/original/f407b7ba85c250c600ed107ad3657c7d.JPG

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