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Home , Microcline, Pleochroism

1 Earth and Space Sciences 439: Igneous Petrology

Laboratory 4: Summary of common Felsic and Accessory Minerals.

You have already studied the optical properties of many minerals. This lab is intended to provide a summary of common felsic and accessory minerals as they appear in . By the end of this lab you should be able to identify the common felsic minerals and accessory minerals in thin section. You will see all of these minerals many times between now and the end of the quarter. You should fill out a “data” sheet for each to reinforce your recall of the key optical properties. The sections can be examined in any order.

Quartz: : Anorthoclase Plagioclase "Perthite" and "Antiperthite" Feldspathoids: Leucite Accessory Minerals: Sphene Magnetite

In many samples, the minerals show evidence of alteration. Alteration may be quite minor in which case it is usually referred to as incipient alteration. In other cases, the alteration may be more extensive. Always note the nature and extent of alteration.

Quartz

Quartz is present in many of the thin sections in this lab. For example, you can look at sections of Jake’s Valley, Cathedral Peak, Adamello, Peekskill, Mt. Stuart, and Caulfeild.

The following criteria are used to distinguish quartz from :

(1) : 2 directions in feldspar [(010), (001)]; only irregular fracture in quartz. (2) Quartz is uniaxial positive; feldspar is biaxial, positive or negative. (3) Alkali feldspar has low negative relief, plagioclase feldspar (An100-20)* has low positive relief, relative to quartz and the mounting medium, which are approximately equal (n = 1.54). Plag of composition An~20-0 shows low negative relief relative to quartz. Observe a Becke line at a feldspar-quartz or feldspar-epoxy boundary. (4) Quartz often exhibits undulatory extinction induced by strain produced during deformation or cooling. Feldspars rarely show undulatory extinction. (5) Feldspars, particularly plagioclase, are often zoned, while quartz is not. 2 (6) Feldspars are usually twinned, quartz rarely so. (7) Feldspars are much more susceptible to alteration than quartz. In many sections, quartz will form clear grains while the felspars, particularly alkali felspars are partially altered, usually to a fine- grained sericitic mica)

Plagioclase (virtually all of the sections in this lab contain plagioclase)

Adamello:

Adamello is a granodiorite containing plagioclase (plus many other minerals). Fill out a data sheet for each mineral you examine. It will usually not be necessary to complete every item on the data sheet. Find suitably oriented plagioclase grains and determine their composition using the Michel-Levy method. Adamello is a plutonic rock. Therefore, you can assume that the plagioclase is in the low structural state. Describe the twinning and zoning carefully.

84-160 B:

Examine a number of plagioclase grains and fill out a data sheet. 84-160 B is a mafic volcanic rock so you can assume that the plagioclase is in a “high” structural state.

In plagioclases, both twinning and pericline twinning is polysynthetic (multiple). The two sets of twin types will be approximately at right angles to each other. As a general rule, albite twinning is much more common than pericline in plagioclase but in some cases it is necessary to distinguish between albite and pericline twins. The distinction is relatively staightforward because albite twins are length fast and pericline twins are length slow. To determine whether the twins are length fast or slow:

(1) Rotate one set of twin lamellae to extinction. In this orientation the vibration directions are parallel to the cross-hairs. (2) Next rotate the dark set of lamellae to the 45° position. (3) Insert your gypsum plate and observe subtraction or addition. (4) Subtraction = Length Fast; Addition = Length Slow

Anorthoclase

Cape Royds

These thin sections contain anorthoclase with "cross-hatch" or "tartan/plaid" twinning that is due to the presence of both albite and pericline twinning that can only occur in triclinic feldspars. "Cross-hatch" twinning occurs in both microcline and anorthoclase. Anorthoclase can be distinguished from microcline by the following features:

(1) Anorthoclase only occurs in rapidly chilled volcanic rocks whereas microcline only occurs in slowly cooled plutonic rocks. (2) Anorthoclase has a low negative 2V (<50°); microcline has a higher negative 2V(>65°). (3) In an a-normal section, anorthoclase will exhibit the cross-hatch twinning, while microcline in that orientation will only show the Ab twin lamellae.

3 Locate a grain with a good figure, estimate the 2V and determine the optic sign?

Caulfield - Unknown Alkali Feldspar

The alkali feldspar in this thin section is distinguished from quartz and plagioclase by its negative relief (best seen with medium or high power and the substage iris diaphragm closed down). Note that the alkali feldspar is interstitial, untwinned, and microperthitic. Microperthites appear somewhat dusty or turbid due to incipient alteration in contrast to quartz which tends to be clear (unaltered) in plane polarized light. Locate a grain that gives a centered optic axis figure (i.e., one with low ) and estimate the 2V of the alkali feldspar. Identify which alkali feldspar is present in this rock, i.e., is it sanidine, orthoclase or microcline—use type of twinning and 2V angle to make the distinction. Which K-feldspar polymorph(s) would you expect in a plutonic rock and which would you expect in a volcanic rock?

Jake's Valley - Three Felsic Unknowns

Compare the following properties of each of the three felsic mineral: (1) relief with respect to the mounting medium; (2) cleavage (angle and perfection); (3) twinning (presence or absence and the twin laws represented); and (4) uniaxial or biaxial, optic sign. Obtain interference figures on grains which are nearly dark between X-nicols [optic axial figures]. Classify each grain in one of the following groups: (1) uniaxial (+); (2) biaxial, small 2V(-) and (3) biaxial, moderate 2V. Identify the three felsic minerals in this thin section and fill out a data sheet for each. Is this a volcanic or plutonic rock and how can you tell just by looking at the thin section? Is the K-feldspar sanidine or orthoclase? Use the 2V angle to make the distinction.

Perthite: Lab 101, 101A

Perthitic alkali feldspars were initially a single alkali feldspar at high temperature (see lecture notes for relevant Ab-Or phase diagram). During cooling, the feldspar exsolved into K feldspar and Na feldspar domains. If the host phase is K rich feldspar then this mineral is called perthite. If the host phase is Na rich feldspar it is called antiperthite. Which polymorph of K- feldspar (orthoclase or microcline) is present in this sample? How can you tell? Note the lensoid shaped lamellae of the exsolved phase and the fact that the lamellae are oriented in approximately the same direction in any given . Make sure you can recognize the difference between exsolution and twinning. The orientation of perthitic intergrowths is controlled by the crystal structures of both of the intergrown minerals (see lecture notes). Exsolution occurs along planes where the two structures have an optimal fit. Sketch and label the perthitic intergrowths.

Peekskill - Three Felsic Unknowns

Identify the three felsic minerals in this thin section—all three show first order gray interference colors. Compare the following properties of each mineral: (1) relief with respect to the mounting medium; (2) cleavage (angle and perfection); (3) twinning (presence, twin laws); and (4) uniaxial or biaxial, optic sign.

Sketch a grain of alkali feldspar showing plaid twinning (albite + pericline twins). Is the twinning uniformly developed in all parts of individual grains? Note that the other feldspar also 4 shows grid twinning (albite + pericline twins), but that it is much sharper and more regular. Which alkali feldspars shows cross-hatched twinning and how might you distinguish among them.

With a medium power objective lens examine the myrmekitic intergrowths. Note the fine wormy intergrowths of quartz and determine, if possible, the composition of the host. Sketch this texture.

Magnet Cove: Nepheline

Nepheline is the abundant low birefringent (first order gray) mineral in this section. Unlike plagioclase, nepheline is untwinned, has poor cleavage and is uniaxial negative. Unlike quartz, it commonly shows secondary alteration (to a fine-grained phyllosilicate) and has a negative optic sign.

This section also contains aegirine-augite, (var. melanite), , apatite, calcite, , and pyrite. The melanite garnet is a rare type of garnet. In thin section melanite is dark brown (almost black) but the edges of grains will usually transmit some light and it is of course isotropic (extinct in all orientations under crossed polars). Pyrite is opaque in transmitted light but it can be recognized by its distinct yellow reflection color in reflected light.

I-18 - Leucite

Leucite is usually developed in equant and often euhedral exhibiting octagonal [stop sign] outlines in thin section. Leucite contains complex and repeated twinning on {110} formed by inversion during cooling. At high temperature (above ~900ºC) leucite is isometric. During cooling leucite undergoes a displacive transformation to an orthorhombic structure. As is common with displacive transformations, the low temperature form shows complex twinning as the structure rearranges itself. The crystals of leucite maintain the external morphology (isometric) of the high temperature form. Leucite has negative relief with respect to the mounting medium, extremely low birefringence, very poor cleavage, and is colorless in plane polarized light. Check these properties of leucite in I-18, which is from Mount Vesuvius, and fill out a data sheet.

Accessory Minerals

Cathedral Peak - Titanite (Sphene), apatite

Titanite is a common accessory mineral in granitic rocks and the Cathedral Peak quartz monzonite is particularly rich in titanite. Note the characteristic shape [not always present], very high birefringence, high relief, and slight . Cathedral Peak also contains accessory apatite, zircon, and opaques as do most silicic plutonic rocks Apatite is very widespread as an accessory mineral in rocks of a wide range of compositions. Because it is an accessory mineral, it usually occurs as small grains and is recognizable by its high relief, hexagonal cross section in grains cut approximately perpendicular to the c-axis, uniaxial negative, very low birefringence (dark first order gray). Apatite forms stubby grains somewhat elongated along c. Because of the very low birefringence, interference figures tend to be fuzzy.

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Caulfield (Accessory Minerals) - Apatite, Zircon, Opaques

Identify each of these minerals in these sections. Zircon has a high relief and forms short stubby prisms with parallel extinction. Zircon is commonly euhedral and has high birefringence (3- 4th order). In igneous rocks, zircon is a common accessory (mayor repository of Zr) but it is commonly more abundant in felsic igneous rocks. Apatite is commonly euhedral and has high relief. In cross section it is 6-sided while in sections approximately parallel to c, it appears as stubby prisms with parallel extinction. Apatite has low birefringence (first order gray), and is uniaxial negative although the interference figure is typically diffuse because of the low birefringence and small crystal size. Apatite is present in most igneous rocks since it is the major repository of phosphorus

Adamello - Titanite, Apatite, Zircon, Magnetite

Locate these minerals in the thin section. Opaque minerals are difficult to distinguish in transmitted light since they are black in all orientations. To properly identify the opaque minerals one needs to have a polished section and to view the section in reflected light. We will discuss this technique in a later lab. In the meantime, it is enough to identify the black minerals as generic “opaques”. Sometimes the shape of the grains can help in their identification, e.g., magnetite grains are isometric and tend to be equant, grains are hexagonal and tend to be elongated, sulfides have a “brassy” appearance when the section is viewed with a hand lens. Note that accessory minerals are commonly found together in clumps.