Edit in JSFiddle

<p>
Mouse over the button below:
</p>

<br/>

<button class="button">
    <img class="icon second-icon" src="//ssl.gstatic.com/bt/C3341AA7A1A076756462EE2E5CD71C11/1x/bt_compose2_1x.png" />
    <img class="icon first-icon" src="//ssl.gstatic.com/bt/C3341AA7A1A076756462EE2E5CD71C11/2x/bt_speed_dial_2x.png" />
</button>

.button {
    background-color: #db4437;
    height: 56px;
    width: 56px;
    border: none;
    border-radius: 50%;
    box-shadow: 0 0 4px rgba(0, 0, 0, .14), 0 4px 8px rgba(0, 0, 0, .28);
    box-sizing: content-box;
    cursor: pointer;
    outline: none;
    padding: 0;
    position: relative;
    -webkit-transform: scale(1) rotate(360deg);
    transform: scale(1) rotate(360deg);
    -webkit-transition: -webkit-transform 150ms cubic-bezier(.4, 0, 1, 1);
    transition: transform 150ms cubic-bezier(.4, 0, 1, 1);
}
.button:hover {
    box-shadow: 0 0 6px rgba(0, 0, 0, .16), 0 6px 12px rgba(0, 0, 0, .32);
    -webkit-transition: box-shadow 150ms cubic-bezier(0, 0, .2, 1);
    transition: box-shadow 150ms cubic-bezier(0, 0, .2, 1);
}
.icon {
    background-size: 24px;
    display: block;
    height: 24px;
    margin: auto;
    width: 24px;
}
.second-icon, .first-icon {
    left: 0;
    margin-left: 16px;
    margin-top: 16px;
    position: absolute;
    top: 0;
    -webkit-transition: all .2s cubic-bezier(.4, 0, .2, 1);
    transition: all .2s cubic-bezier(.4, 0, .2, 1);
}
.second-icon, .button:hover .first-icon {
    opacity: 0;
    -webkit-transform: rotate(225deg);
    transform: rotate(225deg);
}
.button:hover .second-icon {
    opacity: 1;
    visibility: visible;
    -webkit-transform: rotate(360deg);
    transform: rotate(360deg);
}