a bit of quaternions algebra;

CMRotationMatrix+SCNQuaternion.mm

SCNQuaternion CalculateRotation(CMRotationMatrix a){

	SCNQuaternion q;
	float trace = a.m11 + a.m22 + a.m33; // I removed + 1.0f; see discussion with Ethan
	if( trace > 0 ) { // I changed M_EPSILON to 0
	
		float s = 0.5f / sqrtf(trace + 1.0f);
		q.w = 0.25f / s;
		q.x = ( a.m32 - a.m23 ) * s;
		q.y = ( a.m13 - a.m31 ) * s; 
		q.z = ( a.m21 - a.m12 ) * s;
	}
	else{

		if ( a.m11 > a.m22 && a.m11 > a.m33 ) {

			float s = 2.0f * sqrtf( 1.0f + a.m11 - a.m22 - a.m33);
			q.w = (a.m32 - a.m23 ) / s;
			q.x = 0.25f * s;
			q.y = (a.m12 + a.m21 ) / s;
			q.z = (a.m13 + a.m31 ) / s;
		}
		else if (a.m22 > a.m33) {

			float s = 2.0f * sqrtf( 1.0f + a.m22 - a.m11 - a.m33);
			q.w = (a.m13 - a.m31 ) / s;
			q.x = (a.m12 + a.m21 ) / s;
			q.y = 0.25f * s;
			q.z = (a.m23 + a.m32 ) / s;
		}
		else {
			
			float s = 2.0f * sqrtf( 1.0f + a.m33 - a.m11 - a.m22 );
			q.w = (a.m21 - a.m12 ) / s;
			q.x = (a.m13 + a.m31 ) / s;
			q.y = (a.m23 + a.m32 ) / s;
			q.z = 0.25f * s;
		}
	}

	return q;
}

SCNQuaternion mulQuaternions(SCNQuaternion q1, SCNQuaternion q2){

	SCNQuaternion q;
	q.x =  q1.x * q2.w + q1.y * q2.z - q1.z * q2.y + q1.w * q2.x;
	q.y = -q1.x * q2.z + q1.y * q2.w + q1.z * q2.x + q1.w * q2.y;
	q.z =  q1.x * q2.y - q1.y * q2.x + q1.z * q2.w + q1.w * q2.z;
	q.w = -q1.x * q2.x - q1.y * q2.y - q1.z * q2.z + q1.w * q2.w;
	 
	return q;
}

GLKQuaternion GLKQuaternionFromSCNVector4(SCNVector4 v){

    return GLKQuaternionMake(v.x, v.y, v.z, v.w);
}