What causes Perthitic texture?
A perthite is an intimate intergrowth of sodic and potassic feldspar resulting from subsolidus exsolution (unmixing of two minerals). During the slow cooling experienced by a plutonic rock, alkali feldspar crystals have sufficient time to invert to structures that are stabler at low temperatures.
How do you identify a Microcline in the thin section?
Microcline is often characterized by a distinctive plaid (cross-hatched) twinning (also known as tartan twinning), giving a zebra stripe appearance. This type is also called gridiron or quadrille structure, the two sets or lamellae being at right angles.
What is Microcline hardness?
Hardness is 6 – 6.5. Specific Gravity is approximately 2.5 (average) Streak is white. Associated Minerals are quartz, muscovite and plagioclase feldspars.
How do you identify alkali feldspar in a thin section?
In general, keys to identifying K-feldspar are its (lack of) color, its low birefringence, and its twinning. In thin section, microcline, orthoclase and sanidine are distinguished by their twinning, optical sign, and 2Vo.
What minerals contain Perthitic texture?
The largest documented single crystal of perthite was found in Hugo Mine in South Dakota and measured about 10.7 m (35 ft) x 4.6 m (15 ft) x 1.8 m (5 ft 11 in). The gem varieties of potassium feldspar, amazonite and moonstone are variant colored perthites.
How Myrmekite is formed?
Myrmekite, irregular, wormy penetration by quartz in plagioclase feldspar; these wartlike, wormlike, or fingerlike bodies may develop during the late stages of crystallization of igneous rocks if the two minerals (quartz and feldspar) grow simultaneously in the presence of a volatile phase.
How do you identify a Microcline?
Microcline may be clear, white, pale-yellow, brick-red, or green; it is generally characterized by cross-hatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline.
Where is Microcline found?
Microcline is found at Baveno, Italy; Kragerø, Nor.; Madagascar; and, as amazonstone, in the Urals, Russia, and Florissant, Colo., U.S. For detailed physical properties, see feldspar (table).
What is Microcline used for?
Microcline is used industrially in the production of glass and ceramic products. It is used as ornamental lapidary material with Amazonite in green color. Sometimes feldspar is also used in the manufacture of glass.
How is a Microcline formed?
Microcline (KAlSi3O8) is the triclinic low-temperature K–feldspar stable at temperatures lower than 500°C. It is usually formed by recrystallization from feldspar, and sometimes by direct crystallization from magma and hydrothermal processes.
What does feldspar look like in thin section?
In thin section, alkali feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL).
How can you tell the difference between plagioclase and alkali feldspar in thin section?
Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not.
What are the physical properties of a microcline?
PHYSICAL PROPERTIES Microcline Chemical Composition: KAlSi3O8 Class: tektosilicate Crystal system: triclinic; bar 1 Crystal habit: commonly as cleavable masses, lattice (perthitic) intergrowth with albite is almost universal, also as anhedral to euhedral crystals, and as a rock constituent it occurs in irregular grains.
Why does a microcline have a perthitic texture?
Microcline. It means that there are narrow bands of albite (sodium-rich feldspar) in the K-feldspar host crystal. Perthitic texture is often visible to the naked eye. This texture is the result of exsolution which happened after the cooling of originally homogenous crystals which are not stable at lower temperatures.
How are plagioclase and microcline alike and different?
The mineral’s low relief, lack of color and tartan twinning are characteristic. In plagioclase feldspar polysynthetic twinning in two directions can mimic microcline’s tartan twinning. However, the polysynthetic twinning of plagioclase is clearly defined and does not pinch and swell as the tartan twinning of microcline does.
Where can you find microcline in sedimentary rocks?
Microcline is also common in metamorphic rocks in gneisses, and in sedimentary rocks in arkoses and conglomerates. Microcline is distinguished in thin section from orthoclase (the medium temperature polymorph of potassium feldspar which is chemically identical to microcline) by the presence of tartan twinning.