Lesson 3 metamorphic textures

Metamorphic Textures
 Metamorphism literally means to “change form.”
 The degree of metamorphism is reflected in the rocks
texture and mineralogy.
METAMORPHISM CHANGES TEXTURE
 Two textures are formed as a result of metamorphism;
1) Foliated Texture
2) Non-Foliated Texture

Reference:
Tarbuck and Lutgens
Pages


Metamorphic Textures Depends On;
1) Allignment of minerals.
 Directional stress causes minerals to align when
recrystallizing. The orientation of these minerals is
usually perpendicular to the compressional forces.
 This gives the resulting metamorphic rock a layered
appearance.
 This is common during high-grade metamorphism and
often produces a foliated texture. Example: Gneiss.
Gneiss


Metamorphic Textures Depends On;
2) Recrystallization of minerals.
 Pressure causes minerals to recrystallize which
generally forms larger, more dense crystals.
 The recrystallization process causes some

metamorphic rocks to display visible crystals with no
visible evidence of the original mineral grains. For
example, quartz sandstone changes to form quartzite.
Quartz
Sandstone

Recrystallizes

Quartzite

 This is common during low-grade metamorphism and
often produces a non-foliated texture.


Two types of textures:
1)Foliated (i.e. layered or banded)
2)Non-foliated.
• Note that the texture of a metamorphic rock is usually
coarser than the parent rock (i.e. previous rock).

Foliated

Non-foliated


Metamorphic Textures
Two textures are formed as a result of metamorphism;
1) Foliated Texture
 Results when the minerals of a rock are brought into
parallel alignment during recrystallization. Produces a

banded or layered appearance.
Before
Metamorphism

After
Metamorphism

 Granite when metamorphosed forms Gneiss.

Examples:
Slate, Phyllite,
Schist & Gniess


Metamorphic Textures
Two textures are formed as a result of metamorphism;
2) Non - Foliated Texture
 Results when the minerals of a rock are recrystallized
creating uniform sized crystals with no visible foliation.
 This occurs when limestone (consist of pure calcite)
undergoes metamorphism and the smaller calcite crystals
combine to form larger interlocking crystals. This
process of recrystallization of calcite forms Marble.
Marble

Examples:
Quartzite &
Marble

Pairs
Parent Rock

Limestone
(Sedimentary)

Sandstone
(Sedimentary)

Shale
(Sedimentary)

Granite
(Igneous)

Metamorphic Rock
Changes to

Marble

Changes to

Quartzite

Changes to

Slate

(Phyllite – Schist - Gniess)

Changes to

Gniess


Sample Problem
Using examples, distinguish between foliated and
non-foliated metamorphic rocks.
Answer:
Foliated: show layering or banding of different minerals
created by the alignment of minerals during directional
stress and recrystallization (Ex: schist, gneiss)
Non foliated: a mass or network of interlocking crystals
consisting of one mineral with no visible layering or
banding (Ex: marble, quartzite)