GRAIN SIZE

Question 1

equigranular

Answer 1

all crystals in rock approximately same size

indicate entire rock crystallised under single set of P-T(depth) conditions

Question 2

aphanitic

Answer 2

rock with a uniformly ‘finer’ texture in which individual crystals are too small to be seen easily without magnification

indicates rapid cooling

Question 3

(aphanitic) cryptocrystalline

Answer 3

individual crystals too small to be seen easily even with petrological microscope

Question 4

(aphanitic) fine-grained

Answer 4

individual crystals large enough to be seen with petrological microscope

(crystal size 0.032 - 0.25mm)

Question 5

(aphanitic) very fine-grained

Answer 5

individual crystals large enough to be seen with petrological microscope

(crystal size 0.004 - 0.32mm)

Question 6

phaneritic

Answer 6

rock with uniformly coarser texture where all individual crystals are easily visible without magnification

indicates slow cooling

Question 7

(phaneritic) medium-grained

Answer 7

crystal size range 0.25-2mm

Question 8

(phaneritic) coarse-grained

Answer 8

crystal size range 2-16mm

Question 9

(phaneritic) very coarse-grained

Answer 9

crystal size range >16mm

indicates intrusive cooling of abnormally volatile-rich silicic magma