What is the most effective way to differentiate potassium feldspar from plagioclase feldspar?
What is the most effective way to differentiate potassium feldspar from plagioclase feldspar?
What is the most effective way to differentiate potassium feldspar from plagioclase feldspar? Some cleavage planes of plagioclase feldspars have striations, whereas no potassium feldspars have striations.
What are the 3 types of feldspar?
The main feldspars in this grouping include:
- Oligoclase.
- Andesine.
- Labradorite.
- Bytownite.
- Anorthite.
How do we identify potassium feldspar?
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 is the difference between quartz and plagioclase feldspar?
Quartz grains tend to be translucent gray, rounded to irregular blobs. The lack of a cleavage means their exposed surfaces are rounded and bumpy. K-feldspar and plagioclase are blocky, not translucent, and show flat cleavage surfaces. They occur in shades of opaque pink, white, gray, or dark gray.
What is the difference between orthoclase and plagioclase feldspar?
The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white colour. Orthoclase and plagioclase are important minerals. Both these forms are members of the feldspar group.
Does plagioclase feldspar react to acid?
For example, calcite (CaCO3) is soluble in acidic solutions. This reaction shows calcium plagioclase feldspar, but similar reactions could also be written for sodium or potassium feldspars. Those ions can eventually combine (probably in the ocean) to form the mineral calcite.
What type of silicate is K feldspar?
Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are also found in many types of sedimentary rocks….
| Feldspar | |
|---|---|
| Category | Tectosilicate |
| Formula (repeating unit) | KAlSi 3O 8 – NaAlSi 3O 8 – CaAl 2Si 2O 8 |
| Crystal system | Triclinic or monoclinic |
| Identification |
What is the cleavage of plagioclase feldspar?
| Physical Properties of Plagioclase | |
|---|---|
| Chemical Classification | Silicate |
| Luster | Vitreous. Pearly on some cleavage faces. |
| Diaphaneity | Translucent to transparent |
| Cleavage | Perfect in two directions that intersect at approximately 90 degrees. |
How do you test for plagioclase feldspar?
It is usually easy to distinguish plagioclase feldspars because their two planes of cleavage intersect at 90-degree angles, and their cleavage faces often display striations. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks.
What color is plagioclase feldspar?
| Physical Properties of Plagioclase | |
|---|---|
| Chemical Classification | Silicate |
| Color | Usually white or gray. Also colorless, yellow, orange, pink, red, brown, black, blue, green. |
| Streak | White |
| Luster | Vitreous. Pearly on some cleavage faces. |
What is the difference between orthoclase and plagioclase?
The key difference between orthoclase and plagioclase is that orthoclase appears in green-yellow colour, whereas plagioclase appears in white colour. Orthoclase and plagioclase are important minerals. A feldspar mineral is an abundant rock-forming mineral typically occurring as colourless or pale-coloured crystals.
What’s the difference between plagioclase and orthoclase?
What’s the difference between plagioclases and feldspars?
Plagioclase group consisted of more feldspars but the current IMA classification allows only 50% rule, so the plagioclases are basically albite (NaAlSi3O8) or anorthite (CaAl2Si2O8), which form albite-anorthite solid solution (albite-anorthite series). They occur more in calcium rich and mafic rocks = less acidic.
Where are the clear grains on the K-feldspar?
The clear grains with low birefringence left of the K-feldspar are quartz , and minor plagioclase (feldspar) is in the extreme lower left. The feldspars and quartz are difficult to distinguish in the PP view, although the feldspars are a bit cloudier due to incipient alteration.
What are the exsolved lamellae of K-feldspar?
The K-feldspar grain above it, on the edge of the photo, contains fine exsolved lamellae of albite (visible as stripes in XP). Minor plagioclase is also present, and also a few small patches of green biotite.
How to tell the difference between K-feldspar and Perthite?
When untwinned, distinguishing the different K-feldspar varieties may be difficult. · Intergrowths – Perthite is created when an original high temperature feldspar unmixes (exsolves) into two compositions. Perthites grains consist of zones of Na-rich plagioclase alternating in complicated patterns with zones of K-feldspar.
