eosc lesson 7

the chemical formula of a given compound/mineral includes…?

component elements and number of atoms of each, written as a subscript

what is the “atoms per formula unit” (apfu)?

the number of atoms of each element in a compound

eg. quartz (SiO2) = APFU of silicon is 1, APFU of oxygen is 2

what does the 2 in the formula (Mg,Fe)2SiO4 mean, regarding the sum of Mg and Fe?

the total sum of Mg and/or Fe in the compound is 2; either Mg or Fe can take up to 2 atoms per formula unit

(Mg,Fe)2SiO4 is an example of a _____ solution.

solid

what is a solid solution?

its chemical formula is written using special notation that tells us about its composition

• shows the chemical formulas of the pure compounds at the two ends of the “range of composition” that the solid solution can have (ie. the diff possibilities for the formula)

• pure compounds = the 2 or more compounds that, when mixed together in varying proportions, give rise to the solid solution

identify the pure compounds of the following formula: (Mg,Fe)2SiO4

Mg2SiO4 and Fe2SiO4, respectively

what is charge neutrality in a compound?

every positive charge is balanced by a negative charge

anions have a ___ charge while cations have a ____ charge.

negative; positive

which elements are always part of a mineral’s chemical formula?

major elements

Fe in the formula Fe2SiO4 is a ______ element.

major

what are minor elements in a chemical formula?

• present in smaller amounts

• commonly replace major elements

• sometimes part of chemical formula

the dominant element in a substitution is usually listed ____ in the brackets.

first

what are trace elements in a chemical formula?

• found in very small amounts

• can either replace a major element or occupy “holes” in the structure

• mostly not included in written chem. formulas

What is a simple substitution in mineralogy?

occurs when one element substitutes for just one other element in a mineral

What is a coupled substitution in mineralogy?

occurs when more than two elements are involved in the substitution process in a mineral, to ensure charge neutrality

what is the goldilocks principle?

the idea that an element must have a similar ionic radius AND valence charge to the element it is substituting in a mineral

why is substitution important?

• allows for substitution of elements, which affects the mineral's properties

• especially important for gemstones, where minor and trace elements can substitute for major elements in a mineral

what are the 10 physical properties of minerals?

1. color
2. luster
3. streak
4. cleavage
5. fracture
6. tenacity
7. hardness
8. refractive index
9. specific gravity
10. fluorescence

(acronym: CLSCFTHRSF)

describe the property of color in minerals/how its identified.

caused by light absorption/refraction of certain wavelengths from:

1. foreign atom (trace element) presence

2. absence of atom/ionic radial typically present (called vacancy; result is a “color center”)

3. mineral structure

describe the property of luster in minerals/how its identified.

appearance of mineral’s surface in reflected light, with 2 types:

1. metallic

   • untarnished metal surface, opaque to light

2. non-metallic

   • lighter colour, degree of transparency (even if only on thin edges)

   • described w multiple terms: vitreous, adamantine, resinous, pearly, greasy, silky, satin, earthy

describe the property of streak in minerals/how its identified.

color of powder produced when drawn across a surface, useful for diagnosis bc:

• mineral’s streak is more consistent than color (on unweathered surface)

• can distinguish between similar minerals

describe the property of cleavage in minerals/how its identified.

ability of mineral to break along flat, planar surfaces + occurs at weakest points of atom bonding.

• surface is generally smooth, evenly reflect light

• consistent property; useful for diagnosis

• described by direction re: crystal faces (cubic, prismatic, basal) + how easy its produced (perfect = clvg easily produces smooth/lustrous surfaces)

• can have multiple directions or not be present at all

describe the property of fracture in minerals/how its identified.

breaks in minerals in directions that aren’t along clvg planes

* type/appearance of fracture helps in diagnosis
* conchoidal, hackly, uneven/even, splintery

describe the property of tenacity in minerals/how its identified.

set of physical properties reliant on cohesive force btw atoms in mineral structures

* malleability, ductility, brittleness, elasticity

describe the property of hardness in minerals/how its identified.

ease or difficulty that a mineral can be scratched

• important diagnostic property

• measured by Mohs scale (1-10)

  • more hydrous minerals = soft, anhydrous/silicate = hard (>5)

• unrelated to toughness/strength

describe the property of refractive index in minerals/how its identified.

measurement of change of velocity + direction when light passes through trans(parent/lucent) mineral

• ref. index = ratio of velocity of light in air, to its velocity in the crystal

• high index = light disperses into its component colors (gives “fire”)

• identified (by noobs) w special liquids or inexpensive equipment

describe the property of specific gravity in minerals/how its identified.

measure of the density of a substance, using ratio of (substance mass) to (mass of equal volume of water)

• ie. mineral w SG 2 = 2x heavy as water of same volume

• found by using special balances OR liquids that allow minerals of below certain SG to float

describe the property of fluorescence in minerals/how its identified.

when minerals emit coloured visible light under UV radiation

• found by using UV light

• inconsistent; less reliable diagnostic indicator

what are the 6 crystal systems?

1. triclinic: no perpendicular axes, the three axes are of unequal length.

2. monoclinic: one axis perpendicular to a plane of two other axes of unequal length.

3. orthorhombic: three perpendicular axes of unequal length.

4. tetragonal: one axis perpendicular to a plane of two other axes of equal length.

5. hexagonal: three axes of equal length in a horizontal plane and a fourth axis perpendicular to the plane.

6. cubic (isometric): three axes of equal length that intersect at right angles.

what is the polyhedral method for constructing atomic models of crystal structure?

• involves using polyhedral shapes to visualize how different elements interact in 3D to create crystals

  • tetrahedra, octahedra, cubes

• central cation is surrounded by a number of anions, and faces are drawn between the outside anions to form the polyhedral shape.

what are crystal habits?

the external shape or appearance of a crystal, as influenced by its internal crystal structure.